Mini's are very good much better value than the price suggests, there are 4 top tips to get the best out of them, there are 3 small o rings on the power tube the bolt slides back & forward on, these need to be tight enough that the bolt doesn't drop off if you pick the assembly up by the bolt.
There are also 2 larger o rings on the outside of the bolt, these should be fairly tight in the body (put the bolt in backwards to check) & buy yourself some "über lube" spray grease from a Kee action sports retailer, it is the best lube for them no question.
The last is use Duracell 9v batteries, cheaper/rechargeable batteries make them break paint
I shot one for 2 years,