My old load was 100 gr pyrodex pushing 240 xtp's. I was 1.5" high at 100 and the same low at 150 (3"). I would say your 300 grainers would be in the 2" low range at 150(4" range). So if you aim centre of lungs your ok. However as suggested the only way to know 100% is pop a few off at 150 to make sure!