Bolt life question.

[jwperry]

I was pleasantly surprised to learn how little heat is transferred to the bolt, bolt carrier and upper behind the barrel. It was a lot less heat than I thought. I fired 300 rounds in about 20 minutes and none were more than warm. The barrel transfers a hell of a lot more heat to the upper than the DI system. There are claims that DI is easier on a bolt due to the forward thrust of the gasses which works against the rearward force on the bolt. I don't know if this is true or not. I would like to see some hard evidence or calculations.

I tried to find some slow-motion shots of the bolt cycling, to see if there was visible gas venting through the carrier and I couldn't find a shot that had any(so no video evidence to backup anything I have to say). Most of the broken bolts I've seen have been at the cam pin (I've never personally seen a broken lug) so I had always assumed that the heat transfer to the rear of the bolt is what would cause the failures.

I guess I've also never handled the bolt or carrier directly after a course of fire to feel how hot it was.

[Moltke]

For a properly made and tested bolt, it should be fine for about 5,000 rounds even under heavy use. After that, it's going to have cracks and inclusions and COULD break at any time but that doesn't mean that it WILL break or that the expected life of a bolt is 5,000 rounds. There's plenty of old Colt rifles still being shot with their original bolts, but then on the other hand, there's plenty of new gun's being run harder than most AR's ever have which need replacement parts pretty often. Just own a well made bolt, shoot it wet, inspect it during your detailed cleaning sessions and replace it when it's questionable.

As far as piston bolts go, their bolts aren't AS exposed to the gas, heat, and pressure that a DI bolt is constantly subjected to, so that may be the difference, or it may just be another aspect of pistons that someone's hyping up and in reality, doesn't really matter.

[Arctic1]

KAC and LMT are making similar claims with their enhanced bolts, so I dont think its piston vs di as much as bolt construction.

I think you might be on to something.

[skullworks]

I thought we covered that in this thread:

https://www.m4carbine.net/showthread.php?t=101258

You thought, but you were wrong (no disrespect intended). Please read the thread you linked again and you will (well, you should) see that you are incorrect. Yes, timing is important. But if timing is not adjusted (as in the "all things being equal" in my original statement) a piston system will subject the bolt lugs to a higher level of stress, whereby a DI system will be less abusive to the bolt than a piston system. Thus a DI system will incur longer bolt life than a piston system.

[dewatters]

I read the BCG PiP for the M4 has been cancelled because none of the entrants offered any improvements, so this got me curious about why they did not offer any extended life and why bolts seem to break so early.

Perhaps it was more of an issue of the improved bolts not offering enough improvement to justify their extra cost? Some beancounters would argue that if your new widget costs twice as much as the old widget, it better have a service life twice as long, at minimum, to justify the change.

I haven't had a chance to read the original documents, but I also wouldn't be surprised if they wanted backward compatibility with the existing barrel extension. Some of the improved bolt designs require a proprietary barrel extension.

[sinlessorrow]

You thought, but you were wrong (no disrespect intended). Please read the thread you linked again and you will (well, you should) see that you are incorrect. Yes, timing is important. But if timing is not adjusted (as in the "all things being equal" in my original statement) a piston system will subject the bolt lugs to a higher level of stress, whereby a DI system will be less abusive to the bolt than a piston system. Thus a DI system will incur longer bolt life than a piston system.

Thats great in theory but it seems the service life of a bolt is 5,000(before developing cracks) for the M4 while piston guns do indeed seem to last much longer.

I just cannot understand why. At first i thought heat but there is at max 40* difference, certainly not enough to induce such rapid wear. So maybe it does come down to construction, but what makes a bolt like the HK for example better constructed, the design of the bolt face and lugs appears to be identicle aside from finish.

To deawaters i know the BCG's entered had to be drop in, but even LWRCI claimed 20,000 ontheir DI bolt.

[Arctic1]

@Skullworks:

And why is that?

What is the difference between an op-rod pushing on the bolt carrier and the gas pushing on the bolt carrier when it comes to extraction? They both push the bolt carrier back, until the cam pin starts to move in the cam pin track, thus rotating the bolt until it unlocks.

There is no room inside the barrel extension for the gas to push the bolt forward. If there was, the gun would probably be unsafe to fire.

Both systems incur wear on the bolt lugs, as there will be a slight diagonal movement during unlock, where the bolt lugs on the bolt push against the bolt lugs in the extension. And case obturation has ceased by the time the bolt starts to extract the casing from the chamber.

Again, this is probably more related to bolt construction than DI vs piston. Also, like some of the guys with more expertise here said in previous posts, it is very hard to estimate bolt life. It will depend on how hard people run their guns, what kind of ammo they use, if the system is synced in terms of ammo, buffer weight and spring, what manufacturer made the bolt, etc.

Edit:

I just googled "Broken AR-15 bolts", and most of the pictures showed bolts that broke in two at the cam pin slot, and not sheared bolt lugs. Why they break in two in that specific area, I don't know.

We have never experienced that on our guns. The 10.5" guns have had some issues with broken disconnectors and sheard bolt lugs, during courses of fire shooting A LOT of ammo. If there is considerable carrier tilt in the 10.5" guns I don't know. I do know that the 16.5" guns are holding up well, and as a comparison, we see way more broken Aimpoints than guns.

[skullworks]

Thats great in theory but it seems the service life of a bolt is 5,000(before developing cracks) for the M4 while piston guns do indeed seem to last much longer.

Seems? Do you have definitive numbers for both systems?

[sinlessorrow]

Seems? Do you have definitive numbers for both systems?

No definitive number but everything ive read from SME's like BB says to replace the bolt at 4,000-5,000 cause thats when they develop crack but they may go to 7,000-10,000 if your lucky.

On the other hand the same people say piston AR's bolt have greatly increased service life and can go 15,000-20,000 before needing to be replaced

I am just wondering why. What makes the construction of bolts like the HK bolt so much better? The lugs and extractor appear to be the same dimension.

ETA to add: the 4-5k is under heavy firing schedules same for the piston numbers

[skullworks]

There is no room inside the barrel extension for the gas to push the bolt forward. If there was, the gun would probably be unsafe to fire.

Please note that there is a difference between pushing the bolt forward so that is has NO contact with the BE lugs and pushing it forward so it has LESS pressure during contact with the BE lugs. What I am saying is that the DI system pushes the bolt forward so that it has LESS pressure on the lugs during contact than when rotated in a piston system. Therefore less stress is incurred on the bolt and BE lugs in a DI system than in a piston system, whereby the bolt life should be longer in a DI system (from a purely mechanical standpoint).