First, it is fairly simple to see if you have droop. Usually, you can just look along the side or top of the cylinder and if the barrel is not pretty close to parallel to the cylinder it should be apparent. You can also lay a straight edge along the top of the cylinder and measure the gap between the rear of the barrel and the straingt edge and measure again near the muzzle. If there is very much droop in the barrell, it will be apparent.
There doesn't have to be zero droop for the rifle to shoot, but if you are out of elevation adjustment, droop is the prime suspect.
I know that the thought of bending a barrel is scary, but that's how guns were sighted in during the civil war. So, I deal with droop by bending the barrell instead of shimming the scope. To me shimming a scope is way too complicated and even when it is done, and I look down the side of the rifle, I can see the droop. I can't stand barrel droop! To bend the barrel, I just lock the breech in a vice or wedge it somehow and bend it using a pipe slipped over the barrel for a lever. The barrels on chinese airguns are pretty soft and surprisingly easy to bend. I bend, look down the barrell to see if it looks right, then maybe bend again. Just a little push or pull will bend it a little. I have made various jigs, but to me its pretty easy just to do it by hand.
