A while back I asked if anyone had tried Alliant's new 300MP powder (Alliant 300MP Powder for 357 magnum). About four weeks ago one of the local suppliers had some in stock so I bought a pound to try it out. Alliant lists 18.6 gr as the maximum charge for a 158 gr GDHP which I reduced by 10% to 16.7 gr for a starting load and 17.7 as an intermediate load if the lower load did not indicate any excessive pressure.

I finally got the range to test them today.

Gun = Marlin 1894C 18.5" barrel
Bullet = Magtech 158 gr JSP
Case = Federal 357 magnum
Primer = Winchester Small Pistol
Powder = Alliant 300 MP
Ambient temperature = 90F (prox)

Results from five measurements (feet/second):

16.7 gr: average = 1675, SD = 23, Max = 1684, Min = 1635
17.7 gr: average = 1765, SD = 24, Max = 1792, Min = 1736

None of the primers or cases showed any more indication of pressure than normal for a 357 magnum. In fact the Amercan Eagle 158 gr JSP (which clocked at almost exactly 1800 fps) had much flatter primers than the 300MP reloads.

All in all I am pretty impressed with the potential of this powder in a carbine.

No accuracy reports yet. About the time I was getting ready to try some guy started banging away with a civilianized version of an M1919 and kicked up so much dust I just packed up and went home.

One of the bad things about Alliant powders is they don't list pressures with their load data online, and they are generally very conservative in their max charges. The latest issue of Handloader tests 300-MP, but again no pressure data. Hard to say if that max charge really is "max".

Do you have any H110/W296? I would like to see a side-by-side comparison of a max load of this powder to a max 300-MP load, with the 158gr bullet. 300-MP is suppose to be slower, but I have yet to see any test where it beat H110/W296 handily.

I shot some 158 gr XTP a couple of years ago and clocked an average of 1609 fps out of a 22" Handi rifle. I used 12.5 gr of Alliant 2400 in .38 Special brass with a CCI 400.

im loading mp 300 at 18.5 grains under xtp fp 158, brand new 627 snubby loves it, like heat seeking missles in my 686 no dash, 200 rounds of this leaves about the same residue in gun as twenty of same loads with 296, accuracy very good with both. if you get a chance go out shoot em in the darek, try this load versus your 296 158 xtp max load, hardly no muzzle flash less noise and way cleaner than 296 impressive product. nothing wrong with 296 but this stuff is just simply better

Tests from two years ago show that with Lil' Gun (A Brian Pearce load from Rifle Magazine), 158gr XTP out of our Marlin 1894 CS (20" Octagon barrel, similar to the OP's) we were getting right at 2100fps with an SD in the teens.

I do not have any data from H110/W296 from the same rifle though. Posted this for comparison. Hope it helps.