https://pinside.com/pinball/forum/topic/terminator-2-slow-cannon-problem
https://pinside.com/pinball/forum/topic/t2-gun-cannon-speed
I have not dealt with this issue, but you might check the posts linked above. Also, while it doesn't provide instructions, the diagram of the cannon in the manual may provide a little help. If you don't have it, you can download it from ipdb.com.
Good luck!