I use 3 pellets in my 24" barreled 50 and haven't seen any issue other than not acheiving a complete burn on the last pellet. Accuracy with this combo is still outstanding. I use 120gr of pellets in my 26" barreled 45 Remington ML. I couldn't find decent accuracy with a heavier charge.
When Remington first introduced this gun the 150gr craze hadn't started yet. Most guns were listed at 120gr quite simply because you couldn't burn much more with that length barrel. After 150gr became more of a standard, Remington started calling their guns "magnum charge" capable but mechanically there wasn't any difference other than 2" more barrel.
Do you have a 24" or 26" barrel?
hangunnr