ORIGINAL: Omega50 cal
None of my Omegas shoot 300 grain bullets as well as the 250 grain. Not a big difference at 100 yards the 300 grain groups would be 2 to 21/4" as compared to 1" or less for the 250s.
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Some of it depends on powder charge & what powder you're using. Bullet speed at ignition can determine which bullet is right for you.
You really need to try them both at both the shortest & longest yardages you shoot. Every rifle is different -- even the powder/bullet weight & primer selection plays a part in this.
Poster Omega50cal likes 150 grains (3 pellets). If you shoot two pellets, with a slower bullet takeoff compounded by less powder energy & heavier bullet weight, the 300 may work better for you than he.
The bullets are relatively inexpensive & the ones that don't perform as well won;'t go to waste. They will still either be acceptable for hunting or easily sold to someone else because of their popularity.