the black o-ring and heavy spring kit comes installed with LMT and BCM bolt-carrier group kits. just picked up a new LMT bolt carrier group from matt-d tactical for my new CDD m4LE build this week. i fried about 350 rounds of mixed ammo this week between t-storms --not a one hiccup. slurpy smooth to boot.
glad it is working for you.
on a bushmaster i all ways check the gas-key stake to be on the safe side as well. some times they will work a bit loose with use and cause a short stroke or two.
the AR series can be a dependable weapon -- you can download army manual TM-9-1005-319 23&p from some of the AR sites it has about all you need to know to make the m4 run--trouble shooting , repair, assembly it is all there.