I , too , use the 180 grain XTP for the .357 MAX in my Encore. Works real well. Your question refers to the mag though. Personally , I've never got a 180 grain to fly very well from any of my .357 mags. All of my testing has been with revolvers , and the max barrel....perhaps from a mag barreled T/C , it would be different. I use 158 gr XTPs or 125 gr cast (Oregon Trail ) over 2400 in my revolvers, get good results. As Mr.O'neill stated , consult the loading manuals , go from there. good luck
regards , irold
Oh ...and welcome to GBO. Vast amount of information here , lots of good folks.