Lots to say here. I have it on good knowledge that 5 years ago the load was 132 of the equivalent of Retumbo. I have not torn apart a load recently but you ought to be able to identify Retumbo by the bronze hue of the Extreme Hodgdon powders. There is no option other than retumbo in the right burn rate that would explain a bronze hue to the powder.
Since 5 years ago, DT has changed from Jamison and Bertram to Peterson Brass. Peterson has 5-8 grains H2O capacity less than the former. That probably required a load reduction, I'm guessing.
The only real suitable powders for the 375 are Retumbo, RL33, N570 and US869, maybe RL50. n570 adn US869 will be easily identifiable.
I'm assuming you have shot the factory stuff and not had pressure, but you're getting it in your reloads? If that is the case, I would carefully measure the H2O capacity of the cases to make sure they're the same; I would FULL LENGTH resize and bump that shoulder back to new specs to make sure I'm not getting false pressure signs because the case has no room to expand. I'd carefully measure velocities with Labradar and compare them. I'd measure distance to lands and make sure they're not being Jammed causing a pressure spike etc.
Also important to realize that a lot of guys have been running the Cheytacs hot. Almost every post of anybody running a 350 whatever over about 3070 fps complains about brass sucking and pockets blowing. No, you're running too hot.
I used to go with a load that was accurate and fast as long as the pockets lasted for 10 or so loads. Lately I've been trying to avoid all pressure signs altogether. In my latest barrel (I've worked up loads for 5 barrels to date), I'm finding Retumbo in the 125-130 range is about max, n570 in the 130-134 range is max, RL33 in the range of 136-140 is max and US869 in the range of 138-142 is max. Quickload can provide some guidance too.
It also depends on the chamber and the bore. With factory DT barrels, I was able to load a bit hotter than with custom barrels.
