I load for two Browning BARs. As you say it may be the powder, but it also could be the rifle.

I worked up a very accurate load in my 300WSM Browning BAR. One day I decided to give it a good cleaning and took it all apart. When I took it to the range my accurate load was now, not so accurate. With some online help, I found the problem. The screw that holds the forearm to the barrel was to tight.I adjusted the screw so it would hold the forearm securely, but not to tight. That's all it took to bring back my accuracy.

Check out the forearm screw, it could be to tight or to loose.