Gauge pressure is the correct method. The gun barrel is exposed to atmospheric pressure, so there is no need for absolute pressure. Likewise, the barrel will, for our purposes, always be exposed to atmospheric, so differential is the same as gauge, in this case.
The current best way is to procure a strain gauge device, glue a strain gauge to your barrel (actually, a single shot like the H&R is a very fine candidate for a test gun), and follow directions for pressure testing. There are a couple of active threads on this subject.
