Years ago, had a bunch of problematic HK416's.. fixed them with A5 tube & buffer.

This gas system can appear paradoxal (compared to a DI) in so far as if you under spring it or under buffer weight it it will slow the BCG rearward motion. The energy is captured in the (really) short stroke piston up front. In overcoming initial spring/buffer weight, in a very short space, it has enough inertia for the cycle.