AR Thumbhole from fiberglass & styrofoam?

B

backbencher

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Bell & Carlson have stopped marketing their AR thumbhole, & the New Zealanders & the Germans sell their for $500+. The Feinstein Project is trying to build an affordable AR thumbhole stock to reinforce our ABS plastic AR lower. Would it be possible to build a stock out of styrofoam, wrap it in fiberglass cloth, & then seal it with resin? Would such a stock stand up to .223" recoil?
 
Absolutely. But not your typical, home-improvement store "styrofoam".

Boats (and probably aircraft as well) have used high-density foams for structural applications for a long time. Any application where high strength, and low weight are critical.

High-density, closed-cell foam like Klegecell and others are perfect for this.

Used with the correct epoxy resins, and cloth (maybe bi-axial), I don't see why it couldn't be done.

Polyester resins will dissolve foams, and most won't air-cure anyway.
 
Many, many fiberglass stocks made with resin, glass cloth, and epoxy or polyurethane foam for a filler. In fact, that was the standard method used by Chet Brown and Lee Six back in the 1980s. And they will handle much more recoil than a 223 can dish out.
 
I'm actually considering a similar project eventually but in carbon fiber. I have access to huge stockpile of it, so time to start making some parts :D
 
One could be made using a form for the fiberglass, and then, when removed, filled with foam. That is how boat hulls are produced.

Forms are made of wood, etc., with a fiberglass covering, then sanded smooth, and last, it has a gel coat applied, to make it slick, just like a reverse of what you want made. Next, a release agent is sprayed on or in the form, and then a gel coat over that, with glass applied next. Once it has cured, a jet of shop air is used to separate the the object from the mold, and then it is finished off.

In this case, you would want an internal mold, and lay the glass on the outside. Once it is cured, then you remove the internal form, and finish off the outside. The finish ought to be something similar to a gel coat, as it can be built up thick, and it is tough. Once done, fill it with foam.

You can add Kevlar cloth, too, and toughen it up more. However, the problem is, can you make it cheaper than you can buy it? A lot of the stuff to do this, comes in quantities that are way more than needed, and are expensive.
 
So cutting up a styrofoam cooler from the PX ain't gonna work, I take it.
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The foam you want, is the same type that is used as insulation in houses, or the kind that is blown on and expands, or used in boat building, or in packaging. It comes as a liquid, and can be bought by the tank, with a gun, and hoses.
 
I wouldn't use anything other than a high-density, structural foam- even for .223 recoil. While the recoil is not significant in itself, many thousands of rounds could- I suspect- compromise the integrity.

That said, I guess much (how much, is above my pay-grade) of the recoil force is absorbed by the buffer spring. Could always reinforce with a lightweight backing block (structural balsa) if needed.

Did get me wondering though as I hadn't thought much about it before- is the recoil force not absorbed through the buffer just transferred into the lower through the takedown pins?
 
That said, I guess much (how much, is above my pay-grade) of the recoil force is absorbed by the buffer spring. Could always reinforce with a lightweight backing block (structural balsa) if needed.

Did get me wondering though as I hadn't thought much about it before- is the recoil force not absorbed through the buffer just transferred into the lower through the takedown pins?
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The reward force of the bolt is absorbed in the buffer spring, but the recoil force is transmitted from the upper, to the lower, through the hinge pin, and takedown pin, through what is known as shear, or shear pressure or force applied to the pins. This recoil force, then, is transmitted to the stock from the lower, along with the force absorbed in the buffer.
 
Making it cheaper than I can buy it is the idea.

Ze Jermans for $570

Kiwis for $525 ish

Bell & Carlson discontinued.

I'm trying to reinforce an AR lower that will be built out of glued together plastic bricks such as Legos or MegaBloks. If I can build the stock out of fiberglass cheaper than I can out of plastic bricks, I'm for it - but no way can I use some fancy boat quality foam.

If I'm reading some of the above posts correctly, the way forward is - build my stock out of cheap styrofoam, cover it in something impermeable, wrap it in fiberglass cloth, then slather it in resin & let it dry. THEN - pull the styrofoam out? And replace with a filler - either spray in foam (like house insulation?) - light, or fiberglass mat & more resin - heavy. Or Bondo, perhaps?

We're not doing a .458" SOCOM here, nor a .308". Cheap & easy - not fast or pretty. If it cracks a couple thousands rounds down the line, you build another one.

Effort Thumbhole
 
If you're doing production, you do a female mold, and lay-up the glass. This is how most everything glass is made- from swimming pools, to a McMillan stock.

But making the mold is time consuming- and, of course, you have to actually make a pattern of the stock, to use as the male part to make the female mold...

When the glass is pulled from the mold and filler is used, it's got to have enough structural strength to be viable, yet "workable" to be routered/ milled for inletting.

There are multitudes of fillers that can be added to resins to achieve both the desired viscosity, "workability" and strength.

Very light fillers, like microballoons (glass bubbles) add a lot of volume, are very light, very easy to sand/fair- but have zero strength. Great for fairing, but that's it...

Others like high-density filler, or colloidal silica, add a lot of strength, but are heavy and difficult to sand.

There's a reason glass stocks are expensive- they're labor intensive, and if you price epoxy resins and fillers, you'll see they're quite expensive.

http://www.mcmfamily.com/mcmillan-stocks-faq.php
 
There's a reason glass stocks are expensive- they're labor intensive, and if you price epoxy resins and fillers, you'll see they're quite expensive.
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I agree. One would about have to want to go into production, if they wanted to do this. I visited Starcraft's boat plant, and watched them being made, and that was where I saw how much labor went into laying fiberglass, and the stocks would be no different.

At one time, Sears had a boat catalog, where you could buy the fiberglass cloth and resin, but I remember the price from back then, and it was really high. Even a small box of Acraglass is pretty expensive, and if one multiplied that to gain enough to make a stiock, and added the other cloth needed, well, you catch on.
 
Since I buy resin, cloth, and foam all the time, here's a rough idea of what it would take to lay up 4-5 layers to make a sturdy stock as you were describing:
Resin- $50
Glass cloth- $75
Polyurethane foam- $25 OR epoxy foam- $35

Then add time, money for molds, tools (glass cloth will eat up tool steel or carbide die grinder bits about as fast as you can turn them on), rejects, scrapped designs, and prototype work can get costly. Do your prototype work in wood, then make a mold with plaster of Paris (you are only going to make one stock, right?), and be ready to do a lot of filling and sanding. Gel coat to finish it off (probably black?). You can complete your project for about $400 or so, plus the time needed to do the work.
 
Wow - if I do a prototype in wood, then I'm done. @ that point I have a working stock.

I take it there's no way to lay the fiberglass over the styrofoam for a one off? I have to have a mold?

It just seems that if fiberglass works by laying it in a mold, it would work if you wrapped it around something - like a styrofoam stock. It can't be done that way?
 
backbencher,

You could, like using it in autobody work, but its harder to lay this way, and you still need an interior form of something that wont react with the resin, and it has to be slick, too, to release itself once the stock is cured.

Look at the inside of a boat hull, at how uneven, and bad it looks. As the resin hardens, it shrinks down into the fiberglass, and leaves behind a rough surface. That surface is what you would have to try to sand smooth, and fill, to get a surface acceptable to a paint. That is why they spray the gelcoat on first, on a mold, and apply the fiberglass over it. In the case of gun stocks, they spray the inside of the mold first, then lay the glass.
 
Let's suppose for a moment.

I rough my stock out of ordinary el cheapo PX styrofoam cooler, glued together. I wrap it in aluminum foil. Cover that in fiberglass cloth, then slather on the resin. Let dry. 1-3 more wraps of fiberglass cloth & resin, then pull out the styrofoam dummy, chopping it into bits if I have to, then fill the hollow stock w/ spray insulation foam from tubes, or just fill it w/ fiberglass mat & pour in resin.

Sand down the outside, & paint. Doesn't have to look good, doesn't have to be lightweight, just has to be cheaper than the alternatives.

If the best alternative is to whittle the thing from pine, I'm ok w/ that too.
 
backbencher,

You can give it a try, and see how it goes. To do that, I don't know how it will work, as I've never done it, nor have I seen it done. The main things are being able to remove the core, and to get a decent surface finish. If it works, let us know.
 
Curious, have you done much layup work?

If I had to dream up the most difficult glass project I could come up with, a rifle stock would probably be it.

Tight radiuses, near right angles (you can't do true 90 degree angles)...

I think even a expert at layup would have difficulty wrapping something the shape of a lower. It would be much easier with a chopper gun- but that isn't in the cards unless you have a shop nearby.

I would use very light cloth because of the geometry- and get the build from multiple layers of epoxy resin, even adding a filler to increase build.

If you use regular styrofoam, acetone (proper safety procedures) will dissolve the foam out after the glass cures.
 
"Curious, have you done much layup work?"

Lol, none ever. Hence the newb questions. We're not trying to make the lower out of fiberglass - just an AR thumbhole stock. It's sounding like fiberglass is not the way to go. Thanks to all for entertaining my crazy questions - I'm sure to have more later.
 
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