Is this factory ammo, if so what kind. I had similar result using an H-2 buffer with a reload with 30.0 grains CFE 223 and 123 grain Nosler CC. The CFE223 load wasn't building pressure fast enough and my rifle was short stroking. I switched to Carbine buffer and the 16" J&T carbine gas upper functioned. H-3 buffer is heavier than H-2. As the rifle gets hot on a new build might explain why it starts to malfunction after a few rounds. I use a lot of CLP on the BCG on my new builds.
My 16" J&T has a fixed A-2 front sight. Are using an adjustable gas block? Under gassed and over gassed can be hard to distinquish. If you have a carbine buffer you might try it especially if your shooting Wolf or PPU brass or steel cased ammo.
My 16" J&T has a fixed A-2 front sight. Are using an adjustable gas block? Under gassed and over gassed can be hard to distinquish. If you have a carbine buffer you might try it especially if your shooting Wolf or PPU brass or steel cased ammo.