I would bet that the 350 primers are why you are getting a good burn. The cci magnum primers do have more brisance (real easy to test by loading one of each type primer in a case with no bullet or powder and popping them off side by side).
I use the Starline 460Rowland brass and cci300 primers for all of my hot loadings. Generally I don’t aim for the very top of the published 460 data. If I stay in the middle range or lower of 460 data I feel comfortable using that brass over and over. The few times I ran the loads up towards the top I kept that brass separated and after a couple of those loadings set that brass aside to be trimmed to fit my regular 45acp chambers. Once trimmed for those chambers it gets used right along with all my other Starline 45acp brass.
I tried one time (and only that one time) to shoot regular 45acp loads (Starline 45acp brass) in my Rowland chambered pistol and yeah it worked ok but even at regular hardball load levels (800 to 850 FPS / 230gr projectile) the cci 300 primers looked bad. Noticeable flattening like things were way too hot when I knew darn well things were not at all anywhere near hot... The primers flattened because the 45acp brass was short for the 460Rowland chamber giving false pressure signs.
The other member that mentioned True Blue powder
@SeabeeKen - That is a good powder for hotter loads and there is published data for it. I have at least 5 or 6 lbs of that and will buy more if I get down to less than a couple lbs left. If I am loading warm to hot 45acp stuff for a barrel that does not have a compensator - True Blue is the powder I am grabbing. I bet if you tried a lb you would like it too!
Anything with a comp and I am sticking with the AA#7 every time! Not at all economical but it makes stuff go really fast without insane pressure and because it is so bulky it will provide all the gasses you need to properly ‘work’ a compensator.
I’ve got bullseye, zip, true blue and aa#7 all on hand for 45 type stuff and honestly never even bother with the faster powders (bullseye or zip) any more. Yeah the faster powders are more ‘economical’ if only making plinking fodder but you are limited in what you can do with them velocity wise and they sure as heck ain’t going to make enough gasses to work any sort of comp.
Appreciate you sharing your data and experience sir. Wishing you good shooting!