I put together a M1S 20 inch A2 rifle kit, I made sure I picked a 5.56mm chamber rather than .223, and a chrome lining for the barrel, they sent me what I ordered fairly quickly and 2500 rounds of wolf later it broke all my expectations for a "cheap" AR15.
During initial break in I had some problems with case rims being torn away by the extractor, approximately 6 of these in the first 500 rounds. Problem has not recurred. (Bad lot of ammo??)
Had a malfunction where the hammer wasnt cocked when the bolt went forward on loading at approximately round 1500...dunno what caused it, hasnt happened before or since.
Several 29th round bolt-over-base malfunctions...before i trashed the 20 year old USGI mags I had and went to PMAGs.
Broke the charging handle at some point or another, didnt notice until the charging handle wouldnt return to battery when I released it. How long it had been cracked and bent I have no idea. (Keep a spare around, these have a nasty habit of breaking when put under hard use.)
Broke the firing pin dry firing the thing. Replaced, bought snapcaps.
It's been rained on, bashed into trees and corners by my clumsy butt, dumped in the mud, ran 500 rounds a day for two days with no oil or cleaning, left to rust (RUST ON AN AR!?!) and generally abused far in excess of what it probably should've been.
If I was going to grab a rifle for "srs bzns" would I take my M1S AR in preference to my Colt 6940? Heck no. If I was putting together an AR that I wanted to take no chances with as far as performance, and could afford Colt/Bravo/Noveske/etc. would I use M1S parts? No. Would I hesitate if all I had was 600 dollars or so to spend on the rifle? Not a moment.
Go get PMAGs, a d-fender for the extractor spring (or at the VERY least a #60 o-ring from the nearest hardware store. 10 cents, no excuse not to do at least this.) loctite all screws and make sure the carrier keys are staked, don't shoot brass cased after steel cased without a cleaning between the two, keep it clean, and there is no such thing as too much oil with an AR15. These are general AR15 owner tips here, not specific to M1S. Doing the above will help.
Only other thing I might suggest is a bravo/colt bolt carrier group kept in reserve when you're a few thousand rounds down the road. Bolts and cam pins can/will wear out and break, pay attention for cracks and excessive wear on these parts when you clean the weapon.
The "Catch" to model 1 sales and similar "cheap" AR15 parts companies is that you have a somewhat higher chance of getting parts that would've been caught by more stringent QC. I've heard of soft FCG parts and bolt hold opens, i've heard of bolts snapping at the cam pin hole way, WAY before they ever should have, i've heard tales of unending malfunctions...but wether that was because out of something wrong with the rifle itself, or the guy who ordered it trying to stuff a thousand rounds of wolf or 5.56mm into a tight-end-of-spec unchromed .223 chamber out of a decades old USGI armory reject magazine with a black follower and original spring without any thought of terms like "Basic weapon maintenance" or "break-in period" ...well...your guess is as good as mine. As I said before, I wouldn't take my M1S in preference to my 6940 for any serious duty, but it IS rather nice having an AR15 I can treat like an AK and not feel bad about it.
