An IMR Trail Boss load for my .357 Blackhawk

I like my single action Ruger Blackhawk, but I don't particularly enjoy shooting full-bore .357 loads. They're loud, and for paper punching they're excessive. I could use the the mild .38 Special target loads I make up for my S&W 14-3 in the Blackhawk, but if you shoot .38 Special loads in a .357, you'll get a buildup of carbon in the front of the cylinder, as the .38 Special is 0.135" shorter than the .357 magnum  chamber and leaves a gap. The answer is to use .357 brass, of course.

I could have just made up .357 loads that were similar, or identical to my .38 target loads, but I wanted to try something different. My friend Ric had given me a half-full container of Trail Boss, a relatively new powder from IMR designed for low velocity loads and lead bullets. It has a very low density- around half that of Bullseye, for a given charge by weight- and measures and flows well. (It's often described as resembling "tiny donuts"- note the photo below.) That low density also makes it much harder to double-charge a case. In fact, it's just about  impossible to blow up a modern gun with a load of Trail Boss. It's also a very fast burning powder, like Bullseye, which makes it suitable for low-power loads. All these characteristics make it a favorite of Cowboy Action Shooters, who like low-powered loads that are accurate and a bit smokey.

After consulting several loading data books and the IMR web site I decided on a starting load of 3.8gr of Trail Boss under a 125gr round nose lead bullet. IMR gives a starting load of 3.5 grains and an absolute max of 5.3 grains. Normally I'd start with the recommended starting load, but even the "Do not exceed" load is only 14,900 psi, and the .357 maximum pressure is 35,000 ps- and the Blackhawk is probably even stronger than that. I use a Lee Auto-Disc loader, and the .88cc disc gave exactly 3.8gr as verified by my electronic scale. I had 100 new cases, so I loaded 100 rounds.

I'd recently replaced the 2x scope on my Blackhawk with the original open sights, and thought I'd sight it in with these new loads. I took the gun to my club, and set up at the 25 yard line. First shot was almost dead center in the bullseye- about a half-inch to the left. Pure dumb luck.  I fired the  rest of the six-shot group two-handed off the bench and got a pretty tight cluster. I did a few more groups offhand, got similar results,  and decided not to change the sights. Gee, maybe I should buy a Stetson, and a holster, and a kerchief, and check out this CAS stuff after all.