View Full Version : What parts would you use for a rifle intended to fire higher pressure rounds?
This article about m855a1 snapping bolts in half and shortening barrel life made me wonder about this question:
https://smallarmssolutions.com/home/the-m855a1
So clearly if you're going to run 62k psi ammo you need components that greatly exceed milspec since the standard milspec is intended to handle 52k-ish pressures.
What would you look for in a barrel, bolt, etc, that would routinely handle the higher pressures? What other parts need an upgrade?
Right not m855a1 is not available to the public, presumably because it would blow up a bunch of people's rifles if it were.
So, what would handle it well?
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.
I see with interest that the justification for a 0.125" bolt face depth continues to be that the bolt is perceived to be stronger and that the lugs are better supported. I am going to go out of my comfort zone a little here and make a few notes about some of the small details that we sneak into the design to alleviate the effects of the larger rim recess. It goes without saying that the bolts have a good metallurgy and that the heat treatment is improved. The perception is that the alloy is AISI 9310 steel. It is in fact very closely aligned to this but we select an alloy known for its enhanced fatigue life over regular 9310. It is not specifically a stronger alloy just resists crack formation better.
Steel aside, the bolt lugs are very slightly tapered to increase the root area but more importantly stiffen the ring of material that they sit upon. Radii are increased as expected and the minor diameter at the base of the lugs is slightly bigger than one would casually observe. This all stiffens the ring of material that sits under the front of the lug, so too do we reduce the chamfer at the front of the bolt recess leaving more material in this area. Given the bigger rim, the chamfer on the case base is greater allowing the internal radius at the bottom of the bolt face to grow. This material in this area very quickly offsets the extra 0.011" of face depth even without other adjustments. (Remember that 0.011" of material is about 2-1/2 sheets of copy paper in thickness and in this instance it is in an area that is already machined away so the bulk stress raiser is already present.)
There are additional changes in the area of the extractor groove which is always a feature that effectively undercuts the two adjacent lugs.
A correctly machined Grendel bolt is substantially stronger than a 5.56 bolt simply opened out during production to suit a Grendel rim. My only reservation has come from the military requirement that the unit should survive being fired with a breach obstruction of a projectile in the start of the rifling. While this test will always destroy an AR functionally, a good 5.56 bolt will stay intact, though deformed, but a Grendel bolt will break lugs. The constraint of a near standard geometry barrel extension is the driver for this as some changes in shape, not size or diameter in the barrel extension could yield a bolt/barrel extension combination that is magnitudes stronger.
This article about m855a1 snapping bolts in half and shortening barrel life made me wonder about this question:
https://smallarmssolutions.com/home/the-m855a1
So clearly if you're going to run 62k psi ammo you need components that greatly exceed milspec since the standard milspec is intended to handle 52k-ish pressures.
What would you look for in a barrel, bolt, etc, that would routinely handle the higher pressures? What other parts need an upgrade?
Right not m855a1 is not available to the public, presumably because it would blow up a bunch of people's rifles if it were.
So, what would handle it well?
During the Individual Carbine trial, many rifles experienced these same types of bolt failures. Locking lugs have also broken off.
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.
Lysander, thank you for the great information.
What I get from all of these how long does by barrel and bolt last threads.
Depending on the guns use you are either replacing the barrel and bolt on a match gun for accuracy (barrel/head spaced bolt) long before anything breaks.
If a whatever use carbine, if it still groups, replace the bolt at x interval.
Standard ball ammo is costing you, what 6-8k for 15,000 rounds?-what's a bolt cost?. Of course for some here, those 15k last you what, 15 weeks:sarcastic:
Don’t over think it, have spare parts on hand, period.
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 causes the bolts to fail? I am surprised life and bolt thrust are not inversely correlated.
What causes the bolts to fail? I am surprised life and bolt thrust are not inversely correlated.
The bolts fail due to metal fatigue: "weakened condition induced in metal parts of machines, vehicles, or structures by repeated stresses or loadings, ultimately resulting in fracture under a stress much weaker than that necessary to cause fracture in a single application." (from brittania.com)
Take the bolt life with a grain of salt. Lysander makes it clear in the post that the sample size was much larger for the M855. There also may have been continuous improvement changes to processes affecting the bolt.
The data seems to indicate that bolts are not on the ragged edge of failure with M855A1 pressure.
Andy
In another test, they intentionally removed lugs from bolts and tested to see how long they would last.
Two lugs lasted 6 rounds before jamming up and requiring disassembly to unlock the bolt.
Four lugs, one lasted 720 rounds before suffering a ruptured case, the second lasted 1800 rounds before headspace measured excessive.
The five lugs bolts lasted 720 and 1800 round before excessive headspace was noted.
Once upon a time certain Army gunsmiths dictated a 35,000-round life cycle before de-mil and destroying an M16 or M4 Carbine -- not for imminent failure of any components like the bolts (preventative replacement after third barrel), springs (every 9,000-12,000 rounds), or trigger components (a Geissele lasting far longer than the lowers they were mounted in) -- but if/when the lower receiver trigger and hammer pin holes are egged-out enough to affect (detract from) the quality of trigger pull.
Excessive trigger and hammer and pin movement = squishy (bad). Bad means lose. Throw away, get new lower.
Simple gunsmith math.
https://www.ar15.com/media/mediaFiles/18978/gage_holes_jpg-2072443.JPG
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?
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.
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.
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, 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).
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.
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.
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.
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.
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