There has been some discussion here regarding the use of this
propellant. Oftentimes the comments remind us to use magnum primers with this powder.

Is that a “must do” perspective or is the use of standard primers something that has been proven acceptable? 

Thanks for your help!

Tom

The late John Louis filled my bucket with his experiments and experience with this powder. I've accumulated lots of cans of powder over the past fifty years but this is one I've never burned or been able to find. My stash has got this powder covered on all sides of the charts but John took its characteristics and performance a step further and found the secret hand shake. 

Like Olin/WW ball powders MP 300 has a burn retardant coating which generally needs hotter magnum primers to achieve full burn. John spent a lot of time testing primers with this powder. Other true magnum pistol powders like 2400, Blue Dot and AA-#9 burn okay without magnum primers but MP-300 needs hotter primers. John had it figured out and I'm sure those who shot with him in Central CA have the recipes 

Just Spit Balling
Rick

For a bit of reference on what john did, here's one of his targets he shot with the rifle i now have, downloaded it of here a while ago

That target photo was sent to me by Joe Louis a few years back. That is what got me running down the 300MP rabbit hole! I will continue running to see where it takes me. 

Joe was a very experienced and good shooter. I’m guessing he uses a barrel clamp, which I don’t….yet.

Hard to read but I think his load data is:
0.376” dia. 310-grain bullet (Hoch)
CPA .38-55
17.1-grains 300MP

My bullet weighs 345-grains in a 12 twist barrel. So my powder charge weight needs to be less than Joe’s 17.1-grains. I’ve tried 15, 16, and 17 grains but maybe need to be even less than 15 grains ?

Tom

JHand,

It will be interesting to learn of your impressions using
that load combination.

Tom

JHand wrote on Jul 2^nd, 2026 at 4:45pm:


That target creates a bit of a dilemma. 

The primer used was a Large Pistol primer based on what he wrote on the target. John was adamantly against using large pistol primers in place of large rifle, he was convinced using the shorter primer would allow it to accelerate to a speed that would damage the breech face. To compensate for this, he talked about turning the case heads down so the seated depth of the primer below the surface of the case head was not so deep.

This is important for a few reasons and he should have told you if he did this for this rifle. The first is safety. Using a large rifle primer in a large pistol primer pocket, will leave the primer cup proud of the case head. Headspace for the rifle would likely have been cut to compensate for the thinner rim. It’s also possible the chamber was cut to headspace off the shoulder, allowing the use of full thickness rims. If you got any brass with the rifle, this will be easy to check.

The next issue is that a large pistol primer has a much thinner cup. The metal is also softer in most brands. It’s much easier to set off. In a well tuned rifle and load, the excessive force needed to set off the primer, may be the reason for your vertical. The issue is that it’s easier for a large pistol primer to fail with a hard firing pin strike and can’t hold up to the higher pressure of a rifle load. Not likely a problem in the loads you’re shooting.

Last thing to consider is the large pistol primer does not have near the ignition heat of a large rifle. Even a magnum large pistol vs a standard large rifle. This also will change your groups.

Just some things worth considering

Sorry, I got side-tracked into primers above.

Do any of you have a 300MP load with a 320-350 grain bullet in a 12” twist?

Thanks!

Tom