Yes I think your on track with the following exception;
Depends on how hard you run your carbine. If you shoot 30-40 rounds a month slow fire, the gun might be passed down to your offspring with the original factory components.
Granted, lots of full auto or burst fire will increase fatigue. I’m not an engineer but it seems I should also add to my previous post heat from large volume of fire also has a negative affect.
I’m running mine with multiple fast double/triples for at least 100 rounds per session I should expect a longer life cycle than a full auto carbine.
Then again I run it slow at the 200 or 100 yard line to work on my marksmanship every now and again.
Yes, but it’s not only barrel length. It’s barrel length along with gas port location (length of gas system). Take 2 barrels both 16" barrels, one middy, one carbine. If both are quality barrels the middy will have less gas port erosion and be more inherently reliable running at lower pressures and have a longer service life, yet both are 16" barrels. It’s the heat and high gas pressures that are eroding the gas ports, get the port further from the chamber and the erosion and cyclic rate are reduced allowing for better reliability and longer service life.
Good point.
Keep it coming guys, if I have time today I’ll copy/paste everything to summarize it.
From post mortem Crane pictures I have seen, it seems the gas is trying to cut the corner where it makes the sharp turn at the front of the port hole.
I suspect once the area significantly erodes, and the sharp corner is no longer there, the port erosion slows down greatly.
Does anyone make a gas system that ports gas to the outside and therefore lowers gas pressure and heat to the bolt?
Not that I know of.
It would be pretty hard to meter it correctly enough to work but still be reliable in a direct impingment gas system.
With a piston type gun (i.e. SKS, FAL, M14, HK416 etc) this is more easily controlled since a solid mass is pushing against the carrier sending it rearward, unlocking the bolt and when the piston gets to a predetermined spot it then bleeds off excess unneeded gas pressure.
This post is educational, as Marine I just used and maintained (field) an M16. This topic is something normally covered by armors.
Based upon the conversation here I think I can safely conclude the following:
The shorter the barrel (rather the closer the gas port is to the chamber) the greater the pressure on the BCG. Hence a shorter life cycle, same for barrels due to heat induced erosion near the gas ports.
There is an inverse relationship between barrel length to the gas port and the two affects above. E.g., the mid length carbine has gas ports further from the chamber than a standard M4. Therefore they should last longer.
There are devices to mitigate pressure on the BCG but nothing will reduce the erosion near the gas port.
Since I’ve seen evidence from SOPMOD these bolt & barrel failure occurs on issue M4s between 4K and 10K rounds I can expect this from nearly every M4ish type carbine. Of course with a shorter barrel, 14.5" or less the affect happens sooner.
The low barrel life in the 14.5“ barrels is a combination of many contributing factors such as:
- High gas port presser ( Colts desire to use the same gas system parts already in production since the XM177 and not manufacture any thing new)
- Long dwell time at presser ( Army’s desire to be able to mount a bayonet)
- Typical sustained rates of fire in all modern training and war fighting applications is drastically above the original sustained rate of fire ( I don’t recall the exact number right now but it is about 10-20 rounds per minute) . Any use above the recommended sustained rate of fire will result in increased and accelerated wear to the system
- 1-3 above all contribute to port erosion that results in accuracy lose, ever increasing gas being directed in to the operating system and increased wear on the mechanical parts.
On a 16” barrel with a carbine length gas system items 1&2 above would be increased and result in a shorter given barrel life (than a 14.5") and increased and accelerated wear on the mechanical parts under the same firing/ heat cycles.
Out
2011BLDR
I too, find this discussion interesting.
It’s particularly interesting when things like gas port erosion are determining barrel life, and overgassing is causing parts failures, when a perfectly suitable remedy is known, but is being ignored totally by the military.
Gas port erosion only determines barrel life, if there is no means of restoring gas metering to compensate.
Overgassing is a by-product of gas port erosion, and shortened parts life is the result.
The cure is to be able to restore correct gas metering as the gas port erodes.
Thus, an adjustable gas system would solve the problem, to a large extent.
Adjustable gas systems have been known for many years. The FAL had one form of it, and the FNC had another simpler form, and other guns also have had it. The AR15 type weapons have had a selection of adjustable systems from the aftermarket for years now.
The “ostensible” reason that these adjustable things are not fitted to the gun, is that “The warfighter is not sophisticated enough, or trainable enough, to properly adjust this part of his gun”.
That’s not what I think at all. But it apparently is what the bigwigs think.
Maintaining proper gas port pressures and volumes will solve these things we’re discussing, to a significant extent, but there is a refusal to use them.
People say, “What if the guy adjusts it wrong, and his gun doesn’t work?”
And my answer would be, “What if the bolt breaks, or the extractor breaks, or the rim gets ripped off the case, or the gun overheats?”
Same thing. The gun quits working.
The difference is that you can quickly adjust the adjustable gas system further open if necessary, in a second’s time, but not so with a broken part.
A “click adjustable” type of adjuster that can be quickly accessed by the user, and has a narrow band of adjustments to take up erosion-caused changes, would do just fine.
So, pick yer poison.
Either you’re going to operate with a gun that will soon be overgassed, battering, overcycling, harder to control during firing, and that is going to break parts, or you’re going to do something about it.
They are not going to go to a midlength gas system. They already want to have barrel lengths that are shorter than even the short carbine gas system can support easily. They are going shorter, not longer.
I can clearly see that the current “school of thought” is that the warfighter cannot handle this simple task, and that the answer is some kind of “unbreakable”(Ha!) parts, or go to another gun design totally, which will have a whole other set of breakdown and failure issues, and may solve one problem and give you others.
IMO, the problem is refusal to see the problem as it is, and add the proper remedy, and train the warfighter to use the remedy.
Getting off the bolt issue briefly:
I’ve yet to find a convenient/practical adjustable gas tube design.
I’ve been issued an ajustable gas system M16FOW - and dont beleive the system needs it.
Most barrels are done for by the time the port is seriously erroded.
In my experience most units that are on a firing schedule to wear out weapons - also have 1) extra weapons issued to the shooter (6 or 7 different upper/lower combo’s) 2) a robust weapons tech/armorer support system that can replace barrels etc within an hour of a weapon being turned in for work.
And nothing can be done about units of personnel that insist on attempting to use a carbine as a LMG.
Last point – in units that use blanks - they errode throats, crowns, and gas ports at aprox a 10:1 (ball to black) ratio.
The only bolt that I have seen that makes any sort of sense is KAC’s E3 bolt – since it radius the bolt removing sharp edges - and provides more locking surface areato minimise strain and stressors – and with the redesign of the cam pin area strenghtens the bolt.
One other issue with bolts – is in units that haphazardly issue the bolt to the rifle/carbine (I’ve seen it - where security procedures require the bolt to be seperate that some units simply put bolts enmasse in a locked cabinet – with no though to serialising the bolt to the weapon.
This increases wear as well…
I’m going to guess that the gas must exit some place on the out side of a piston operated upper…
Bushmaster now makes one.
http://www.bushmaster.com/shopping/uppers/pre-ban/bura3b16m4-gp.asp
“with Gas Piston reliability eliminating excess heat, carbon build up, and gas leaks (as well as gas rings, gas key, and gas tube)”
You can always get one custom made out of titanium. It only costs about $500-1000.
Thanks everyone for the great information:D
Because of this thread, I have had to add a 20" gov’t profile rifle to my list of items to buy.
Aristogeiton
How about a stellite bushing in the gas port?
I was talking with the current .Mil supplier for spare bolts about what they do to them (as far as testing, etc). They said that they do not HPT or MP test them (batch MP test is what they do). They also do not shot peen them.
I then spoke to Sabre Defence about how many bolts they have ever seen fail MP and they said MAYBE 1-2% (if at all). I asked them what their acceptance criteria was and they said zero flaws.
Next on the list was Stag Arms (that MP tests all their bolts as well). I asked them the same question as SD and they gave me the exact same answer.
To add my own experience to it, most of the bolts I personally use are not HPT/MP tested and have over 7 thousand rds on most of them without a single issue.
So the million dollar question is, if the bolt is made out of the proper steel (158) and is properly heat treated (which is the most important part IMHO) do they need to be HPT/MP tested and shot peened??
C4
The HPT/MP does not sound necessary if the mfg.s do their jobs, which it sounds that most are doing. What happens if a bad batch of steel gets through? YOu could end up with a batch of bad bolts that the HPT/MP would catch.
If we would ever have to rationalize our production of the units again, the HPT/MP would be extremely necessary, too much of a chance of a mfg without a firearms background dropping the ball.
All bolts are batch tested so if there was a bad run then they would catch it. I should have said that in my post, but forgot.
C4
Could this possibly be the reason LMT’s gas port on their MRP barrels are cut at an angle?
ok, so is this the way it works? A Colt m4 leaves the Colt plant headed to the Army. Its got all the C marked stuff in it along with the MPC marked bolt. After getting the hell shot out of it in the Army, a armorer decides it needs a new bolt. So in goes the unmarked bolt form the secret suplyer that Grant has contacts with. Is that a likley senario?