Issues with my slug gun. Advice from an outside prospective?

[Pete112288]

I have a Mossberg 535 Slugger that I put a Bushnell 1.5-4 power scope on.
I have never had issues with bushnell scopes even on heavy kicking weapons.
I had sighted it in for 100 yards using Winchester Partition Gold 2 3/4 slugs.
I had a decent 3 inch group when I sighted it in. Then today I went out to try some Hornady SST slugs that I got to see if they were any better grouping with my gun than the partition golds. First group was 3 shots of Hornady. 6 inch group 6-8 inches high 4-6 inches right. Then I did three shots of Partition gold and got a clover leaf dead on the bullseye. After that it was all downhill. My groups spread out and went high right with both slugs. And it just kept going. The last group was over 8 inches across, cant even call it a group, and it wasnt even on the 12"X12" paper target, they all hit the board the target was stapled to.
I dont understand. Could it be the scope? But then why would my groups spread so much rather than just move. I know my sight picture was good. I am positive my trigger squeeze was good. I know I jerked once or twice but I went through almost 30 rounds and it was consistantly doing the same thing, spreading groups and going high right.
Any ideas?
Thank ya much

[winshooter88]

In my experience slug guns kick pretty hard so it could be a couple of things, either your scope mounts have loosened, your scope is damaged or the more you shot the more you flinched. If I shot thirty rounds out of a slug gun my shoulder would hurt for a week. Check all your scope mounts for tightness, if they are good and you have different scope try that. Beyond that what you described at first is just difference in ammo types, your shotgun likes the Partition Gold and not the Hornaday SST.

[ICEMAN]

Agree, I bet your mounts have loosened. Shotty's with scopes are hard to keep tightened down.

[Pete112288]

That might explain it. And would for sure explain why it kept getting gradually worse. If it kept loostening. I will have to check that. I thought I had checked it but I may have just thought it rather than done it.