The structural integrity of rifle stocks, part 1 of many

Or, playing with Fusion 360’s Generative Design feature, which is available for free until January 2020!

I spend a fair portion of my every day at the office subtracting material from gun stocks to make receivers, barrels, and bottom metals fit. To be very clear, a firearm is a tool, a tool where the forces involved in the standard operation represent a hazardous condition if things aren’t right. More than that, they are a tool someone may bet life and limb on at some point.

Given the serious nature, and not discounting the fact I many times have 20 minutes to let my mind ponder these things while I watch a machine run, a great deal of thought goes into the forces at play and how my actions affect the structural integrity of the end result.

Autodesk has been spamming me non-stop about a free period of generative design in their Fusion 360 product and I decided it might be fun to validate a few assumptions about rifle stocks and play with Fusion 360 at the same time.

I probably should have spent a little time brushing up on long-forgotten basic physics before I began. Specifically, I should have made sure I had a good recall of the information required to make accurate inputs for the “Structural Loads” portion of the generative design. These loads are how the forces of recoil would be accurately interpreted by the analysis and enable it to generate something resembling an accurate output. I didn’t do that, I just pulled a few things that sounded semi-reasonable out of the air and ran with it. I will have to circle back on this.

For those not familiar with the construction of rifle stocks they are many and varied, but generally speaking, high-end stocks come in two forms outside of injection-molded stocks and “chassis” systems. There are wood stocks, in which the species of wood, the particular cut of wood, grain orientation, and etc. are all very pertinent to the structural integrity of the stock. Laminated wood is a bit different animal. Most of the stocks I deal with day to day, though, are of the composite variety. They are constructed of a fiberglass or carbon fiber shell filled with a “fill” material. The shell on these stocks is exactly what you would expect of a fiberglass or carbon fiber part, they are various layers of cloth material molded into the appropriate shape. Typical shell thickness ranges from 0.025″ to 0.125″, largely depending on fabric type, the brand of stock, location on the stock. This otherwise hollow shell is stuffed full of a combination of epoxy and a variety of other filler materials, most commonly little pieces of chopped or milled fiberglass or silica.

You can clearly distinguish the thin carbon fiber shell, two different types of fill material, and stainless steel pillars in this photo of a Manners elite carbon fiber stock

The assumption typically made is the strength and overall structural integrity of a composite stock is largely in the shell. The fill is typically there to support the various features and accessories, such as the recoil lug area forward of the receiver, sling studs or flush cups attached for carrying, etc. Ok, cool, seems reasonable. Except, when we install a barreled action, bottom metal, and everything else into a stock to make a complete rifle, we tend to gut the thing. The entire top gets hogged out, almost to the edges, for the receiver. Likewise, underneath the receiver, most of the bottom gets hogged out for the bottom metal. Most of what is left of the shell are the two sides of the stock, which doesn’t seem to lend as much structural integrity as one would like… Especially directly rearward of where the recoil gets absorbed into the stock.

I wasn’t intending to be very scientific, I was just hoping to start getting an idea of how the forces are transmitted through the stock. So here we go — to start, I modeled up an extremely simplistic representation of a barreled action. The bolt and bolt handle was modeled in very simple form both closed and fully open.

Extremely simple barreled action

This generative design functionality in Fusion 360 requires 3 major elements. You must define all of your required features, in our case things such as where the recoil lug will transmit force into the stock, and where the recoil pad will attach / where the stock will transmit force into your shoulder. Fusion refers to these features as “Preserve Geometry”. You must also define your “Obstacle Geometry”, these are all the places you don’t want the software to put any material. In our case, we have a receiver, recoil lug, bolt, bolt handle, barrel, bottom metal, trigger, etc. We don’t want the software to create structural elements of our stock that can’t actually exist, because another component of the rifle has to live there.

Fusion 360 Generative Design “Obstacle Geometry”

I created only four features the software was asked to preserve, the rear of the recoil lug well, the recoil pad attachment face, and two pillars with lengths typical of an M5 short action DBM.

Fusion 360 Generative Design “Preserve Geometry”

Loads must also be defined. The force of gravity is pre-defined for you by the software, but it can be nuked or modified if necessary. This is probably one of the more critical parts of the whole deal, garbage in – garbage out, as they say. Accurately defining the loads your part/object will be subject to will allow the software to generate a model able to withstand those loads. Not accurately defining these constraints will put you out in left field, guessing — but today I ignored this fact I already knew and think I learned a few things anyway, TBD.

I also ignored any form or fashion of stock “user interface”. In theory, once this design is generated, you could install a barreled action and bottom metal, pull it to your shoulder, and fire it. Doing so would be a bit of a trick, however, as there are zero provisions for actually holding onto this generated stock, nowhere to rest your cheek, none of what makes a stock — well, a stock. A stock really has two jobs, it has to hold onto the receiver and all the other components, and deal with all the forces that happen when fired. The stock also has to serve as a comfortable and intuitive user interface, allowing one to use the firearm effectively. Yes, we are absolutely ignoring that second one, to be continued…

Fusion will also allow you to constrain your simulation to particular materials and production methods. My general sense is the materials are accurately simulated at least for parts that will ultimately be machined, perhaps not created additively, such as with 3d printing. I also have the impression the various options related to how the part will be produced is somewhat hand-wavy. At least in my experience thus far it seems dubious to think you might actually end up with a part you can turn around and produce using this method. You will be money ahead if you simply use it as a means of idea generation.

I ran the generation process with a variety of materials and production methods, but I will only be sharing the results of the CFRP (Carbon Fiber Reinforced Polymer) material. CFRP adequately describes the construction of a composite stock, but considering a stock is a shell with fill I will be taking any output with an additional grain of salt (or in the case of Fusion, adding in some additional “Safety Factor”).

I ultimately ran two different simulations with different loads and settings. One a bit on the side of having this big piece of thing that needs to carry a barreled action around, but doesn’t necessarily need to withstand much recoil. There are a variety of settings related to the design objective at your disposal in this process. For the first simulation, with low recoil loading, it was directed to maximize stiffness at low target weight.

As you can see from the images below, the generative design process resulted in a stock a bit exoskeletal in nature, for lack of a better term. You can imagine this trellis-like structure would result in an overall part that was fairly stiff at a low weight — around 200 grams in this case.

Simulation 1 with Preserve and Obstacle Geometry
Simulation 1 rear view
Simulation 1 front view

The other simulation was performed as if the stock was going to have to eat a ton of punishment coming through the recoil lug area. The objectives for this simulation were a bit in opposition to the previous one. It was instructed to minimize mass while handling the specified forces, with a defined safety factor.

As you can see in the following images of the result, this design was very simple and architectural. The generative process resulted in what looks to be as close as it could come to a linear beam between the lug face and the recoil pad. This thing would very likely eat recoil quite well, but might also be a bit of a wet noodle to handle — but also at around 200 grams.

Simulation 2 with Preserve and Obstacle Geometry
Simulation 2 rear view
Simulation 2 front view

What’s the take-away? I have some thoughts, but I’m not sure these particular simulations spell out any sort of improved stock design on their own. You can grab my working file below if playing with this sounds like a lovely way to spend an afternoon!

The following are step files of the two results.

After a bit more time playing with generative design, it might be fun to bring things full circle by modeling a good representation of a “real” stock and playing with Fusion 360’s simulation tools. It would be interesting to see, with different material types applied, what manner of interaction there is between the shell and the fill material.

To be continued, just so long as I don’t get distracted by too many shiny objects.

So I built my father a rifle…

Read Part 1 first…

So who to actually build this thing? I wasn’t going to be running chamber reamers on a drill press or thread the muzzle with a hardware store die set. I might stoop so low as to Krylon a stock, but not on this rifle… definitely not on this one. There was only one choice, it had to be LRI-built. Half high-end gunsmithing shop and half machine shop, they bring the weird and impossible to life… and they’re local.

I decided to go with barrels made nearby in Rapid City, SD by K&P Gun Company. These barrels are well priced, match grade, cut-rifled, and made of 416R stainless. They didn’t need to be all that, I liked that they are manufactured effectively just down the road. The fact they are some of the finest traditional match grade barrels made certainly does not hurt, though. I got these barrels in a varmint contour, which is a bit on the heavy side, but I rationalized that a few different ways: they would be fluted to take a bit of weight off, they would handle trips to the local prairie dog town well, they would leave plenty of meat to support the 5/8-24 threads and suppressor, and the weight would moderate recoil a bit. I had both barrels finished at 19″ so the rifle with 7″ of suppressor would handle somewhat like a rifle with a typical 26″ barrel. Finishing these at 19″ also took quite a bit of weight off. I figured they would be approximately the same weight as a 26″ #5 contour barrel once they had been shortened, and the fluting would lighten them further to about the weight of a 26″ #4 contour. This should lead to a rifle that carried about like one of the modern sporting rifles (Bergara, Howa, etc) which have a #5.5-6 contour barrel once the suppressor was installed.

A variety of custom touches were added, such as engraving on the barrels.

I had the McMillan stock bedded using LRI’s pillar bedding process, where the receiver is mated to the stock by a layer of epoxy 0.05″ thick. The receiver is dunked into the stock that has had an oversized receiver-shaped bathtub carved into it, and that void filled with epoxy resin. After curing, the receiver is popped back out, and all the various ports are milled through the epoxy. This is a fairly different process than typical bedding, where the complete inlet is done first and a thin skim of epoxy is used to mate the receiver. It’s tough to control the epoxy, so the traditional method typically ends up looking pretty messy when you pull the receiver out, whereas the LRI method looks like art.

The ARC Nucleus being pillar bedded in the McMillan HTG

To take a bit of the guesswork out of swapping barrels I tracked down a torque wrench just for the application. The barrels both had one of LRI’s contour-matched thread protectors on the muzzle, but this rifle was a bit different… with two barrels one of them would need to be wearing a thread protector on the other end. A custom tenon “Thread Pro” was in order.

The bottom metal was a choice I thought should have been easy but really wasn’t. My dad has big hands and needs something a bit more “tactical” to work well with his physiology. The problem with this is that most of the bottom metals in this vein either have a huge magazine catch lever that would catch on everything while being packed in a hunting situation, or they have features a bit silly for a hunting / all-around rifle, like barricade stops. I eventually settled on a Badger M5 footprint enhanced DBM. The Badger has the magazine release integrated into the trigger guard but does so in such a way the paddles are still easy to push with gloved hands (or very large fingers). The barricade stop would have to be milled off prior to assembly.

Easy stuff, just mill that barricade stop right off of there…

I decided on a two-stage trigger for no other reason than I have a preference for them, especially in an all-around firearm that will be used for hunting. There is a bit of a lack of good two-stage triggers for Remington footprint rifle actions, and I settled on the newly-released Geissele Super 700. I have used and liked Geissele triggers in AR-platform rifles and liked them quite a lot, fingers crossed.

That led me to magazines. Because the bodies of the two different cartridges are significantly different in diameter (even though the case head is the same size) they would need to be fed from different magazines. I guess I should clarify that… it is definitely possible they both could have been set up to feed from a common, shared, magazine but the 243’s would have sat much higher and caught a lot more of the bolt face during feeding than the 284’s. Ensuring 100% reliable feeding the first time, every time, meant setting up magazines for each round. In a perfect world, your action and bottom metal magazine catch are the right distance from each other so when the magazine registers against the catch the bolt pushes against just the right amount of case as it comes forward to feed reliably without rubbing on the magazine feed lips. The 243’s feed well from unmodified factory magazines. The 284’s will feed, but ultimately the magazines were modified by opening up the feed lips slightly to ensure reliable feeding over the long haul.

I decided on a pair of Accuracy International 5-round magazines for the 284. These magazines hold 4 of the larger-diameter 284 rounds. I figured 4 was perfectly adequate for what would likely be used on hunts for larger game. This magazine was also fairly low profile, which would be beneficial when packing the rifle. The binder plates at the nose of these magazines were removed and replaced with a soldered-in steel plug. This was to ensure the hand loaded 284 rounds could be loaded out a bit longer to really take advantage of the larger case capacity the 284 offers.

For the 243, I settled on a pair of American Rifle Company’s double-stack center-feed magazines. These magazines are a bit taller than the 5-round magazines, but not so much taller as to be problematic. To ensure reliable feeding of these magazines they were equipped with LongRifles, Inc. aluminum followers. The 10-round capacity of the magazines intended for the 243 seemed appropriate, as these may be used for the occasional target shooting or varmint session.

Down to the details, rings, bipod, sling, paint… American Rifle Company makes amazing actions, they also make great scope rings. Choosing them was a no-brainer. For the bipod, I chose a 9-13″ Harris with the swivel and leg detents. The Harris is classic, functional and lightweight, hard to beat. For the sling, I decided synthetic material would be the best match, and an easily adjustable but fairly traditional sling would be the best match. I settled on a hunting sling from Viking Tactics.

Final test assembly before painting.

The paint was a matter of much deliberation. The stock and metal were to be Cerakoted at the end of the build by LRI, who have the capability in-house to do almost anything from mild to wild. When looking at some of the amazing stencil work they regularly publish online, it can be a bit hard to reign oneself in and remember… this isn’t my rifle, what would my dad really like? In the end, I settled on a grey theme, with the stock coated with Sniper Grey with dual webbing colors on top in Graphite Black and Stainless Steel. All of the metalwork was finished in Graphite Black sans the action and bolt, which were left in their factory black carburized finish.

Finally fully assembled, and just in time. This build was wrapped up on December 21, 2018. (Yes, it took me almost year to post these details).

Ready for Christmas morning.

Putting together something very custom has a way of spiraling out of control, but that can be enjoyable too — just so long as you don’t do it every day. As nice as it looks I sure hope my dad treats it like the tool it is.

I wanted to do something extra rad for my dad…

I am not much of a gift giver and I don’t remember ever having been (as an adult at least). I think that’s largely because I’m not much of a gift getter. I generally know what I want, have prioritized those things that are important to me, and work to make that happen. So I have everything I need, and almost everything I want. Anything I might want but don’t have that can be purchased … well, if I don’t have it already, that’s probably because it’s expensive. By expensive, I mean unreasonable to gift. At this point everything on my personal list of wants costs at least as much as a decent used car. My personal wishlist probably wouldn’t make a great Amazon Wishlist to circulate to my friends and family. Totally unreasonable.

I think I get this gift aversion from my Dad. For as long as I can remember he has instilled in me the virtues of only having to buy something one time. I always remember him being very particular about what material things he chose to spend his hard-earned money on. It had to be the right brand and have the right features. It had to have a reputation for durability and availability of good service or warranty. What it didn’t need to be was any particular color, or cheap. My dad also has everything he needs, and most everything he wants. How do you reasonably buy a person like that a gift?

As an aside, before I tell you about the unreasonable gift I gave my father, I will tell you how you can make the holidays and birthdays work for people who seem to have everything. It’s very simple, people who are particular about what they own are also particular about what they consume. Consume we do, however! Pay attention to the things we use as a consumable, maybe it’s drill bits, coffee, wine or beer, or a very particular variety of toilet paper. It doesn’t matter, we will appreciate the gift and we will appreciate the thought, especially since you noticed our high standards in whatever the consumable might be. I would be absolutely elated to receive a case of Charmin Ultra Strong Double Rolls with a nice note for the holidays. Or drill bits.

It’s festive right there in its own packaging, no need to wrap or even add a bow.

I have known my father long enough to have learned a few things about him apart from his ability to be impossible to buy gifts for. He enjoys doing things outside, enjoys hunting, enjoys the solitude hunting entails, enjoys the planning, and enjoys the bounty it can reap. He doesn’t necessarily love firearms. He grew up with them, is comfortable around them, but he doesn’t embrace them the way many from the midwest who grew up hunting do. He doesn’t like the recoil, he doesn’t like the noise, he doesn’t like headache-inducing muzzle blast.

My fathers favorite tool is usually a bit traditional, heavy, and hard to hurt. If we’re moving dirt together he will naturally tend to pick up the biggest mattock, and take nice slow swings, letting the tool do the work. If we’re felling trees together his preferred tool will be in the 70-80cc class, and he’ll never complain about hauling around the bit of extra weight that entails… he’s happy to have enough saw when he puts it into the tree.

My father is also actively trying to reduce his footprint on this earth. Having spent his adult life maintaining dozens of engines on everything from saws, to mowers, to log splitters, multiple vehicles, multiple trailers, tractors, he prefers nowadays a bit of consolidation. He is the target market Miller and Lincoln are going after with the new multi-process, multi-voltage welders on the market. Why own, store, and maintain three welders when you can have one?

I started this rifle build with a set of rough specifications. It was to be suppressed from the start as a suppressor helps moderate much of what my dad doesn’t enjoy about firearms. It should be multi-caliber capable, so it can work as well on varmints and deer as on moose and elk. It need not be a complete lightweight but should be light enough to be fielded for a half day. It should pack and carry as well as a typical rifle with a 26″ barrel. It should have a larger for-end, length of pull, and trigger guard to accommodate my dad’s long arms and large hands. It should be easy to switch between calibers, both the actual barrel swap and the dope.

The rough concept and bullet points were written down toward the end of 2017. Not knowing for sure yet what caliber(s) it would be chambered in, I decided on a 30 caliber suppressor with the common 5/8-24 thread. I also decided a Titanium suppressor would be the best choice. Weight was not the primary concern with the build overall, but it made sense to limit the amount of weight hanging out at the end of the barrel. Of all the suppressors on the market, I narrowed it down to a couple of manufacturers. It would either be a Thunderbeast ULTRA-series manufactured in Cheyenne, WY, or a Dakota Silencer Varminter manufactured in Sturgis, SD. Both hit my goals for weight, construction, and sound reduction. I was leaning toward a 9″, and originally ordered the Varminter 3.0 for no other reason than they are manufactured closer to home. By the time I had my paperwork in, the Varminter 4.0 which could be run as a 7″ or 9″ suppressor was on the market and that is what I ended up with.

Based on the decision to run a 9″ suppressor the rest of the rifle needed to be short to be able to pack the whole package around, in and out of vehicles, or through the woods. I decided it needed to be based on a short action, and that there were plenty of big game capable short action cartridges available to make the package work. I was originally planning to build the rifle around an American Rifle Company Mausingfield action. Having seen these actions in person alongside numerous other tuned up and custom actions, there really just wasn’t anything else that could hold a candle to it. The Mausingfield is really just as good as it gets and the only negative thing anyone can say about it is that, well… it’s expensive. About the time I was ready to order the action, in early 2018 the American Rifle Company Nucleus was announced. The Nucleus retained many of the amazing combinations of features making the Mausingfield top-tier while adding some great additional features and reducing the price. Around the time the Nucleus was announced American Rifle Company also announced what they call the Barloc. This device makes it possible to quickly and fairly easily change barrels. It removes the need for a barrel vice, action wrench or any other special wrench such as for a Savage-style barrel nut. It even eliminated the need for a headspace gauge. If the barrel is re-installed to the same position, or with the same amount of torque, prior to tightening the Barloc, the headspace will be consistent every time the barrel is removed and put back on. Perfect. The Nucleus with a Barloc it is, and they were available at a discounted price at the time.

American Rifle Company receiver with Barloc. Simple physics at its finest.

One of the next major decisions was the stock. Dad and I are both big fans of wood, but in terms of practicality, there really is no beating fiberglass. After considering all the options, I decided it had to be a McMillan for a number of reasons. One reason was lead- time, McMillan was able to supply an unfinished/blank stock within a reasonable timeframe. The second reason was quality, the McMillans I have seen on average seem to have a better fit and finish out of the mold than some competitors. Lastly, I wanted a stock with fill only, that wasn’t built around a bedding block. This is so it could be pillar bedded using my choice of pillars. After debating which of several stocks in the McMillan line would be best for several weeks, I finally ran out of time and pulled the trigger on an M40A1 HTG. This stock has a fairly traditional look and handles like a fairly traditional stock as well. It doesn’t feel or look out of place in a hunting or field application. It balances those traditional attributes with a bit of tactical thrown in, it has a larger forend, good for larger barrels as well as larger hands and can be had with an adjustable cheekpiece and spacer system to adjust the length of pull. I ordered the stock with both of these options, but without any adjustable cheek hardware. Staying on the classic side of the fence, the stock would be configured with 3 sling studs in lieu of tactical rails or flush cups. The stock was ordered with a glass shell and standard fill, not the lightest but durable, cost-effective, and practical.

McMillan M40 HTG

Enough decisions had been made that I could hardly move further forward without deciding what the barrels would be chambered in. The Nucleus, with its removable bolt face, would have allowed for nearly any option. Initially, I was leaning towards a 6.5mm or 6mm Creedmoor for one barrel and a 300 WSM for the other barrel. I wanted a good amount of capability delta between the two, with the smaller optimized for game up to and including deer, and the larger being more appropriate for elk and other large North American game. In the end, I opted for a 6mm cartridge and a 7mm cartridge, the venerable .243 Winchester paired with the venerable .284 Winchester. The advantage of pairing the 284 as the large cartridge lies in the rebated rim of the 284. It will run with the same bolt face size as any cartridge in the 308 family, including the Creedmoor’s or the 243. This meant there would be fewer steps and less complication when switching from one barrel to the other. Somehow it seemed natural to lean on these longstanding cartridges for this build.

Problems solved, dominos falling, but oh-so-many decisions to go. Barrels and barrel contour. Bottom metal. Magazines. Rings. Optic. Trigger. Bipod. Sling. Ammo… Oh yes, what about the ammo? I was originally wanting both cartridges to dope out very much the same, at least out to 400 yards. In theory, one should be able to switch barrels, re-zero, and go shoot. Whatever mechanism one was used to for range and wind drift compensation could, would, should apply to both cartridges. Some research suggested this was feasible with load development. Middle-weight bullets in 6mm and 7mm have comparable ballistic coefficients. If the 284 were loaded hot and the 243 were a bit on the mild side… you could match the ballistic trajectories under an inch from 0 out to 400 or more. Perfect. This would take some work on the load development side but should be do-able. Onto the next decision…

Optics. Let’s see… Most of my father’s hunting experience has been with sub-10x scopes. 2-7x, 2-8x, 3-9x. Tiny objectives. I initially thought perhaps I should get him something in this ballpark, maybe with a modern reticle and turrets, and/or with a bit more zoom range. There were some good options for hunting optics in the 3-12x ballpark, with 40mm or larger objectives. Then I remembered, the one rifle he has been spending some time shooting is my .17 rimfire with a cheap 4-16 on top and a big objective. It doesn’t have great glass, but he likes having that range, likes being able to zoom way in and see some detail even if he zooms back out to take the shot. That settled it. It should be a scope with a big zoom range, and a big objective. Given that, it still needed to be a hunting scope. So big zoom range and objective, without also being crazy heavy or having too little (or big?) minimum zoom. I decided the Zeiss Victory V8 2.8-20×56 was the best option meeting those requirements and restrictions. Huge objective and zoom range, very useful zoom range for hunting, without weighing an absolute ton. At 29oz this scope is not a lightweight by any means but is more than a pounds lighter than some tactical scopes with a similar zoom range and objective while being only slightly heavier than other much less capable hunting optics.

Zeiss Victory V8. That’s some pretty amazing glass.

After ordering the Zeiss scope it shipped and showed up very promptly. Upon inspection of the optic, accessories, and manual I realized it changed my assumed ammunition plan for this rifle. The Zeiss Victory V8 comes with an MOA-based turret for elevation adjustment pre-installed. It also comes with 9 different meters-based (Yes, meters…) turrets designed around common drop rates for ammunition. These alternative turret bands are easy to install in just a few seconds. I played with the calculator on the Zeiss website for some time with some of my proposed loads and found one of the bands was a very close match to each. After this, I realized dope wasn’t a problem. Figure out which band goes to the load for the 243 and the 284 and run without compromise. YES. Now we’re cooking with gas.

It’s all well and good to have a plan, but it was coming time to execute on it and that is a whole different animal. The receiver, stock, optic, and a wish list in hand… What next?

On to Part 2…