Sounds like an ammo problem The primer is either damaged or missing from the round, and it tied up the gun, if there is no damage and the gun dryfires fine, I would get the rest of the box of ammo and call the ammo manufacturer. They will send you a label to send the ammo back for evaluation and they will replace it with ammo or money, and if there is any damage to the gun pay for that as well, if you insist.
What brand of ammo and type was it? Lot number on the box?
He mentioned that it got tied up on the MAGNUM ammo, and said that that was Blazer ( CCI) which was more than likely the Aluminum cased ammo, since he didn't say Blazer BRASS.
I never had a problem when I have used this ammo.
Anyone have thoughts on the aluminum case being a problem??