M4 Carbine Bolt Life and ways to help extend it?

AR/M4 AR General Discussion
1

I’ve read enough about the advantages of the mid-length gas system here that I’m thinking of adding at least one middy to my collection.

But I can’t afford and don’t want to trade off my carbines for middys.

Are there ways to improve the bolt life in a carbine length gas gun?

Does it help to fire mostly .223 in the gun instead of 5.56 to keep the pressures down?

About how many rounds does a bolt last in a carbine gas gun?

Those of you who have had to replace bolts, were they all MP, HP and shot-peened bolts, or have you had to mostly replace “doubtful” spec bolts?

How difficult is it to headspace a new bolt to the existing upper? Are we talking “be prepared to return several before you get one that headspaces properly”, or are we talking “nine times out of ten a bolt will headspace properly.”

Lots of good info on this board. I learn something new nearly every day. :cool:

2

what causes the extra wear is the overgassing because of the gas port to muzzle distance, cant get around that without switching to a different gas system.

3

If already have a carbine don’t sweat it too much? If I had to guess you will be ok for thousands of rounds. Just buy a few extra bolts for $60 or so a piece and call it good.

4

I have a carbine and plan on building a midlength in the near future. Get an M16 carrier and the heaviest buffer that will still allow the bolt to lock back on the last round with the weakest ammo you shoot. You will probably make it well over 20k rounds before ever having a problem with the bolt but an extra bolt from bravo company is ~$70. Also as long as you’re buying parts from a reputable manufacturer, headspace will never be an issue.

5

I would just have a spare BCG and bolt or two on hand (would do it anyways with any gun) but I wouldn’t sit around worrying about bolt breakage.

6

my experience with bolt breakage is limited to crappy/unknown bolt manufacturers. i don’t think i’ve broken a single bolt since switching to quality bolt makers. the two biggest things you can do to avoid having a broken bolt, in addition to using a quality bolt, like LMT, colt, or BCM, is to replace your cam-pin every 5k and to keep it clean and well juiced with an LP (lube, preservative).

most bolt breakage occurs at the lugs and cam hole. every bolt i’ve broken/seen broken has broke at these two places, IIRC. they break at the cam hole because of wear creating play between the bolt and cam-pin- this extra play allows for extra jarring during action, which can create cracks that eventually snap the bolt in half. the lugs break due to cracks that generally form from micro putting behind the lugs- keeping it soaked in LP, generally requiring keeping it CLEAN, prevents the pitting from starting to begin with. using an LMT bolt, which has radiused lugs, further preventing pitting and adding a bit of spring-tension to the equation, in my opinion, further prevents lug breakage.

as to expected life- who knows. every bolt has it’s own grain structure and fit in it’s host action. a bolt may be destined to break after 5k, and no amount of effort will prevent it. this is why HPT/MPI combination is important- it weeds out most of the bolt destined to premature failure. a bolt SHOULD last the 20k hoped-for service life of any weapon, but PM is imperative to this… and, once a bolt has over probably 15k on it, it should be checked for headspace periodically to prevent losing the gun entirely. bolt life is also going to depend heavily on how the weapon is used- even a complete piece of shit bolt will probably go 30k+ if the weapon is only fired 1,200 rounds a year, even more so if those 1,200 are slow-fired.

i’m not sure how many bolts i’ve broken, but it was a pretty common thing to have happen once upon a time. gun-show and mail-order catalog bolts generally didn’t last all that long. i also shot a lot more, and did a lot less PM, back then. i eventually figured out that Colt bolts lasted longer- in fact, they didn’t seem to break at all. later still, i learned just what exactly the TDP was and how to apply it, and i’ve not used a non-HPT/MPI bolt since… i’ve also kept my guns a little cleaner, and replace cam-pins at 5ish K… i can’t recall breaking a bolt in the last 4ish years, since getting better educated about these things. as i said, i don’t currently shoot as much as i once did, but i still have at least two bolts that currently have between 10-15k, and don’t have any plans to replace them. they’ve each/all been through 2-3 cam-pins by now, and are cleaned after at least every other use, and coated in a lubricating steel preservative.

7

Don’t worry about it. Keep an extra bolt or two along with some spare parts on hand, and go shooting.

I see bolts break for two reasons. First, and by far the most often is bolts that are broken because they are junk to begin with. Your cheap stuff is always going to break quicker.

Second are the good bolts that are taking huge amounts of use and abuse. Abuse isn’t a bad thing in and of itself, but you need to take it into account. If you are shooting a few thousand rounds of ammo at a time, and much of it is full auto or doing CQB drills where you do not allow your weapon to cool down, thats abusing it. Again, thats not a bad thing, but you need to keep in mind that the parts will fatigue quicker. Abuse in this context is doing something that increases wear, which is fine because we know the end result.

Expect a bolt to last 30k or so depending on how its shot, though many will go well over that. Replace the rings once in awhile, and you are good to go.

To be honest, I think you are worrying about it too much. By the time your bolt is dead, your barrel will most likely be shot out, and its time for a new upper anyway.

8

Excellent posts, guys. Somebody should “sticky” this.

I’m not worried about it; I just like to be prepared.

Interesting that wear on the cam pin seems to be the more important factor than number of rounds fired. I’m going to print out that post and keep it as a guide.

The above posts are exactly why I decided this time around that I was going to stick to quality “left side of chart” rifles with HP/MPI shot-peened bolts. Being old school I had it drilled into my head for years that if it ain’t Colt it ain’t an AR15. Luckily the chart shows that there are other manufacturers that are up to snuff these days. I grew up knowing that you bought your Colt for your SHTF weapon and tore up your DPMS (Doesn’t Pass Mil Spec) or your Bushmaster at the range.

I’ve got a BCM Carbine and a Daniel Defense Carbine, and I know that I’m going to weaken by next spring and buy another Colt M4 as well. Should never have sold mine three years ago.

I’ll probably add a BCM middy eventually, but as long as a carbine doesn’t self-destruct in 5,000 rounds or so then replacing the bolt is a small price to pay for being able to shoot Ol’ Shorty.

I take it by the above posts that headspacing a new bolt usually doesn’t involve much other than verifying headspace to be sure? I’m reading between the lines and inferring that you guys are pretty much saying that the weapon’s tolerances are such that any quality bolt should headspace to your upper unless there’s a problem with either being out of spec, right?

I’m so used to bolt guns that may require the barrel to be set back, or weapons with pinned barrels like the AK that may require a bucket full of bolts for you to find one that headspaces that it takes a little thought for me to get used to the idea of expecting a bolt to headspace right out of the box most of the time.

Please correct me if I’m wrong!

9

Well, you got some nice rifles. No need to sweat it, especially with those brands. That said, always good to have spare parts around because you just never know…

10

The only guns I have seen break bolts are Colt M4A1s.
Not because they are poor quality, simply because that’s what we had, and we shot them a lot.

Most common breakage was the lug beside the extractor.
All of the broken bolts were from well used/abused guns, with round counts in the 10s of Thousands of rounds. Exact round counts were unknown as the guns were already well used before we started to (loosely) track round counts. I have personally witnessed at least 5 bolt breakages, and have seen about a dozen in total. All were from carbine length gas guns or piston guns.

Our 20" guns had the bolts replaced when the barrels were replaced, but I never saw a bolt break in any of them anyway.

I have never seen a quality bolt break in a middie or longer gas system, but I have dealt with exponentially more carbine length gas guns than middies, and about triple that number with rifle length gas systems.

11

Not to throw cold water on the warm fuzzies, but are the mid-length guns being shot in sufficient numbers/round counts/battle conditions for a genuine statistical comparison? Or are they a little new to have a really good handle on whether the improvement over the carbines is that significant?

I only ask because I never heard of mid-length guns until a few months ago. Granted my interest in the AR platform took a ten-year break, but M4’s and full-length rifles are sort of a known quantity.

Are middies?

You would think that another two inches of gas tube wouldn’t make that much difference. Maybe I’m wrong. You’d expect the rifle bolts in guns with the full length gas system to last the longest because that’s the way Stoner designed the weapon. The fact that even carbine bolts apparently survive tens of thousands of rounds is definitely encouraging.

(I’m not saying the middies not better than carbines, just do we really know yet)? I’m certain that I don’t know the answer.

12

The odds of a broken bolt are slim as long as your bolt isn’t complete ass. I wouldn’t worry about it at all.

Shoot till it stops working, and that will be awhile if you have a decent quality weapon.

13

terms like “better” are hard to quantify.

there are no mid-lengths in mass-service- so there’s not only not enough info, there’s probably not going to be.

but we do know that they shoot softer, which some equate to “smoother.” the ratio of dwell to gas time in a carbine 14.5 is .517, in decimals… mid’s ratio is .655, which is closer to the .625 of the rifle-length system. i don’t know why .625 was chosen for the original design, i don’t know how much experimentation was done. but we know that the 20" guns are very “smooth” shooting, very reliable, and very durable, relatively speaking.

so, by the numbers, midlengths should, and so far appear to, behave more like rifles. whether or not this will result in significant durability improvement is probably a question that’s gonna remain unanswered for a long time.

14

I think I got my first middie in 2004. They have been around for a while. They didn’t really become popular (and by that I mean within niche users, such as the general readership here) until about 2006. There are many hundreds, maybe even thousands of middies that are run through high-density training by people that care enough to pay attention to and track problems. Unfortunately, we tend to track things that go wrong instead ofthings that work. Thus is how we have tracked the problems with Oly, BM, DPMS, etc through the years. In AARs they kept popping up as “Had another BM (or other manufacturer) with a loose gas key (or other problem(s)”, and the like. AARs would take a whole hell of a lot longer to write if everything that was ok needed to be documented. Scientific process? No. But nobody is asking for or demanding it.

So, unless somebody wants to pay instructors to tack each and every model pof weapon, with notes on any modifications made, along with ammunition, weather, round-count, etc, the data points aren’t going to follow the scientific data gathering/compilation process anyway.

Basically, what I am saying is that you have to take the “no news id good news” as long as the items have been represented in significant numbers. Middies fill this, and the gas system itself has been found to be compeltely viable.

FWIW- 2" of gas tube, like .004 inches of gas port width, is significant in the AR when it comes to operating pressure, dwell time, and gas volume, all of which are critical to proper operation. The closer an AR is brought to the 20" original design the better the gun performs. Any deviation causes some detriment to operation and function. Sometimes the benefit of the deviation outweights the cost to the operating system, and sometimes slight progress back toward the standard will greatly improve function without being detrimental to the mission.

15

I completely agree…just like you have several carbine length gas systems, there are hudreds of thousands of carbine systems out there and they run just fine…is there something better? Possibly, in the middy, but is it worth trashing a carbine over or even worrying about? Not IMO.

If you wanted to upgrade your bolt you could take a look at LMT’s Enhanced or even LWRC’s Advanced bolts…both are designed to improve some of the weak points of the exisiting/conventional design. LWRC also makes an advanced cam pin.

Further, LMTs Enhanced carrier alters the gas porting to do at least some of what a mid length gas system does: increase dwell time and decrease pressure.

I think anyone would be hard pressed to determine a quantifible difference in bolt/component life of a carbine vs. middy and if such a study did exist the engineer in me would be interested in seeing it.

Even if such a test were attempted it would be extremely difficult to control all of the other variables in order to say yep, the middy did (or didn’t?) last longer.

We (myself included) are assuming it will be better based on lower pressures, etc. but by how much?

16

Headspace is determined when the barrel extension is installed at the factory, so you will most likely never need to worry about it. Barrels/bolts/upper receivers can be swapped back and forth without worrying about headspace. Just throwing out a number off the top of my head, using a field gauge after 5k rounds or so might be a good idea.

17

Play it safe on this one if you change out your bolt have the headspace check, where there you have the field gauge or gunsmith. You don’t want your $1000.00+ gun damaged by the lack of spending $35-50 for checking headspacing do you?

18

I could not agree with you more. I have a 16" Daniel Defense CHF CL Pencil Barrel with M16 carrier, stock buffer spring BUT I run an H3 buffer and it does not short stroke with any ammo. Cyclic rate is slower but I keep her well lubed and still can fire as fast as I can pull the trigger. TIMING is EVERYTHING and I believe the heavier M16 bolt carrier and my H3 buffer is certainly easier on the Bolt and will contribute to the longevity of the rifle. Funny how 99% say to go with just an H buffer. Again, I believe we got it right with your statement in blue above!

19

Buy a spare bolt, shoot and enjoy.

20

Bolts last longer if they are otherwise built to the mil-spec but not proofed with a high pressure round.

A shorty bolt can be expected to fail at 6-10k. An M4 isn’t too far behind. M16s are 15,000 plus. ROF has more to do with it than anything though. Personally I’ve not had bolt failures but I’ve been on the range for them. Normally commercial style bolts fail at the cam pin and military style bolts at the lugs. The latter is often times not a critical failure.

Ideally the bolt should last as long as the barrel, and as the length of the barrel life is extended it is desirable to increase also the bolt life to match. A good point to aim for would be 20,000 rounds.