I disagree about not getting the gun fitted. But I don't think that you should spend a lot of money getting your first gun fitted.
If the shorter stocked BT-99 felt better by all means shorten the stock. Double check your gun by removing the recoil pad and mounting the gun. If the shorter length feels better then shorten the stock. It is a wood stock and not plastic right? I am not sure if you can shorten a plastic stock. You could put a thinner recoil pad.
Once you get the length of pull to your liking or at least acceptable. Fire couple of shots at 13 yards free hand with your Full choke. Focus on the aim point and slap/pull the trigger like you are shooting clays. Hopefully both shots impact at about the same place vertically aligned with the aim point. Rough estimate is if you are 1" high equates to about 60/40, 2" = 70/30 etc. If your gun does not have an adjustable comb use the shims to bring it as close to 1 to 2" high.
Next step get a trap to yourself and have it locked to throw straight away from post 3. With the Full choke find tune the comb to smoke the targets. If you do not have an adjustable comb, learn the sight picture that gives you smoke balls. Hopefully you can keep your focus on the target, yet be aware of the bead/barrel coming up to it.
Once you get the gun and/or you tuned in; move to post 2 then 4, 1 then 5. We are now getting into serious practice. Have the trap set to throw hard lefts. Start on post 5 and work your way to post 1. No cheating with your hold points because you know were the target is going. Then lock the trap to throw hard rights. Start at post 1 and work your way to post 5.
Finally, get back to the patterning board or a card board box. Use a rest and actually aim and squeeze the trigger. Fire couple of shots. The shots should impact at the same place. This documents where your gun shoots. If you were to put a mid bead on, now is the time to find the place to get a fig 8.
When you get your new gun you should make it shoot to the same POI as a starting point.
Jason
