If my rifle is shooting big groups, how can I determine for sure whether it is the rifle's fault or the scope's fault?

H. C.

You'll have to put a second scope on, and see if the groups improve. You should also check the scope bases, and rings to see if they are loose at all.

Henry,

I'd switch scopes, using a scope of KNOWN reliability, i.e., one that works well on another rifle.

I'd take that opportunity to check the integrity of the rings and bases, too.

George

It could be the rifle, ammo, scope, conditions or even you!

The most common reason was given in that something is usually loose. Rifles will even group, believe it or not, with iron sights so if it's easy to put the existing scope on and off at the range and it has irons take your tools with you and give it a try. Check the scope at the distance you are shooting for parallax also.

I have seen shooters rest forends on hard wood surfaces, flinch or shoot in high winds and get big groups.

I hope this helps.