What parts would you use for a rifle intended to fire higher pressure rounds?

[okie]

They've tweaked the A1 since the initial soft rollout, and it doesn't seem to be a problem anymore. The only reason it's not widely available is because the Army owns the patent for it, and they've made sure it's not sold on the commercial market. Gotta love how a patent that we as the taxpayers paid for is being used to prevent us from having what's already guaranteed to us in the constitution. They've also made sure the EPM (magazine to feed A1) is restricted. Believe it or not, they won't even let us have the brown follower mags. They made OKAY Industries agree not to sell those to civilians.

[okie]

I question the idea that an AR bolt is weak. The design is not as strong as an AK or M249 bolt, but it is strong enough for anything within reason.

During the 2007 baseline reliability test of the M4 with M855 ammunition, 36,000 rounds were put through each of 20 M4 carbines, that's three-quarters of a million rounds fired. The total number of failed bolts was 46. On average, each bolt lasted 15,652 rounds.

In 2015, the test was repeated to get the new reliability baseline with M855A1 ammunition. This time only 6 weapons were used and only 18,000 rounds per weapon were fired. There were a total of 5 bolt failures. On average, each bolt lasted 15,730 rounds.

The peak forces imparted on the bolt face of an M4/M16 by M855 and M855A1 ammunition was directly measured. The average forces were:

M855
2041 pounds @ 70° F
2296 pounds @ 12° F
2520 pounds @ 160° F

M855A1
2414 pounds @ 70° F
2527 pounds @ 125° F
3012 pounds @ 160° F

So, even with an 18% increase in bolt thrust, the current bolt life is the same, I would say the bolt strength is adequate.

What seems to really stress bolts is what pressure they unlock at. I'm just guessing that the weapons unlock at around the same pressure for both rounds. Like maybe the A1 burns faster resulting in higher chamber pressure but is the same pressure at the port as 855.

[Pappabear]

This was posted by Bill Alexander on 65Grendel.com at 03-12-2017, 12:24 PM. A new bolt and barrel extension geometry within the existing envelope sounds very appealing. Knights E3 bolt is one execution of this but I don't know how it compares to what he had in mind.

LMT for the win in this regard.

PB

[lysander]

Lysander, during those M4 tests what was the barrel life like? If re-barreled once, then it’s about 17,000? How did bolt life compare to the M16 with rifle gas system?

Barrels were lasting about 9,000 to 10,000 rounds.

Just remember the Army's definition of a "worn out" barrel is fairly specific. A barrel is worn out when:

1) The velocity decays is 200 fps or more.
2) 20% observed keyholing
3) Dispersion exceeds 7.0 inches at 100 yards (91.4 m).

[markm]

Barrels were lasting about 9,000 to 10,000 rounds.

Just remember the Army's definition of a "worn out" barrel is fairly specific. A barrel is worn out when:

1) The velocity decays is 200 fps or more.
2) 20% observed keyholing
3) Dispersion exceeds 7.0 inches at 100 yards (91.4 m).

Good Lord. Those are all atrocious levels of performance.

[okie]

Good Lord. Those are all atrocious levels of performance.

There's always been a big disconnector for me between Army lifespan findings and what I see in the real world shooting civilian AR variants. I think maybe for one they're harder on them than is typical, and maybe they're a lot more nitpicky than is truly necessary. I think even they have stuff last longer though. Like I kind of doubt they're replacing most barrels at less than 10k rounds. Unless you're mag dumping on the regular 15k seems more typical. Idk though, I've heard stories of those guys flipping on the giggle switch because they're required to burn a certain amount of ammo, and even doing that once can seriously F your S up by heating the barrel to the point where it's undergoing metallurgical changes that will rapidly decrease barrel life. It's pretty amazing the difference between a typical semi auto firing schedule and what full auto and bump stocks and binary and stuff will do in terms of lifespans. These guns just weren't made to be run that hard.

[lysander]

The firing schedule for the typical endurance test is:

30 rounds semi-auto 1 round per second
30 rounds full auto in 3 round bursts one second apart
30 rounds semi-auto 1 round per second
30 rounds full auto in 3 round bursts one second apart

After 120 rounds allow barrel to cool to the touch

After 5 of the above cycles (600 rounds), wipe down and relubricate.

After 1,200 rounds, complete disassembly, clean and inspection of all parts, MPI bolt (in some cases bore scope or measure bore wear).

Repeat until 6,000 rounds, or however long the test is.

So, yes, the firing schedule is pretty harsh, but it is standardized and repeatable. The results from one test are pretty comparable to other tests. And, it kind'a has to be harsh or it would take forever to burn through 6,000 rounds at the typical rate of most shooters.

[okie]

The firing schedule for the typical endurance test is:

30 rounds semi-auto 1 round per second
30 rounds full auto in 3 round bursts one second apart
30 rounds semi-auto 1 round per second
30 rounds full auto in 3 round bursts one second apart

After 120 rounds allow barrel to cool to the touch

After 5 of the above cycles (600 rounds), wipe down and relubricate.

After 1,200 rounds, complete disassembly, clean and inspection of all parts, MPI bolt (in some cases bore scope or measure bore wear).

Repeat until 6,000 rounds, or however long the test is.

So, yes, the firing schedule is pretty harsh, but it is standardized and repeatable. The results from one test are pretty comparable to other tests. And, it kind'a has to be harsh or it would take forever to burn through 6,000 rounds at the typical rate of most shooters.

Oh wow, yea that certainly doesn't leave any room for mystery then does it. People don't understand how even a little full auto is so much more wear and tear on these guns that just weren't made for it. It's a testimony to their durability that they're able to take that and still last as long as they do, and how durable and long lasting they are for us who aren't abusing them like that. I'm really glad I was fortunate enough to be able to put back a nice little stockpile of the TDP spec LE6920s and parts. That's another thing, people go out and buy some commercial rifle or parts that aren't made to TDP spec and then project the problems they have with it onto the milspec AR variants that were and try to claim the platform isn't reliable or durable or whatever. I think in a lot of cases those mystery parts are probably rejects from batches that didn't make the cut to be sold under contract.