I started at the mininmum 72gr. for the 300 gr bullet.Worked up slowly chronographing as I went up ..Broke 2500 fps at 76.5 gr.in wuarter grain increments the velocity did not increase and in some cases decreased up to 78 grains ..Imr 4350 powder. My speer manual shows 78 gr at max charge as does the imr website..My nosler book shows 75 grains..No pressure signs were evident. No hard extraction,primers looked fine,new ones seated nice,not loose at all from what I could see.no marks on the case from ejector slot..I havent seen this before..Hiwall stated on the big bore forum that it sounds like high pressure..It may be but no signs of it . Rifle is a cz 550.. Thanks for any help..

I am puzzled by the leveling off of the velocity. How many rounds did you load and fire for each weight? You should load at least 3 and some say more at each loading weight unless using extremely expensive components. Sometimes you just get bad readings from the chrony. Especially if you got one of the less expensive chrony's. The more rounds you fire at a given charge the better statistical average you will have. There are many things that can cause one particular cartidge to behave in a way you would not predict. You might have a bullet that was slightly undersize that slipped by quality control at the factory. You might have a primer with a little less product in it and fired with less intensity such that the powder only burned partially. When you reach very high pressures the powder sometimes become unstable and acts erratically. The pressure from round to round jumps all over the place. Others on this forum, with more experience than I, may have other ideas. Good hunting. "D"

The Swift manual says 76.5 and 2600fps is max. If you have a 24 inch barrel you should be able to reach 2550 and anything past that is cream in the 375 H&H case. Good hunting. "D"

IMR-4350 From 63.7 grains to 81.0 grains

Remington 9-1/2M Primer



I found:

H-4350 From 72.0 grains to 81.5 grains

Remington 9-1/2M Primer

a little better.

machinistbutler
I have saw this several times with different calibers. The velocity and pressure(I have equipment to measure pressure) will level off and more powder does nothing. The last time I saw this was in a 340 Wby with 225 grain bullets with H4350.
Using GM210M primers, starting at 75 grains of H4350, from 75 to 78 grains increased pressure and velocity. From 78 to 85, nothing happened.
Switched to GM215M primers, ran the same test, pressure and velocity increased but I had to stop at 83 grains.
I wish I had the exact answer, sorry but I dont.
I post a lot of these test on the other forum if you're interested.
Charlie