Pressure curves and moment; updated

See, I can too speak techno babble.

A little.

Ok, very little.

What brought this on was part of what Gerry put in comments on the casting post:
I tried my tried and true Unique powder in my M1 Carbine. According to the books the velocity was fine. Unfortunately it wouldn't function the rifle making it a straight pull bolt action*. It's OK though, I have an adapter cartridge so I can shoot .30 Carbine rounds in my Ruger #1 and one of my Military bolt rifles both chambered in .308 Win (7,62x51mm NATO). For loads for the carbine, it's back to 2400. Something about pressure curves and moment. Techno engineering babble.
In the general interest of spreading knowledge I shall now exhibit some of my somewhat incomplete and possibly slightly-flawed knowledge on this subject.

When he speaks of the pressure curve, he’s hitting on one of the things behind the design of any semi-auto or automatic firearm that’s gas-operated; that there has to be sufficient pressure in the right place and for the right length of time to operate the action.

Take a look at the old M1 Garand. Way out near the muzzle is a gas port, a small hole drilled into the bottom of the barrel. Attached to the barrel is a combination gas tube/sight base, and sliding into the gas tube from the back is the operating rod. When a shot is fired, as the bullet passes the port some of the high-pressure gas is siphoned off into the gas tube, where it pushes the op-rod to the rear, which unlocks and retracts the bolt, ejecting the empty case. Then the op-rod spring pushes the bolt and op-rod forward to strip the next cartridge into the chamber, lock the bolt and position the rod for the next operation. This action, including the size of the hole in the barrel, was designed around a 150-grain spitzer bullet moving about 2800-2900 feet per second and being pushed by a particular powder. It’s been known for a long time that shooting bullets heavier than 180 grains, or using a slower-burning propellant, can produce pressure in the system high enough to bend the op-rod and/or cause other damage. Bad Thing. This is one of the reasons why, when it was decided the Garand had to be in .30-06 instead of the .280 cartridge that it had originally been built around, it took a lot more than just a different bore & chamber to make the change. It’s also why, with cast bullets, you can use some powders that are too slow-burning to be used with jacketed bullets; cast have less resistance to being pushed down the bore, and a powder that can damage things with jacketed bullets can, with a heavy cast, be just right to produce the needed pressure at the gas port for the necessary time.

The AR15/M16 family is similar in that the gas port is out front under the front sight; in this case a gas tube takes the high-pressure gas all the way back to the receiver where it pushes on the bolt carrier to operate the action(‘direct gas impingement’ I believe it’s called). Again, there has to be enough pressure, for long enough, to cycle the action.

The early M16 cartridge story also has one of the reasons it got a reputation as a jamming POS. It was designed around the military version of the .223(called the 5.56x45mm), which used x grains of a certain powder. Then it was decided, without sufficient consideration for the consequences, to change powder(I believe because the new was easier/safer to manufacture). I can’t remember if went from a ball to a flake or the other way ‘round, but while the new powder produced the right pressure, it also produced more fouling. Which clogged up either the gas tube or the receiver/bolt carrier/bolt. Which, combined with lousy instruction on cleaning and few or no cleaning kits, produced lots of jamming problems that got people killed. (Update: more information here; it was going from a stick powder to ball)

So a gas-operated firearm of any kind has certain limitations on ammo. Not as many as once, because there are ‘self-regulating’ gas systems that can compensate for different shells(in the case of semi-auto shotguns), and some machine guns and semi-auto rifles have an adjustable gas plug that lets you compensate for ammo variations and/or dirt in the system from sustained firing. For that matter, you can get a gas plug for the M1 that is adjustable, to allow the use of other than M2 ball-standard ammo without damaging the works.

The M1 Carbine has the gas port much closer to the chamber; part of the reason was to have the gas at high enough pressure and temperature as to keep the port free from fouling. There’s no way to adjust that gas system unless you decide to do some machining and modifying, which means there’s a definite limit on what can be done with the cartridge as far as pushing velocity up without hitting dangerous pressures.

Basic 'how it works' according to my understanding; if you've got better info, say so, I have no problem with being saved from continuing error.


*I'll note that some people, for plinking, don't mind their semi-auto turning into a straight-pull bolt. I've read of the '150 to 170-grain bullet and 16.0 of 2400' load in the M1 Garand; it won't work the bolt, but it will, in many rifles, give good accuracy with light recoil. And working the bolt by hand means the fired cases don't go far.