I just bought a Marlin 1894C with the 18.5" barrel .375 Magnum.
I shot my new Marlin for the first time on Saturday the 20th, and brought along a Chrony chronograph to test ammunition. I tried Sellier & Bellot, Mag-Tech and American Eagle cartridges, all JSP 158 grain.
All ammunition was fired more than 10 times. The barrel was cleaned between each brand. One of each, the highest & lowest velocities were discarded to normalize the results. Here they are:
Sellier & Bellot
Avg Velocity 1434.69 - Extreme Spread 46 - Std Deviation 15.98
Avg Velocity 1689.25 - Extreme Spread 41 - Std Deviation 12.16
Avg Velocity 1853.50 - Extreme Spread 21 - Std Deviation 7.36
The Sellier & Bellot had the accuracy advantage at 25 yards, but opened up to a larger group than the American Eagle after that.
The Federal American Eagle is a really great ammunition, considering it sells for nearly the same price as the other two, but out performs them both at 75 yards for velocity & accuracy.
If anyone would like to recommend a better ammunition, in a JSP, 125 to 158 grain, I would probably test it. I’m not interested in any ammo heavier than 158 grain for reasons of trajectory.