Don't be surprised to find your 100 yard shots "all over the place" with BM3 using those heavy bullets. The one thing in your favor thouis the loose fit of the Powerbelts. So perhaps, you may be alright with your 100 yard groups.... I dunno!

If those were normal 400 gr. sabot/bullets with a moderately-tight fit, I could almost assure you that your target holes would be alot more widespread.

We recently (yesterday?) had a APP thread where the shooter questioned why so much soot existed in the breechplug & around it when he took it out. Well... that's a result of a weak take-off powder like APP that exhibits more back-pressure when exertingenergy tothrust the bullet. Some powders take off like lightning -- some take-off the few few inches of the bore like a turtle.... well maybe a turtle is a bad example...lol [:@]

BM3 is another slow-starting powder that usually works best with light to moderate bullets. But with Powerbelts, your back-pressure should be somewhat less -- due to the slip-fit of the conical.

I'll bet if there was snow on the ground & you shot downward, you would see an abnormal amount of powder in the snow using the combination of powder & bullet that you are using.

Just my opinion... my findings.
Perhaps your findings will be different.