I have been using 300MP for breech seating for quite a while now with excellent results. I use it in calibers from 25-20 Win to 40-65. I have noticed that it seems to reach a point when increasing the charge results in a failure to burn all the powder. I am working up some loads for a new 38-55 build and plan on mostly using BP but I decided to revisit using smokeless since I acquired some cast bullets from Bob Zimmerman’s estate. I loaded some 248gr bullets with 17 gr of 300MP to start out low. Velocities were around 1000 fps and not consistent. Yesterday I tried using 22gr which is what I use in 38-55 breech seating. BIG failure!  The first shot was around 1150 fps but the second was close to 950 and the third in the 600s. Looking down the barrel revealed lots of unburnt powder. It even reached the point where rounds were hard to insert. I finished off the last few shots by running a patch through the barrel between shots.  I think I will stick with my decent supply of 4759 in that caliber. 

Jack

bpjack wrote yesterday at 2:08pm:


I found with 300mp, it needs mag primers for full, consistent ignition.  Then again,  I have only done breach seating with it