Recoil springs, gas systems and caliber conversions

[Miroslav]

So my mind wandered off the other day and I started thinking about bolt carriers, recoil springs and caliber conversions. I feel kind of ignorant for not getting this completely straight, but I'm wondering if I'm missing something.

Let's say you'd have an AR-style rifle chambered in .308, and you'd convert it to .223 with a swap of the bolt head and the upper receiver. Let's ignore the magazine issue for this discussion. I'd imagine that the optimal bolt carrier velocity is the same regardless of cartridge (within some reasonable limit). Thus it should be perfectly possible to compensate for the new cartridge only by changing the gas port location or size, and leaving the same bolt carrier mass, the same bolt head mass, the same recoil spring and buffer in there. 
 

For some reason I've always had it in my head that a larger cartridge requires a heavier bolt carrier, but I just realized that that's not right. A larger cartridge requires more space on the bolt face, more space in the receiver, and a sturdier lockup. This tends to lead to a heavier bolt carrier group, but there is no need for a heavier bolt carrier per se. Is my understanding correct? Of course there is less volume to work with when running a gas system on a .223 versus having a larger cartridge, but it should be perfectly possible to fiddle with the gas port size and location to compensate. I could also imagine the larger surface area of the larger cartridges to increase friction during primary extraction, but the difference between different calibers should be negligible compared to the difference between dirty ammo and slightly oily ammo.

The Saiga rifles use the same bolt carrier and virtually the same bolt for all of the difference cartridges.

The Knights Armament SR-25 uses the same springs and buffer as the M16a2 (although they have a heavier carrier and had some issues)

The DPMS genII small frame .308 rifles use the same buffers and springs as the 5.56 rifles.

Bonus: Check out this thread from arfcom on bolt carrier velocity
https://www.ar15.com/forums/general/for_AR_fans_and_engineering_nerds__I_calculated_the_bolt_carrier_velocity_profile_for_an_AR_10/5-1150725/

[Sturgeon]

3 hours ago, Miroslav said:

Let's say you'd have an AR-style rifle chambered in .308, and you'd convert it to .223 with a swap of the bolt head and the upper receiver. Let's ignore the magazine issue for this discussion. I'd imagine that the optimal bolt carrier velocity is the same regardless of cartridge (within some reasonable limit). Thus it should be perfectly possible to compensate for the new cartridge only by changing the gas port location or size, and leaving the same bolt carrier mass, the same bolt head mass, the same recoil spring and buffer in there. 

Correct.

 

3 hours ago, Miroslav said:

 

For some reason I've always had it in my head that a larger cartridge requires a heavier bolt carrier, but I just realized that that's not right. A larger cartridge requires more space on the bolt face, more space in the receiver, and a sturdier lockup. This tends to lead to a heavier bolt carrier group, but there is no need for a heavier bolt carrier per se. Is my understanding correct?

A larger cartridge has more inertia and more surface area creating more friction between it and the magazine feed lips and rounds further down in the stack. So, it needs a bigger bolt group to overcome those things, because there is a peak BCG acceleration and recoil spring mass you don't want to exceed, so that means your theoretical BCG velocity is "fixed", and the only way to increase the momentum of the bolt group during feeding is to increase its mass. So no, you could not just drop in 30x173mm rounds even in a hypothetical context, because they'd be too heavy for the BCG to reliably feed. However, the difference in inertia and friction between 7.62 and 5.56 is relatively small, and rifles like the POF Revolution show beyond a doubt that the AR-15 itself has more than enough moving mass to reliably feed 7.62 cartridges. Compare the POF Revolution BCG (left/top) to a standard AR-15 BCG (right/bottom):