"A" Preventative Maintenance Program

AR/M4 AR Technical Discussion
1

I have been in the process of re-building my “M-4” carbines for the last couple of years, and have also been reading/ re-reading a lot of threads trying to come up with a good parts replacement schedule.

So to re-cap, I wanted to throw this out there and see if it sounds about right. But first, just so we’re all on the same sheet of music, my basic guidelines are as follows. I want a carbine for civilian self-defense, regardless of how likely or unlikely that may be. I want something as close to a mil-spec M-4 as possible w/o select fire.

So the rifle we are talking about will be 14.5" bbl, 1 in 7 twist, chrome-lined, MPI and HP tested, “F” marked FSB (with a Predator FS for leagality); 5.56 chamber, chrome-lined; bolt MPI tested w/ HD extractor spring, insert, and Crane O-ring; bolt carrier with proper staking. Lower with std LPK (i.e. stock trigger); grips, rails, stocks, BUIS, optics, etc. of your choice.

Magazines will be USGI’s (Okay, D&H/DSG, etc.) with Mag Pul Gen II’s.

Ammo will be the best 55/62 gr available at reasonable cost for training.

I know there are a few different choices out there that meet these criteria and I don’t want to get wrapped up in all that. Suffice it to say that if you can put together a package that meets these requirements, using good quality components (Colt, LMT, CMT, etc.), I think you would have a good solid carbine that closely resembles a M-4 in performance and reliability.

Now if all that doesn’t sound too far- fetched, then let’s look at a possible maintenance schedule. I’ve been re-reading a lot of posts about this subject and the one that sticks out is the CQB-R report that puts 10K rounds through several 10.5" bbls. If you want to use this as an extreme worst case scenario, I think this gives you a fair upper limit to baseline off of.

So if we are running our 14.5" “M-4” in semi-auto mode, unsupressed, at a “moderate” rate of fire, we might ASSume that we can get 10K rounds through the bbl.

So if this is a reasonable baseline for bbl life, then it would be nice if the bolt/ bolt carrier, buffer spring, etc. could also last this long. It would be a simple matter of re-building your rifle at a certain round-count and driving on.

But methinks the bolt and various springs might not make it that far. So it might be a good idea to have another re-build point short of the bbl change. It sounds like a good MPI’d bolt should be able to make it to at least 5K.

Now we get to springs and other stuff. The extractor, spring, insert, and O-ring; the ejector and spring; the gas rings; and the buffer springs all take it in the shorts pretty hard, so I’m thinking these might be changed at 2.5K to make our sked even out.

So the maintenance sked might look something like this:

2.5K: Springs and things replacement

5K: Bolt replacement, plus springs ‘n’ things

10K: Re-barrel, plus bolt, and springs ‘n’ things. (plus at this point I’d go ahead and replace all springs, detents, etc.)

Disclaimer:

This is based upon the best of my knowledge on the subject. I do not claim to know everything (or much of anything), nor do I have any hard data to back this up. I am merely trying to sift through the best available info and draw some conclusions. I am not trolling, nor do I want to argue endlessly with anyone about it. Tax, title, and license extra. See dealer for details. :smiley:

2

Thanks! I was looking for something like this! :cool:

3

So Diz, what exactly do you sew anyway?

This is gonna be a hard topic- are you asking a question, asking for clarification or just making a statement? Seems you just want clarification, right?

Seems what you said is what I remember reading for the last few years. -The correct threads I’ve read anyway…

This is going to be your first AR15, your first M4gery? Your first gun?

4

Ok by the numbers.

I prototype design and do limited production of tac nylon gear.

I have been going through a few threads here and at 10-8 trying to pull together the best info into some kind of maintenance schedule.

I am putting out my best SWAG and asking for comments. Too high, too low, about right?

The main references for me have been the Crane reports on the CQB-R and SOPMOD programs, Pat Roger’s comments/articles on his experiences, and a host of others here and at 10-8 who have shared their experiences.

Is this my first? No. I have owned or been issued a M-16/CAR-15/AR-15/M-4 since 1976. I have built a few along the way. What I have attempted to do recently is update my weapons systems to a M-4-like standard, using the best info I can find, from this site and others.

So now that I have some pretty decent rifles, I want to establish some kind of manitenance plan for them. I have to admit it’s only been recently that I’ve really started putting good round counts through my weapons.

5

Totally agree with what you’re trying to do here and it’s appreciated.

This being said, I think this is a very aggressive schedule. Springs and things could easily go 5K without replacement, and bolts around 10K (if properly lubed, especially the lugs, and replacing the extractor/ ejector and springs at 5k), and the barrel if chrome lined, and properly maintained around 15K. I spoke to LMT about my MRP barrel and this was there recommendation, but emphasize regular and proper cleaning. They also believe their bolts could go 10K. These are for a 16" MRP barreled system.

I have all the replacement parts on hand including bolts, and barrels just in case I’m being too conservative. You’ll be able to tell when the bolt and barrel start to go through the examination of spent brass, or failure to lock up because of head space wear, and the fact that your groups will begin to balloon when the barrel starts to go. Both not catastrophic failures, and predictable.

Thanks again for posting this!

Tack

6

Thanks for the come-back. That is exactly what I’m trying to do here. Get some dialogue going about this issue.

You have a good point, this may be too aggressive a schedule. It would be nice if damn-near everything could make it to 10K for a nice major overhaul. A lot of what I’ve read seems to point to this as a good figure of merit.

I was going off round counts as a quick and dirty way to keep track of it. Perhaps we could consider some periodic diagnostic checks as well. Go/No-Go gages in the chamber. Visual inspection of the chamber and the bore. Obviously function checks of the rifle. Test firing the rifle for groups.

Not trying to re-invent the wheel here. If somebody knows of a good (open) source for all this, let me know. I have read a lot of the manuals out there, including the GI ones. Speaking of which, the -23&P is a good starting point for diagnostic checks but it seems to be based on quarterly inspections “at the unit level”. I see no mention of average round counts, although I have read that 7.5K is supposedly the GI limit for a bolt (Sweeny, “AR-15, vol. 2”).

So you have a choice of round counts (which is where the military seems to be going, at least SOCCOM), and maintenance intervals (traditional). Or a combo of both?

7

We’re on the right track here. I agree with specific round counts and keeping a detailed log of the types of ammo used, new springs, BCG’s, FCG’s, Barrels etc. being tracked. I also track all of my other implements like Aimpoint battery hours, Light battery hours, etc. I also support using Go/ No Go gages to really define wear and tear.

As I mentioned, I have all the replacement parts for preventative maintenance, and really load up on bolt parts, as well as having an additional bolt ready to go and stowed in my TD VFG with extra firing pin, Cam Pin, and retaining pin. The one thing I’m working on now is a complete lower assembly, and additional MRP barrel that has the gas block and gas tube included ready to be “dropped In”. These barrel assemblies (like all LMT Gear) is expensive at $466.00.

But we also need to keep track of what preventative maintenance items that might be banned in the future, and load up on these.

Tack

8

Depending on your firing schedule I would check the the throat every 1500 rounds, as well as action spring length. The barrels could last 50 or 60 thousand rounds, but the chambers go in as little as 8 grand. Even less if your doing alot of Frangible shooting.

Since Crane says you can have bolt shear in as little as 4 grand with a harsh schedule its going to be up to you to modify the schedule depening on your use.

But your baseline is good.

9

Good responses, guys, thanks.

I guess you could do a light/med/heavy use schedule as well.

And a suggested spare parts load out makes sense.

Along with tools and supplies.

We are re-plowing a lot of ground but it would be nice to gather it all up into one place, with a suggested maintenance schedule.

BTW, anybody know of a shop manual for the AR-15 like Kuhnhausen did for the M-14?

10

I’ll argue with you!!! :stuck_out_tongue: :smiley:

I think that good quality chrome lined barrel should make 15k-20k rounds (on semi-auto fire), but I may be overestimating. 10k sounds a little low to me. LukeMacGillie does bring up a good question about the throat erosion though. Something to keep an eye on. The biggest thing I focus on at present, is spring replacement. I check the bolt, cam pin, etc…, usually after every range session, and definitely at every cleaning for any signs of trouble. Extras are kept on hand.

11

Throat erosion is something to be concerned with, but I question how important it is in CQB weapons. When I shot Bench Rest, we would bring the barrel back about 1/8" every 5,000 rounds or so to have “Fresh Land and Grooves” to contact the bullet bearing surface. Those were .308’s.

I have 6400 rounds on my MRP barrel, and even though I haven’t checked the throat, it is still shooting sub MOA (checked it 3 days ago), which tells me something. The head space is still well within tolerance.

Tack

12

I dont know this for sure, but I really do think that the throat erroision was the reason that the Rangers were back to the old Vietnam SOP of having a cleaning rod taped to the weapon in the early days of the GWOT.

Throat errosion is the first thing I check when Agents have extraction issues, and 8 times out of 10 its the throat, allowing the round to seat too deep, not a bad extractor/ejector.

But I end up replacing the entire upper and bolt.

Some of the folks who I support are on the bolt a week and upper a month plan :eek:

13

Yeah, this can get real technical real quick!

I think the thing to shoot for is a nice average, sorta in the middle of the bell curve, as it were.

It sounds like a good chrome lined bbl is capable of much more than 10K and the chamber might be the critical wear point. On the CQB-R report I believe they had something on the average of 1-2 thou larger headspace after a hard10K rounds. On the other hand, there was also significant errosion at the gas port which you normally might not think about.

I believe you are right, in that the initial focus should be on hard wear items that typically wear out much sooner than bolts or bbls. The bolt parts, especially the extraction system, the buffer spring, the mag springs, etc. Then there is the bolt carrier keyway staking, and the reciever extension staking.

Then you have to cull out any problem mags. And try to find some decent ammo.

And last, but not least, find a good lube and cleaning system.

Aye Carumba.

14

D,

Just keep in mind when reading those reports, to differentiate between full auto fire and semi auto. I’m sure you are, but just saying it for posterity. Full auto fire is going to wear on the throat, gas port, bolt, etc… harder/different than semi auto fire.

15

Diz these questions have been asked quite a few times on here in the past with quite abit of decent replies. If you cant find them in a search let me know and I will help you look. Also when your done please post you findings as this is Goos Sh-T!

16

Absolutely right on the full auto part, as well as suppressors. I know these two factors are gonna really fry your piece.

It’s kinda like shooting for a target here. We really don’t know how many rounds we will get out of our components, but we have a pretty good idea of how many it takes to trash a rifle under extreme conditions.

At the other extreme, we have all heard stories of guys getting 20K or more through a rifle, with the same bolt and bbl.

17

OK I did do a search for PM. And I think gotm4’s reply should be a sticky. (I did read through all of those, BTW.)

-3K for complete bolt re-build, head space check, and buffer spring check.

-Bolt and barrel until a headspace down check, then replace as a unit.

-Complete overhaul at 10-15K including all the above, plus FCG springs.

Sounds pretty good to me. My apologies for not checking first.

18

You can’t really fix a number to barrel life.

Assuming that the other parts of your shooting system are in order, when the barrel stops hitting what you are aiming at, it’s probably time to replace it.

For highpower shooters, this might only be a few thousand rounds.
Threegun shooters will go longer on a barrel.
Slow-fire plinkers will go a long long time before wearing a barrel out.

If you don’t shoot much beyond 100-150 yards, you probably won’t even notice when your barrel IS getting worn out.

19

Good point. I never specified that I am talking about using my carbine as an in-close defense weapon, probably not shooting past 100m. So even with significant chamber/bore wear, I can still get 1 minute-of-bad guy.

What got me thinking this way wuz the Crane SOPMOD report where they were talking about going to round-count -vs- maint intervals for M-4 inspections. Which makes sense for high-mileage short-bbl guns that are at high risk of failure before std maint interval.

Another point seems to be the gas port errosion on a carbine length gas sysytem. We are pretty much stuck with it. Although heavier buffers seem to help by increasing dwell time. It would also seem to make sense to drill the gas port before chrome lining, with a careful finish reaming to size?

20

Diz, the minor issue with the carbine length gas system is one reason I have become a fan of the midlength system.