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.
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