On Wednesday afternoon I had a little time and went out and shot three rounds loaded with 35 grains IMR 4350 and three rounds loaded with 37 grains H4831, both using the 156 gr. Lapua Mega soft nose, Federal 210 Match primers, at an OAL of 3.005, not crimped.
Velocities with the 4350 load were 1969, 1983 and 1934. The H4831 loads were 1881, 1852 and 1866.
I measured the base diameter of the cartridges before firing, new Norma cases, by putting the edge of the micrometer spindle (?) at the top of the extractor groove, so the measuring point was one half of the diameter of a Starret mic spindle up from the extractor groove.
In all cases the primers were pushed back by firing by an easily visible amount which I have not measured yet....more or less 20 to 50 thou.
The before and after head measurements for the 4350 loads were: .4430 and .4497, .4433 and .4497 and .4432 and .4494. I did not measure the 4831 loads but assume they would have been similar.
Opening the bolt and the extraction process was easy and smooth...no resistance. Accuracy at 100 yards was about 7 inches for the 4350 loads (very old powder but still smells OK) and about 3 inches for the H4831 loads.
About .006 case head expansion and way too much headspace makes me wonder if re-barreling is not the best course to follow.
As a 6.5x55 was mentioned above I looked up the head diameter of a 6.5x55 and found it to be .4803 on the 6mmBR website.
I would be very interested in your thoughts on this.