I was shooting my 500 today blastin rounds and it goes threw birdshot no problem real quick shots then when I used the federal classic 000 buck the gun would lock up for a split second but would function. any ideas of why the higer power loads are causing the action harder to work?

My guess is the birdshot you were using was low brass. The 000 buck has high brass shells. When you switched over to the 000 buck the higher brass is thicker then the plastic and is tighter against the chamber walls. The blow by of expanding gas leaves carbon deposits in the chamber down to the top of the low brass shell. When you went to the 000 buck you were forcing the taller brass part of the shell into the chamber further, where it has the carbon build up.
Kind of like the .38 switch to .357 scenario
When I would shoot about 100 rounds of .38 thru my .357 I couldn't get a .38 in the cylinder until I cleaned it.


Clean the chamber and you should have no problem.

I would strongly suggest that you check the headspace on the Mossberg. I suspect that it may be excessive. Good luck. Charles.