This document is intended as an introduction to shooting roundballs in smoothbores. This stuff probably won't take you to the winners' circle at Friendship, but it may help you fill your freezer. You probably aren't going to get this without some work though. If you are serious about getting this kind of accuracy out of your gun, there are a bunch of factors you need to consider.
Most modern barrels are OK, and a surprising number of original guns are fine too.
One way to speed up the lock is to replace the cast mainspring found on most modern locks with a forged mainspring. A well made forged mainspring is usually faster than a cast spring of equal quality. But a good cast mainspring is better than a poorly made forged spring, so caveat emptor. Most people define a good trigger as one having a pull of 3.5 pounds to 5 pounds, with little creep or overtravel. It should be pointed out that the number one complaint of the smoothbore shooter I have talked to is a poor trigger, usually due to excess weight of pull.