MP 580 Smokeless Rifle Powder

A little while back I saw some MP 580 smokeless powder on American Reloading’s site and it was labeled as “Similar to IMR 4451”. The reference data for it was for a 243 Winchester with a 100gr SP. I figured I would give it a try since the IMR Enduron powders are well regarded. At ~$200/8lbs, why not?

I looked up a starting load of 4451 with a 80gr Hornady CX. It was listed in one of the Hodgdon manuals in the app. It’s probably obvious that I like monolithic bullets. I haven’t really tried the CX line yet, but I like Barnes bullets. I decided to try the CX since they had a box at the store. I settled on a starting load of 38.7gr and loaded up five of them to see where they end up with the velocity. I also ran the load through QuickLOAD to get an idea of what I could expect if the MP 580 was similar IMR 4451. Looked like 38.7 should get me 2,761 fps from the 18″ barrel of a Ruger American All Weather Compact, same rifle as the 308 in this post, but in 243.

Here is what I got with the first five. It was much lower than expected.

I ran the propellant table with some tweaks to set the grains and velocity to see what looked close to these results. H4350 looked pretty close to the average, so I loaded up a pressure test series at 0.5gr increments starting with 38.2 and up to 43.7. The first shot was close to 0″ horizontally and about 2″ left. The next nine were all in about a 2.5″ group about 3.5″ above 0″ and the group was centered pretty well on the 0″ vertical. The 10th was high and about 3″ to the left. I didn’t get heavy bolt lift, ejector swipes, or flattened primers. I measured the case webs before and after and they were all pretty close to the same starting size and they were all about 0.002 larger after firing. The last few were about 0.0025 larger, so I think I was getting close. Here are the results compared to QL estimates for H4350.