For an air gun you are going to want an A/O scope, that is airgun rated. Best "budget" scope I have found is a Hammers.
I Love their 4-12X40 AO. Have two and they are "magnum springer rated" great for the money IMO.
Hammers 4-12 X40 AO on AmazonHawke is upper crust, but has a lifetime warranty. I have had to use it once and it was painless... A+ company in my opinion.