My Guess is 1. Measure the neck of a fired case. Has the neck expanded to its full normal diameter? If the neck has Not expanded fully, you have a low pressure condition. This is caused by light bullet pull because of bullet design? The full diameter bearing surface of the bullet is to short or the bullet is seated to long?? 2. The gas port , the hole drilled into the barrel, might not be at the best location or be the correct diameter for the lite bullets. I know on M16A1's the carbine compared to the rifle have much different gas port size and location. IMR 4198 runs fine in my 3 different auto with Sierra 50 or 55gr bullets. I do NOT like any of H's copies of the IMR's. There was another poster on here using IMR 4198, same problem, very strange. Just a guess.