http://www.lewismachine.net/product.php?p=52&cid=7&session=744d6492b33b2ecebbbc8043b037b5f7
NOTE: Full-Auto Enhanced (to be used with a 14.5" barrel or longer)
They told me why when I set up dealer account with them and ordered my personal 10.5 upper. Don’t remember why now but he told me DO NOT use it with the short MK18 type uppers. So I got their normal spec BCG and it runs perfect…
Best option would be to try a few different ones and see what happens. I want to say LMT use to provide a carbine buffer in some of their 10.5 factory sbr’s but I’m sure someone can chime in.
When I built mine(10.5 lmt on an lmt lower I did a form 1 on) I started with an h buffer but had a carbine and h2 buffer with me. Gun hasn’t given me any problems related to the buffer both unsuppressed and suppressed so I’ve left it as is. I do have the h2 buffer in its bag in case it needs it down the road when the gas hole opens up some from wear.
My factory LMT 10.5" SBR came with a carbine buffer and it ran fine for 1,000 rounds. I changed to an H buffer in hope of less bolt bounce and pounding on the bolt its been good to go for 1200+ rounds. I just got an H2 in from BCM but haven’t tried it yet.