When is a bolt "old"?

[26 Inf]

send me your email address via PM, and I will be happy to send it to you as a pdf document

Joe Mamma

Got it! Thanks!

[Joe Mamma]

I'm still having trouble figuring out where to find the roll pins for the ejector assembly, though buying a JP ejector kit seems like it would do the job.

You might want try White Oak Armament or Damage Industries. I have bought ejector pins from both in the past, and they were good to go in my opinion. They were also both the 3/8" mil spec length, not the 7/16" length which is also very common. Here are links to the ejector pins at both places:

https://www.whiteoakarmament.com/sho...-roll-pin.html

http://damageindustriesllc.com/store...ch=ejector+pin

I have no ties to either company, just a satisfied repeat customer of both places.

Hope this helps.

Joe Mamma

[opngrnd]

You might want try White Oak Armament or Damage Industries. I have bought ejector pins from both in the past, and they were good to go in my opinion. They were also both the 3/8" mil spec length, not the 7/16" length which is also very common. Here are links to the ejector pins at both places:

https://www.whiteoakarmament.com/sho...-roll-pin.html

http://damageindustriesllc.com/store...ch=ejector+pin

I have no ties to either company, just a satisfied repeat customer of both places.

Hope this helps.

Joe Mamma

Thanks! Order placed!

[AKDoug]

Agree, my dad always instilled in me that parts that wear together should get replaced together. I was about 8 the first time I heard it when helping him replace the races and bearings on his front hubs. Being a kid, I asked why he doesn’t leave the races and just replace the bearings. He also explained why you never mix up the bearings while repacking them. Has always stuck with me. I would get a new bolt and carry the 2746rd as a spare.

I'm in that camp with most things, but AR bolts are not one of them. If you shear a lug off a bolt at 6000 rounds and the barrel is still good, are you going to toss the barrel? No, you are going to throw a new bolt in and rock on (if it passes a field gauge). Old extension and new bolt isn't really any different than old bolt and new extension/barrel.

[1_click_off]

I'm in that camp with most things, but AR bolts are not one of them. If you shear a lug off a bolt at 6000 rounds and the barrel is still good, are you going to toss the barrel? No, you are going to throw a new bolt in and rock on (if it passes a field gauge). Old extension and new bolt isn't really any different than old bolt and new extension/barrel.

To me bolts fall into the consumables pile. I would simply replace the bolt as you have stated in the above scenario. But.... why mate the foundation with a worn part from day one when it is so easily replaced? Going back to cars, to me this is like reusing pads on new rotors. Will they work, sure will.

I just think this is a one way street.
New barrel = New bolt
New bolt = reuse barrel.

Ultimately, all that matters is if it headspaces correctly. The rest is just a possible risk of accelerated wear.

[titsonritz]

From the manual:

If excessive headspace, first replace bolt assembly and then recheck. if headspace is not corrected, replace barrel assembly; then recheck
with the original bolt to determine if the bolt is still good or if the bolt should be replaced also.

[Uni-Vibe]

To me bolts fall into the consumables pile. I would simply replace the bolt as you have stated in the above scenario. But.... why mate the foundation with a worn part from day one when it is so easily replaced? Going back to cars, to me this is like reusing pads on new rotors. Will they work, sure will.

I just think this is a one way street.
New barrel = New bolt
New bolt = reuse barrel.

Ultimately, all that matters is if it headspaces correctly. The rest is just a possible risk of accelerated wear.

I'd be inclined to do this.

[Triple X]

Diamondback, send me your email address via PM, and I will be happy to send it to you as a pdf document (or some other format if you want).

I have thought about posting the information from time to time because I have never seen a good preventive maintenance and parts replacement schedule *anywhere*. That's why I created my own.

I'm sure many would find it helpful. But I'm also sure it would start all sorts of arguing (because these things always do on the internet), and I don't want to deal with that. So I don't plan on posting it anywhere.

Joe Mamma

I'd like a copy also please.
Sent you a PM

[lysander]

Deep pockets, ha ha.

So let's look at my numbers in the context of the numbers in your post:

With a rifle length gas system, you say it is on average 13,000-15,000 before a bolt failure (based on the numbers you quoted). So if someone plans to go to 14,000 rounds on the same bolt, half the time they will not make it because they will have a bolt failure. Replacing it at 10,000 rounds as preventative maintenance seems like a very good time to do it.

With a carbine length gas system, it is more complicated. The M4 military carbine length gas system is used with a 14.5 inch length barrel. Outside of a military setting (in the United States), a carbine length gas system is almost always used with a 16 inch barrel. The slightly longer barrel, and specifically the significantly longer dwell time (about 20% longer if my memory is correct), is much harder on the bolt. It's one reason why there were often extraction problems with 16 inch barrels until different inserts, o-rings, and better extractor springs came into common use. Another reason is larger than M4 spec gas ports on 16” barrels (to help cycling some lower quality consumer ammo) which is also harder on bolts.

Again, the 10,000 to 12,000 round number means (theoretically, in this data set, etc.) that if you try to go to 11,000 rounds, half the time you will not make it because of bolt failure. So as preventative maintenance, someone should go for what number, maybe 8,000 rounds? And that would be for a more gentle military carbine system with a 14.5" barrel.

But with a carbine length gas system and the much more common 16" barrel and often larger gas ports (which have been shown to be significantly harder on bolts than the military M4 setup) that round count number would be significantly less. Also, if you spend some time looking around, you will see evidence from reliable sources (military and civilian) of bolts breaking with carbine length gas systems at a significantly lower numbers (and no I am not going to research that for you). So the 5,000 round number still sounds very good.

One thing I did not mention is that my parts replacement schedule has a 1,000 round “cushion” built in to everything. This means that if you should replace a part at let's say 2,500 rounds, you can probably comfortably go to 3,500 rounds. I know that sounds like a big difference. But this covers a situation where it's right before you should replace the part, but you unexpectedly want to grab it and go to a shooting class, weekend of shooting with friends, SHTF situation, or whatever without replacing it. I think 1,000 rounds is a reasonable (and reasonably necessary) amount.

Regarding your “top 4” list, I am not sure if you are suggesting barrels should be replaced before bolts or if you simply cut and pasted from an alphabetical list. But I think it would be wrong to replace barrels before bolts unless you consider things like rate of fire, barrel material (stainless, chrome moly, chrome lining, nitriding, etc.) ammo type, etc.

Joe Mamma

A few things:

1. I stated that on average bolts for M4s are replace every other trip to the depot. And, they go to the depot approximately every 6000 rounds, at which time all bolts are MPIed, So, most make it to 12,000 rounds a few need new bolt after 6000 rounds, which brings the average down a bit.

2. The top four list is in order by round life:

Barrels - 10,000 to 11,000 rounds
Bolts - 10,000 to 12,000 rounds M4/13,000 to 15,000 rounds M16
Ejector springs - 27,700 rounds
Hammer springs - 42,000 rounds

Some more statistics of part life and probability of failure:

For an M4 bolt
10% probability of failure (rounds) - 7,034
90% probability of failure - 17,342

For an M16 bolt
10% probability of failure - 10,771
90% probability of failure - 18,585

[lysander]

The USAMU changes M16 competition rifle bolts usually at the end of the third barrel's life, or around 8,000 to 12,000 rounds. Any longer and you start seeing bolt lugs crack and break off.

And these numbers are not completely out of line with the average life numbers from depot replacements. The AMU probably MPIs their bolts every year.