With the buffer installed with a 9mm upper, the rifle had a weird double recoil impulse. It was sort of a ka-chunk effect. Since it didn’t have any noticeable effect on performance, I went back to using the RRA 9mm buffer.
Shooting .223Rem ammo, I didn’t really notice the double recoil, but it still didn’t seem to have any noticeable effect on recoil control. My 16” LMT M4 upper ran okay with it installed, but the 10.5” LMT upper was hanging up. The 10.5” upper only ran well with a carbine buffer; I had the same problems with the “H buffer” though.
The 10.5” upper was run on full-auto. Again, I still didn’t see any really noticeable increase in control. Rate of fire didn’t seem to be slowed down much, although I didn’t try to measure it.
None of the uppers I tried with the Endine buffer would consistently lock open after the last round. I tried the buffer in another lower, and got the same results.
My suggestion would be to skip the Endine buffer, and run the heaviest mechanical buffer that will still allow your rifle to function properly. You can just about buy a carbine buffer, an "H buffer," an "H2 buffer," and a 3-pack of buffer springs for the price of the Endine buffer.
The 10.5” upper showed an obvious preference for the standard carbine buffer. My 16” LMT upper will run on the carbine buffer, but seems to run a little better with the “H buffer.” Some rifles will run with the “H2 buffer,” but I personally haven’t come across one yet. When running a 9mm, you should run one of the 9mm buffers, but an H2, or H3 buffer might work.