The fact it was used on someone elses gun isn't an issue and would have no affect on using it on your gun.
You will need to install a tapered scope base in order to have more adjustment on the up side after zeroing the gun.
The only way to get it is with the scope base. If you have 12 minits left at this point, installing a 20 minit base should give you 32 minits total.
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