I finally completed my load development for, 35 Bergers, 32 SBKs, 34 Dogtown, 39 SBKs. I had the tightest groups with 35 Berger and A2230 at 27.6 gr in Nosler Brass. I decided to hook up a magneto V3 chronograph and see where my velocities were. In other calibers I've been pretty close to powder manufacturer's listed FPS, so I really didn't worry about testing the 204R.
Well... I shot 10 rounds with an average of 3730 FPS. At least 200 FPS slower than what I expected. I had some 34 Dogtowns loaded with same powder, case, primer but a lower charge 27.0 vs 27.6 in the Bergers. 10 shot group averaged 3,868 FPS. This was almost perfectly consistent with published FPS and my expectations. Tried some SBKs and they were consistent with expectations. It was only the bergers that were off.
I use a RCBS Charge Master, and after the low readings, went back duplicated the load and tested it on a mechanical scale. Dead on. Return to the range produced identical results slow results.
Anybody experienced this? I'm seating at mag length, 2.260". 27.6 A2230 is max load for 32gr, so it should be hot for the 35s. No pressure signs, I worked up from 27. Maybe I need to seat deeper? Maybe insufficient neck tension? They shoot about 1/4" groups, so I'm not complaining too much, but I'm just confused on why the 1 gr lighter 34s with the same charge is so much faster.