You may have heard of the "tape detent" or "nail polish detent" people have been using in lieu of small bore barrels. Here is a short tutorial on how to do both.
Shake your nail polish (or your mom's) up very well to get it mixed. This is why it's a good idea to simply man up and buy a fresh bottle.
Use the brush to apply three dots of polish evenly spaced around the bore of the barrel. Paint them no deeper than 1/4 inch for Snipers and Cockers, as seen here.
Be careful not to let it run!
Each dot should look like this:
Start by cutting a piece of tape (electrical tape works very well) roughly this size:
Slip it into the barrel as shown. Notice it's only about 1/4 inch deep.
Fold it over like so, and make sure it is smoothly adhered to the inner and outer walls.
Last, create an identical piece on the opposite side. Failing to do so can cause paintballs to spin erratically.