Mossy 500 POI way high

Iv'e had my Big 5 special for a couple of yrs now. It has around 2,000 rds through it mostly bird shot. I have three barrels the 28",and 18" fixed chokes. I also bought a 28"ported with adjustable choke barrel from Mossberg. I've shot almost exclusivly the 28" fixed mostly trap. Have used the ported barrel some. Both work well. I hit 65-75% of the trap birds not bad for a relative newcommer. I took the gun to a 4 day tactical shotgun course at Frontsight. It was working fine towards the end of thefirst day the Barrel bolt bracket solderd joint broke off the 18" barrel. I thought how strange. I switched to the 28" ported barrel and promptly it broke as well. Lucky for me the staff at Frontsight loaned me a 590A1 that's a nice pump. Those ghost ring sights make 4" groups with slugs at 100" yds laying prone with no time pressure no big deal. Anyway back to my issue, Mossberg was nice enough to send me two new barrels gratis, problem is at 25yds. with slugs I'm not hitting paper on a two ft. target at 7yds I'm over 12" high. The gun looks fine I field stripped it it looks fine something must really be amiss. Anybody have any Ideas before I send it away in the brown truck?

Jack