So, I just bought an 18 inch upper with "rifle" gas system?

[jeffk14]

Rifle gas system was originally designed for a 20" barrel length. Based on my experience with a CRANE spec SPR barrel and an 0.0995 gas port, you could probably go up to an H buffer or a Vltor A5H1 like I use. But the wildcard is the gas port size and I have no idea how your barrel is actually gassed.

^^^This^^^ I have that same lower with a carbine buffer under an 18" rifle gas BCM upper and it purrs like a little kitten. Unless your upper is over gassed, you should be fine. If you're curious, you could always throw an H buffer in there and see if it locks back on an empty mag. Never bothered trying it with mine, since it shoots so soft already.