I just bought a cheapo sniper rifle too, just to dip a toe in the water...
I bought some Geoff's .40s and the gun shoots quite nicely. But it's inconsistent. I'll get 3 shots looking bang-on, but then I'll get one or two off to the right - either going straight right or curving off a bit. At 15m I think might be missing by 4 or 5 inches - I'll stick a paper target up and measure it properly.
The barrel looks clean all the way through.
I'm guessing that sort of behaviour warrants a new hop rubber and nub as a starting point... but I'm also aware it might be the start of a long and expensive journey, so I've resolved to leave it alone until I can test it at greater range and put a decent number of shots through it.
Edit: a very quick string of 6 shots.. 1 flier, low-left, the rest in a 1.75" group - which goes to show that shots may look worse through the scope than they actually are...! I did dial in some more hop after a few warm-up shots away from the target, so perhaps my previous shots weren't back-spinning enough to stabilise them properly?