After several requests from students about an affordable piston conversion vs getting a manufactured piston gun(my personal coice) I picked up a bushmaster kit for an LMT m4 14.5" gun.
The kit installed fine but it has several failure to chamber stoppages.
It will shoot, eject and close but its like it doesnt have enough energy to drive the bolt to the rear far enough to strip a fresh round after ejecting a spent casing.
My first thought was to change the buffer spring and hammer spring.
I’m waiting for parts now.
My second thought was to investigate whether the shorter barrel length makes a difference in the gas volume cycling the mechanism.
I’ve had one of these kits for over two years now, I had the same issue with a bushmaster barrel. I changed the barrel back to a wilson and the problem disappeared. I believe its a gas port issue, wether its to small of a diameter or it doesnt line up correctly with the hole in the spigot of the piston kit.
I believe the gas port was to small on my bushmaster barrel. You could also try getting a heavier buffer, maybe something like an H or H2 anti tilt buffer… When they become available of course. Hope that helps…
FYI if you open the gas port up to “spec” it should run fine. Bushmaster barrels seem to be notorious for having an undersized gas port. These builds were:
Rock & Roll time after opening up the port… not a single issue since no matter how crappy the ammo used. Now these are NOT GPUs. However, point was that the ports were under spec. The shop gun with the Bushmaster piston kitted shop gun had the same issue.
Ok I have put a Bushmaster retrofit kit onto my 14.5 (Del-ton 1-7 chrome lined barrel) carbine and it runs like a Swiss Watch. Also I measured the gas port before putting the gas block on and it measured .069.
I am right at 500 rounds down range with NO problems what-so-ever. I did do a minor stress test by running a full Beta C-Mag through it followed by three more full mags as fast as I could pull the trigger and all I did was cook my Duracoat paint job a little.
I was getting short stroking of Wolf 75 grain steel ammo (which is reputed to be under powered lately) but I had a PRI fatboy gas tube in so that may have been the problem there.
Anyway the conversion kit runs great. The only headache was getting the Troy rail to fit over the top of everything. With the help of Seth Harness, that turned into a non issue. Thanks again there ! ! ! I also covered the top two vent holes with a piece of brass to keep the hot gasses away from the (very close at this point) underside of the upper rail. I did open the lower two vent holes a scosh as well.
Time and rounds down range will tell but so far so good. I think the clamp that was added from the Aries days is a big improvement. On that note I positioned it under a vent hole in the hand guard. I did this so could see the screw and get at it if necessary, and I scribed a witness mark into it and the screw to see if it backs out. I did use some red loctite but I am afraid the heat will cook that.
Again Time will tell. All in all I am quite pleased with the results and I really do dig not having to scrape carbon off the bolt.
I have 4 Bushmaster 14.5, 1 in 7 twist barrels that I am REALLY not wanting to pull the FSBs to check gas port sizes but I sure don’t want to build them only to try them and discover THEN… that they are undersized.
Hummmmmmmm The things that make you go Huuuummmmmmmmmm
When they become available of course. Hope that helps…