Adjust the nut behind the barrel.
Seriously, how about a more verbose description. I 'assume' that you're saying it groups
inconsistently. If thats the case, my advice is to hand it to a seriously experienced Glock shooter and see how they do with it.
Most of the problems I see with Glocks are either A) aftermarket parts or B) poor fundamentals (sight alignment, sight picture, trigger control, grip, etc).