3700 fps from the 7/375 Ruger even from a 28" barrel seems hot.
I had a 26" 7 RUM a while back and the absolute max load with a moly coated 140 nos bt was 3665 fps.
The case capacity of the 7/375 Ruger is less than the 7 RUM, very close to the STW.
I happen to have some virgin 375 Ruger brass, some fired/fl sized 7 RUM brass and fired unsized 7 STW brass on hand. I filled each case with water:
375 Ruger 100.9 grains
7 STW 100.6 grains
7 RUM 114.8 grains.
A 7/375 Ruger should hold less than a full size 375 Ruger but I don't have the means to neck one down.
The 375 Ruger has 12.1% less case capacity than the 7 RUM. Using JB's 1/4th formula the non necked down 375 Ruger case would yield 4% less velocity compared to the 7 RUM. .96% of 3665 fps is 3518 fps. Add 50 fps or so for the longer barrel and that makes 3568 fps.
I won't deny you got 3700 fps but it seems to me you are working with some very high pressures.
If anyone can produce the actual water capacity of the 7/375 Ruger case I'd like to hear about it.
