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I would suggest shooting it off a bench and find out if it is you or maybe it doesnt like the ammo. If it makes you flinch quit shooting it before it begins to effect you shooting other guns.
Have your buddy load your gun for you and don't watch him. Have him not load a round randomly. You'll know if you're flinching that way.
Start simple too...make sure your base and ring screws are tight, etc. What kind of rifle?
My advise is to sell it, give it, or leave it in the gun safe. Developing a flinch is a very bad habit to break, and you already have a 30-06 which is more than adequate for most everything you would use a 300 on. Once you start flinching with your 300, you'll most likely start flinching other firearms. Shooters that develop a bad flinch habit can even flinch with .22s.