Some fantastic bullet velocities caught my eye while recently browsing through the Reloader’s Guide on Alliant Powder’s website, www.alliantpowder.com. For instance, it listed 300-MP velocities of 1,686 fps with 158-grain bullets in the .357 Magnum and 1,394 fps with 270-grain bullets in the .44 Magnum.

In addition to those two cartridges, Alliant lists 300-MP loads for the .22 Hornet, .454 Casull, .460 S&W, .480 Ruger, .500 S&W and 2½-inch .410-bore shotshells. So naturally I had to try some 300-MP.

The Reloader’s Guide gives one weight of 300-MP for each bullet listed. That must be a maximum load, because the guide states “REDUCE RIFLE AND HANDGUN CHARGE WEIGHTS BY 10% TO ESTABLISH A STARTING LOAD.” The guide does not state cartridge case brands used. To compensate, my top loads for the .357 and .44 Magnums were .3 to .5 grains below Alliant’s.

With that slight reduction in propellant weight and the fact Alliant used a 10-inch barrel to shoot the .357 Magnum loads (most likely on a closed-breech gun like a Thompson/ Center Encore), I didn’t figure velocities from the 4-inch barrel of a Smith & Wesson Model 19 .357 Magnum would be as high as Alliant’s. No barrel length was given for the .44 Magnum loads.

Alliant’s 300-MP is made in America and is slightly slower burning than Hodgdon’s H-110. It is a double-based spherical powder with 10 percent nitroglycerin content. The kernels are flattened and vary quite a bit in size as studied under a magnifying glass. After dispensing five charges or so from a powder measure, the fine-grained propellant metered within 0.1 grain and often less from the set weight.

Spherical propellants in magnum handgun cartridges usually require a relatively sustained flame provided by magnum-type primers to reliably ignite. So it was somewhat surprising Alliant’s data lists standard velocity Federal 100 Small Pistol primers used for .357 Magnum and Federal 150 Large Pistol primers for .44 Magnum. I didn’t have those exact primers, so I substituted standard Winchester Small Pistol primers for the .357 and Federal GM150M Large Pistol Match primers for the .44 Magnum loads.

The three .357 loads produced quite a bit of blow-back of unburned powder. Velocities from the Smith & Wesson Model 19’s 4-inch barrel were quite a bit slower than from the 10-inch barrel Alliant used to shoot its .357 loads, but the velocities should not have been that slow. Perhaps the reason was the standard primers I used.

R.H. VanDenburg, Jr., in Handloader No. 264, used small pistol magnum primers and got 1,308 fps with 158-grain jacketed bullets and 17.5 grains of 300-MP from a 45/8-inch barreled .357 Magnum. My load with the 158-grain Sierra Jacketed Soft Points (JSPs) came up about 200 fps short of VanDenburg’s velocity and nearly 500 fps slower than Alliant’s. The 125-grain bullets were equally short of Alliant’s velocities.

Most troubling was the .357’s quite high extreme velocity spreads over 10 shots with the standard Winchester primers: 170 fps with the 125-grain Sierra Jacketed Hollow Cavity (JHC), 175 fps with 158-grain Sierra JSPs and 145 fps with 157-grain bullets cast from an RCBS 38-158-CM mould.

I went back to the bench and loaded the three different .357 loads again using CCI 550 Small Pistol Magnum primers to determine if magnum primers would help reduce unburned powder blow-back, narrow velocity variation and increase velocity.

The first time each powder charge was weighed. The fine granules of Alliant’s magnum pistol powder dispensed consistent weights through a powder measure. So the second time loading the .357, I set the powder measure to dispense the correct weight amount of 300-MP then dumped the powder charges directly into the cases from the measure.