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Revenge of the previous "gun guy" AMA

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So the 416 is overbuilt for 5.56 NATO

The 416 is actually under-built for 5.56 NATO. Granted, this is a problem the AR has in general- the bolt is undersized for how hot we load 5.56 NATO today (and the 416 is compatible with standard bolts).

This is part of the reason guns like the Bren 2 have significantly thicker bolt lugs- so that you can run the hottest possible ammunition through it (and if you're running a short barrel, loading it that hot is what you have to do to get the fragmentation you're looking for) and it'll never break. Battlefield Vegas' forum posts on Arfcom have nothing but praise for the SCAR-L's durability (another gun with a much beefier bolt), and if you're a small military force that wants your rifles to last a long time because your government really doesn't like the fact the military exists at all having what are essentially forever-rifles that can still do some light but sustained automatic fire it's a wise idea to sell them on the slightly-overbuilt models to avoid having to ask said government for money for new parts later.

What kind of stress does the receiver undergo here?

The Stoner design originally expects that driving force to come from the center of the BCG, so when that force is applied the carrier does not tilt into contact with the receiver (while there probably is some effect based on the gas key it's also going to be minimal relative to everything else).

Better piston conversions, like the SIG MCX (and derivatives; all AR-18s and most AKs do this too), have a steel guide rod in the upper that the bolt assembly rides on, so while there's still going to be a torque generated it's kept in line by a material that's far more resistant to wear.

The 416 (along with most of the early piston conversions) just blank off the gas key with no other modifications, so when the piston contacts the carrier to force it backwards, it also imparts a counterclockwise moment (view the gun from the side with the muzzle pointing to the right) about that blanked-off gas key (the fit isn't perfect; there's a bit of space between the receiver body and the carrier). The net effect of this is that normal use drives the bottom-rear of the carrier into contact with the receiver and wears on it over time if steps aren't taken to mitigate this (to their credit HK's BCG is tapered there, but they wouldn't have to taper it if they did more than just the bare minimum).

H&K making big bank on a gun that's not really that stellar and maybe even flawed

It's probably worth noting the context: the memes about AR-15s when the 416 was created were that standard "direct impingement" (even though it really isn't) were unreliable guns because "shits where it eats" with a side of "muh Vietnam". The AR was never not an excellent rifle, and I think the 416's success is mainly due to that and succeeded despite "fixing" a problem that didn't actually need fixing.

Ah, didn't know about all that, thank you. I think it's only relatively-recently that people have realized that the AR-15 might be the closest thing to a Platonic Ideal of firearms, though there are probably good reasons why First-World nations are adopting all new rifles that use more proprietary short-stroke systems (when they're not buying 416s), as you note.

there are probably good reasons why First-World nations are adopting all new rifles that use more proprietary short-stroke systems

It all ultimately comes down to unit cost.

Contrary to what US market prices might have you believe (and the worst import laws in the world help keep foreign gun prices high), AR-15s are one of the most expensive modern rifles to make- they require more machine time and labor than any other modern rifle. Sure, forging helps get the rough shape right and saves a good chunk of process time, but you still spend a lot of time drilling and tapping holes and milling into that final shape and that gets expensive fast.

You know what's dirt cheap by contrast? Aluminum and plastic extrusions.

Every modern rifle is made this way. They're not nearly as outwardly blatant about it as the Bushmaster M17s is, but aluminum and plastic extrusions require vanishingly little post-processing time: the tubes need to be cut to length and have a few holes cut into them, and the plastic lower assembly needs nothing else (if it needs to be made in 2 halves, like the KE15 does, it can be automatically welded closed). Install the other parts (doesn't require trained labor, even for the barrels most of the time) and the gun's out the door. The upper half doesn't even need to be aluminum if you're smart about it (Beretta was, HK was not) which means even less cost and weight (for the cost of significantly less sustained fire capability and slightly less durability, like the AR).

So why's the AR not made that way too? Because you can't make an AR any other way without losing its unique advantages. The BCG and buffer has to sit where it does mainly for balance reasons- all piston guns except for the AR are front-heavy, and because they usually pack on a pound and a half for reasons related to that aluminum extrusion the balance is worse than a similarly-equipped AR. The Perun X16 is a really good try, but I think most reviewers are confusing more weight with better overall handling. This isn't even something the KE15/WWSD solves, being that it requires more reinforcement material because the stock and lower are one unit and, if you're not using the thinnest barrel and lightest forend you can manage, compromises the balance just the same.