Hi:
You may have a non-scope related problem.
I can't tell without knowing a bit more about your setup. Firstly what rifle, how old, what ammo, new or 'very' used?
The 22.250 is a hot round and can burn out a barrel in 2,000 rounds, and a typical 'shot out' example may well group up
nicely with 2>4 shots, then go right off the paper when it gets hot, does this sound familiar?
Anyway as a general comment, Nikko Stirling scopes are only OK, not rubbish but not that good for a centrefire rifle either.
Sorry to run this past you but you need to spend at least £500 on a centrefire scope, a