03 December 2022
I look at the match results for 30BR shooters. When loading my 30x1.625 which is slightly more capacity I almost always find an accurate load at a much lower powder charge. ie; 30 br using 30 grains N135, me using 26 grains N135.
There are a lot of variables that can mess up a head to head comparison between two people shooting the same powder. Assuming both (or all) have match grade casting, reloading and bench equipment and match grade casting, reloading and bench practices it could be something as simple as the lot of N135 your burning. You probably know more than me on more than a number of things about cast and shooting cast accurately but if their velocity matches yours (and you're using 4 grains less N135) I'd guess that it would be a wild difference in the lot of powder you're using. If your velocity went in the same direction as the reduction in powder charge you're using maybe it might not be a problem. If it's shooting groups comparable to match results I'd work with it.
In my way of thinking finding the node/harmonics of the barrel are probably easier on a 66 grain jacketed bullet running at 3300fps than a 200 grain cast bullet running 1700 to 2000fps. For my lack of experience in shooting cast the biggest takeaway from match results is velocities. Within each class there are pockets of powders that are common but it sure ain't like jacketed BR where the only variation in powder is a few tenths of a grain of N133. With cast I get a feel for the velocities being run with the cartridge I'm shooting and give a few different powders a shot at accuracy. For the BR30 the most popular powders seem to be N135, Varget, 2495 followed by 3031 and 8208. And there are probably another handful of powders that would shoot just as well. That monkey see, monkey do things blinds us from seeing other options. But at least the results give you a starting point.
If I were starting out on unrestricted or Hvy I would go out to the range with N135 and Varget and work up the load to get in the vicinity of the 2000 fps most are shooting. If it took a grain or two more or less to get that 2000 fps I wouldn't sweat it. And 2000 fps is not magical either. It may be between 1900 and 2100. As Ken and others point out, there's only one rule to cast accuracy and that's bullet to throat fit. I'm new to cast accuracy but that one variable, bullet to throat fit, quickly rose to the top of the "got to get it right" list.
As a side note I am a little amazed that 4 out of 5 competitors in this years nationals loaded at home and not at the range. In jacketed BR that would be unheard of. (setting aside the breech loaders and a unique peculiar species) J BR shooters will load at the range and they will tune their load group by group taking out the vertical. But your talking about 4 grains of N135 not .4 of a grain. So that has me scratching my head.
For what its worth this is the 2021 and 2022 Nationals data. In total there were 19 competitors those two years. Of the 19, 16 shot three different powders; N135, Varget and 2495. Bullet weight ranged from a low of 170 to 220 grains with the average being around 200 grains.
Not sure if this will help but here is what the shooters put on the data sheets for 2021 and 2022 shooting those three powders chambered in BR30. Good luck and if you have time give the velocity, weight of bullet you're using. That and on what platform. Again good luck, Bill C.