I have reloaded the 350 remington magnum for my 15" encore barrel, which is probably a rare gun for this caliber, but I have the following advice:
AA2230 was simply too fast a powder, even in my 15 inch barrel. When I approached the limit, according to Lee's data, I would get brass separation at the base and difficult extraction. I found that AA2460 was just the right speed. In a regular rifle barrel, I suppose you could use slower powders, but you'll need to watch for pressure problems. AA2520 was too slow.
More than any other rifle round, the pressure in my 350 remington mag was highly dependent on seating depth. I found that when I seated close to the lands, I would get either high or low pressure with velocities varying by 300 fps. I now seat the bullet so that there is plenty of freebore and get good standard deviations on spread. Maybe a product of my encore chambering--I don't know.
I have both full length and neck sizing dies and found that I have to full length size after every two shots or the brass will be too big for insertion and extraction. I don't like the Remington brass and have thought about making brass from Lapua 300 win mag, but have not done that. The remington brass was inconsistent in weight, seemed relatively soft and only lasted 4-5 shots with moderate loads. I have not figured the brass problem.
My 15" encore likes the 225 grain sierra gamekings best. I could not get consistent performance from Hornady's 200 grain spire point.
All in all, I really like this load, especially in the 15 inch encore. I can get fairly close to rifle ballistics and very good accuracy.
hope this helps.
Barry
