In his Shotgunner's Notebook the late Gene Hill had this to say about the subject:
"When I talk to a shooter, after a consistent series of misses, about the possibility of his gun barrel being crooked, he usually smiles and marks me down as a crank.
"Well, let me tell you of just a few who didn't. One is an Olympic hopeful who had a lot of misses at trap on left-angling birds. I suggested he bench-rest and pattern his gun. Guess what? The upper-left quarter of the pattern was virtually blank, and the bulk of the load was in the right half. Fifteen minutes with a gunsmith and that problem was solved.
"Another friend was breaking every target--of those he broke at all-- on the left side. Both barrels of his over-under, when properly patterened, were a full two-thirds of a pattern width off to the left...
"A gun barrel is a lot more fragile than most people believe. A careless toss into a car trunk or someone knocking it out of a rack at a shoot can bend it or take the barrel out of round, creating eccentric pattern placement. I've asked several prominent choke experts how many barrels they got to work on that shot where they should. The answer? Very, very few.
"Any competent gunsmith can bring the barrel back to shooting where it should, either by straightening it out if it's bent or getting the muzzle back in round if that's what's wrong."
Please, take Gene's advice. Then, if necessary, have it checked out by a competent 'smith. What do you have to lose (besides those annoying misses)?
Good luck, and good shooting!