I bought a stoeger double defense, coach gun in 12 gauge and a 20 for a girl I shoot with. The 20 is great, but the 12 misfires about 20% of the time(I have fired at least 200 rounds from it and it fails about 1 in 5 times) I never use remmington ammo, so it ain't the ammo. Any suggestions as to the cause? It seems well finished; opens with gravity alone and has a very smooth trigger pull; it just fails alot