What would cause leading in only 1 groove?

[crgaston]

Strangely, I just observed my 625-7 Mountain Gun .45LC leading in one groove the full length, the one next to it just near the forcing cone, and none in the rest of the grooves.

I was shooting a new load today, finally making it up to 18 grains of 2400 pushing an RCBS 270 SAA cast from wheelweights, unsized (they dropped at .453x.454), and lubed with LLA cut with mineral spirits. I started with a clean barrel (I thought, anyway) and only fired a dozen rounds.

They shot into about 1.5" at 25 yards backrested with wrists on my knees, first and last shots SA, the rest DA. That's about as good as I can shoot, period, so I'm not complaining, just curious. The lead brushed out with a few strokes of a brush wrapped in a wet patch.

I am super excited about this combo. This is the best this gun has ever shot for me (no factory ammo or J-words yet) I've been trying different loads and working my way up the power scale with this MG and a 625-6 MG off and on for a couple of years now. Finally, no sooty cases! The fired cases just fell out of the cylinder, too. Not a hint of stickiness, whereas with Titegroup loads were sticky well below maximum.

I just got the custom bullet seater stem fot this bullet from Lee this week so I'll get to use it on my next batch. Maybe I'll try sizing them, too. [smilie=1: My logic is, soft as they are, they'll either be round or not by the time they leave the gun no matter what I do before they're loaded.

Anybody have any observations or ideas about what might cause the strange leading pattern?

[2ndAmendmentNut]

First off I would double check your load data. 18gr of 2400 under a 270gr boolit seems a bit stout, a little to stout for a Smith IMHO. The Alliant Powder website lists 15gr of 2400 as maximum for 250gr bullet.

As for your leading problem. Does the gun do this with other loads? You might want to try lubing your boolits with undiluted LLA. I am no expert but if I had to guess it is something up with the way the rifling got cut in your particular barrel. Perhaps a better lube would help, or better yet fire lap your barrel.

[Bullshop Junior]

might need a gas check, and some Bullshop speedgreen.

[wallenba]

Chamber to barrel misalignment? Just tossed that out there because all the other bases seem covered. You would be shaving lead at the forcing cone, so probably not the problem.