I have a Smith & Wesson 642 I bought 7-22-2013, from a local gun shop. I shot 50 rounds of (Target factory ammo through it on 7-28-2013 no problem, cleaned gun). Went to the range 9-30-2013 and shot (50 rounds of Target factory ammo and had 4 misfires) also shot 5 rounds of Magtech Guardian...