The "Screws and Loctite" rifle trend and modularity

Posted by StrangerOutrageous68@reddit | ForgottenWeapons | View on Reddit | 38 comments

Modularity has a price

Why do some manufacturers , not just FN or CZ and other manufacturers use screws to hold rails or certain receiver elements on some of their rifles.? Not to mention entire barrel assemblies of different caliber barrels even. (Think of rifles like the SCAR, BREN 3, ACR, even the new 30mm grenade launching weapon the MTL-30 , the EVOLYS MG and many others.)

CZ BREN 3 receiver.

https://imgur.com/a/ttQV8sP

(Image credit: Matthew Moss, thefirearmblog .com )

Well yes of course the rails which the Bolt Carrier ride on can now be made out of a stronger materials than Aluminum which in turn will reduce receiver wear and may or may NOT facilitate certain lightning cuts or using thinner receivers withotut compromising receiver integrity.

But in reality, and without other weight saving measures all that would actually constitute to a gain in weight and not in a lighter receiver. As now you have multiple screws, steel rails and a barrel to screw assembly.
And on top of all that if you also want multi-cal capability, that will further add weight to the receiver.

FN EVOLYS, where using steel rails can actually be more beneficial for weight and durability of the receiver itself, but the amount of screws is astonishing.

https://imgur.com/a/7zkX1lK

https://imgur.com/a/dqRZ2ud

(Picture credit: www.fnherstal .com)

Points of potential failure

FN MTL-30 A30mm low velocity grenade launcher.
https://imgur.com/a/ppZhL20

(Picturecredit: fnamerica. com)

I think most people are aware of loctite is not a permament solution for use in such an aplication. It will of course degrate and break up overtime depending on the use case and intensity. It has to deal with heat cycles and significant vibration. And this can absolutely cause: even catastrophic issues. Such as losing headspace because some or all of screws holding the barrel decided to say "SCREW IT! I'm out of here!"

This introduces extra maintenance steps where the screws has to be checked and if loose, removed, cleaned ,re-loctited and torqued to spec. How many extra points of failure are there? What's the inetrval between the checks?

For a civilian use case this complexity is perfectly acceptable. It is a big deal for some and not a big deal for others.

However for military use this is not optimal on a large scale and especially in times of conflict.

The civilian version of the M7 rifle. SIG SPEAR LT's barrel to receiver assembly.

https://imgur.com/a/sig-spear-lt-receiver-screws-8lb7F85

(Picture credit: www.americanrifleman. org)

Of course if you really want screws/bolts and make them almost impossible to walk out you need to go bigger ,and more complex geometries, with retainers even springs in some designs, and dual locking screw arrangements among many other solutions. But these would make a gun even heavier and more costly to manufacture.

Perhaps some people want to think of screws in the reciever as a sign of "advanced modern manufacturing practices". Very much in the same way some people also like to think injection molding, CNC machining and metal extrusion are also "advanced modern manufacturing processes"

All in all everything points to screws and loctite on rifles being more of a cost saving measure than anything else. Pretending to be peak modularity, in some people's minds.
But who would actually need, not want need multi caliber compatibility and modularity apart from civilians and special forces?

(Disclaimer: I have no control over those images and links from _imgur.com. In case _imgur.com or its would be successor site decides to reassign the links to someone else, the links might get replaced by something not relevant to this topic.)

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Temporary_Border7233@reddit

Just make everything you can 6mm head. THERE. modularity

-some exhausted engineer crying

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Ares4991@reddit

Hi, I'm a mechanical engineer in the defence industry. Probably also the only person in this thread who has actually held all of the weapons you mentioned, including the MTL-30.

So why do manufacturers use screws? Because screws work. They've been around for centuries in firearms design. Arguably the most important connection between parts is that between the barrel and the breech, which has often been done by a thread: usually a thread on the barrel and receiver. Mauser, Stoner, you name it. Obviously, scope rings still use screws in most cases, as do most picatinny rail mounting options. Hell, precision rifles like AI and Barrett also use screws for quick change systems, and those hold accuracy very well - even repeatably with disassembly.

What you are describing is the phenomenon of using screws to mount sticky outy bits to a receiver, such as a piece of rail or acase deflector. These components can be adequately mounted with a screw connection, provided the connection is designed properly. There's books upon books written about this: Machinery's Handbook is the object of choice for designing weapons while providing a nice blunt weapon to hit interns or management with.

Now, making the sticky outy bits seperate partsakes it easy to localise certain material properties (such as hardness, toughness, wear resistance), but mainly serves to make a receiver that is as simple as possible: a tube. Or at least, something that can be extruded with aluminium. If you have not done machine work, you simply cannot appreciate how much cost this saves, both in time and money.

You mention lightening cuts. Well, extruded aluminum is lighter than steel, so it might not even need lightening cuts. If you want something to be stiff, to resist bending, you want a lightweight material as far away from the neutral/bending line (basically, center axis). Aluminium housings are perfect for that. Go look up stress and I-beams if you want to learn more. All this to say: hollow aluminium light, steel heavy (even with cuts).

Then you write a lot of critique about the Evolys and MTL-30. Have you handled a Minimi? And the Evolys? The weight reduction they got is astounding, and even better because they don't use a huge amount of exotic materials. No, it's actually a design that can be manufactured at scale. So, yes, it has screws, yes, they are properly designed, no that does not make it worse.

Thread locking compound can be a sufficient solution for these purposes, that can exceed the lifespan of the barrel. But, thread locker is only a part of the equation: you also need the right bolts, with the right amount of preload, which is applied using a torque spec. And yet, you do not critique a Mauser barrel which is held in without thread locker, just by preload alone. If that barrel were to walk out, it would definitely increase headspace, possibly leading to catastrophic issues! However, I have never even seen that happen - so why are screws a problem?

The M7 you point out has two screws that hold on the barrel. What if they walk out? Well, they are retained by the handguard, so before you lose the screws, the front of your rifle would have fallen off. In the defence industry, we try to design stuff where the front does not fall off.

You make it a big deal that militaries might have to tighten some screws. Well, go look up how the barrel swap for a Mk.22 (Barrett MRAD) works. That thing is designed to where the user can just change barrels with a single screw, no thread locker, just use the torque wrench provided in the deployment kit. Backing out the screw a bit would not lose headspace, since the bolt locks into the trunnion, which is part of the barrel. Sure, you might have accuracy issues, but that is what the torque spec is for.

What you want for properly designed screws is... To properly design them. You generally don't need complex retaining mechanisms unless there is an extreme use case, which firearms are generally not in the broad scheme of things.

Screws in the receiver are nowadays a sign of using modern, optimised manufacturing techniques that allow mass manufacturing, use efficient manufacturing techniques, generic production machinery and yet allow for customisation and adaptation.

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Candyman__87@reddit

I take it “sticky out bits” is an industry term? 😂

I kid, I kid. Absolutely brilliant response!

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Ares4991@reddit

No, it's not an industry standard, we don't have enough fun jargon. It's actually a term used in F1 Tech Talk, although they are most commonly known as sticky uppy bits there.

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Candyman__87@reddit

I knew it sounded familiar. Gotta love those sticky uppy bits!

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existentialdyslexic@reddit

Maybe we've just been spoiled (not sure if that's the right word) by things like the AR... which does have screws and threading it still. Just in non-obvious locations. Unless you've got a free floating handguard or a set screw gas block or...

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Ares4991@reddit

Or the barrel. Or the receiver extension. Or the grip. Or the mag catch. We can go on, I have the TPD on my work PC.

What OP is on about, is that screws are now visible and, according to him, that is bad. It is not.

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existentialdyslexic@reddit

I mean, aesthetically, I find it displeasing, but, overall, it's difficult to argue with it from any other grounds.

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WindstormMD@reddit

Excellent post, and exactly what I was thinking as a quality engineer that had worked for a defense contractor lol

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Pirat_fred@reddit

Main reason is because you don't need specific machinery to produce the weapons. For most of the old weapons like M1, M14, G3, Cetme, MG42, AKM you need machinery that only can do this one weapon or weapon family.

With the new designs you can just trow them in any multi axis CNC Milling machine and hit start. The machine is leased and will be sold after 5 Years for a new one, Wich then again can be written off for a tax benefit.

Also you can produce a family of weapon on the same Reciver, even when no one is really using the modularity that that theoretical exist. Most gouverment agency will buy multiple rifles in all the caliber they want and don't bother using the modularity aspect.

And privat shooters will use the modularity until the novalty wears of or the get their hands on the next thing, then the, put it in the caliber that the new thing is or isn't and leave it at that.

It's mainly a Marketing buzz word and military loves "modularity" because politics is stupid and it makes it easier to get new stuff, if the new stuff can do more than the old stuff.

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Confirmed_AM_EGINEER@reddit

You clearly know your stuff.

A large part of this modularity just comes down to tolerance stacking. As tolerances that we can machine become smaller and smaller it is easier to make a modular design because your tolerance stack up doesn't turn the weapon into a useless rattling pile of parts.

The new thing in precision rifles is quick change barrels. It's nothing new but being able to machine down to .0001" reliably and without huge cost makes this possible.

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Pirat_fred@reddit

Yeah that's a big part, also acessability to precision manufactureing is becoming easier and cheaper.

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fendtrian@reddit

Quit the yapping and start servicing your weapon regularly like you service your car, you’re not complaining everything on you car is screwed in

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Pirat_fred@reddit

I don't yapp, I just stated facts, ultimately I don't care how a service rifle looks, it has to be cheap, reliable and fast to manufacture, in the Past this was done by having a drop down forge and sequential milling, Turning and drilling the receivers, then came stemped metal and now extrusion and milling again.

Everyone has benefits and disadvantages.

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Brown_Colibri_705@reddit

Extruded aluminum is not CNC machined

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thatARMSguy@reddit

It gets stuff machined into it. The top rail, the holes for the screws, slots and sockets for interior components, etc

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Brown_Colibri_705@reddit

True.

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Stairmaker@reddit

Also look at the scar. It's just a funnily shaped tube that has some holes and bolted on stuff.

Wanna make the siderails and brass deflector integral? While still doable on regular cnc machines, the stock they would have to start with would be a lot beefier and require a lot more machining to cut out the finished reciever.

Using an extrusion is much cheaper than using a billet block. So is using a slimmed down extrusion compared to a befy one to.

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MaverickTopGun@reddit

That's why B&T just makes basically all their guns from the same long extrusion method that they can just cut to different sizes.

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Dr_Arnie@reddit

Lightness is in demand, that used aluminium, aluminium can be extruded, but aluminium is too soft for high wear parts.

High wear parts are made of steel and heat treated accordingly.

There’s only really two ways to attach steel to aluminium, threaded fixings or rivets. Threaded enables changing of components easier as they wear and tightening if they loosen and galvanic corrosion would cause a rivet to fail prematurely

Thread locking compounds have come on a long way.

If lightness isn’t a priority, then steel standings, rivets and welding and you’re good to go. And still enables heat treating of key components.

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StrangerOutrageous68@reddit (OP)

This is not an absolute truth that such a design needs steel guide rails or even reinforcing bolts and very much so dependant on the way the receiver itself is set up and the method of manufacture.

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Ares4991@reddit

Third option for steel and aluminium would be friction stir welding.

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SovereignDevelopment@reddit

The lock screw system such as on the Browning Auto 5 would be an improvement, and with modern CNC techniques the threads could be timed sufficiently that screws do not need to be hand fitted for lock screws.

For applications where the lowest possible profile isn't necessary, safety wire is very reliable. The non-QD M203 mounts use safety wire, as just one example already used to great effect in the small arms world.

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StrangerOutrageous68@reddit (OP)

I've seen safety wire used to great effect in the automotive worlds but I did not know about it's application on firearms and grenade launchers. Good info!

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aphocks@reddit

I will start off by stating my credentials because this will be controversial. I am an r&d mechanical engineer for hydraulic systems and heavy machinery. I regularly design custom fasteners for some pretty gnarly applications, and we use a similar process to NASA and other similarly anal organizations.

A screw should not loosen or break if chosen correctly for the application, regardless of thread locking compounds. A thread size should be selected based on the loads expected and the corresponding torque required to supercede those forces by some safety margin. I.e. if there is a 500 lbf load on a flange, then a thread that can be torqued to a spec that generates a tensile force that is say 2.5 times greater than that needs to be chosen. Additionally, the fastener should have an aspect ratio of at least 5:1 of length to diameter, this ensures that the fastener will have sufficient fatigue life and low enough stiffness to not loosen under cyclical loads. You want the fastener to act as a spring, which cycles or stretches under loads. while ensuring that the low end of the cycle is still higher than the static loading, and the high end of the cycle does not exceed the safety limit of the fastener.

This is how threaded fasteners SHOULD be designed. If your assembly cannot accommodate the size and length determined by this process, then you should seek another method for fastening. For guns especially, I do not see threaded fasteners as a smart way to connect parts. There just isn't enough room, and the loads are very high compared to the total mass of the gun. I vote for welding!

With that said; size, weight, convinience, and cost all end up ruling over true engineering wisdom in the real world. That's how we get stuck with less desirable solutions like "just glue the 50 screws in". I understand that practicality and outweighs "ought to be" in most cases, but my tism wishes it could be otherwise.

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StrangerOutrageous68@reddit (OP)

Absolutely true.

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dynamoterrordynastes@reddit

This just shows a lack of understanding of how economized the SCAR's extrusion is for manufacturing. Extrusions were used all the way back with the SR-25's upper, and lots of "billet" uppers are machined from an extrusion that takes away quite a bit of the machining, just not as much as the SCAR does. The SCAR extrusion requires a minimal amount of metal to be machined.

If you want to look at an extruded upper that has way fewer screws, look at the XCR. That has a lot more machining than other designs like three SCAR, ACR, APC, etc.

The headspace will only change if the barrel comes loose from the barrel extension/trunnion.

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StrangerOutrageous68@reddit (OP)

What exactly that I wrote showed a lack of understanding of how economized specifically the SCAR receiver's extrusion process is? Please do quote.

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dynamoterrordynastes@reddit

All of these styles of receivers are pretty much the minimal size aluminum box to house the components. There needs to be some way to hold all of them together. That method is screws. The alternative is to add to the extrusion section profile somewhere, which either adds to machining time and increases scrap or results in a heavier receiver, both of which add cost. Scrap really isn't a big cost, but tooling and number of setups/fixtures is. The SCAR type receivers minimize that.

The biggest issue with screws is barrel axis coming out of alignment with the top 1913 rail. There are ways to overcome this, including designing the trunnion/extension so it interfaces with the extrusion profile so the fasteners don't counter the torque, spacing out the farthest distance between the fasteners so they don't turn as much under load, and just adding more screws.

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StrangerOutrageous68@reddit (OP)

Perhaps you might want giving it another read because you may have missed some of the point(s) of it.

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dynamoterrordynastes@reddit

Which points don't you think I read?

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StrangerOutrageous68@reddit (OP)

I'm still waiting for your answer as to which point in my writeup was an indication of quoting you, "A show of lack of understanding of how economized the SCAR's extrusion is for manufacturing" give it another read and indicate your points. We can discuss this after you've read through it again and then we can talk about guns.

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