My T91 shoots very reliably, however, even unsuppressed, accuracy is all over.
I too was shooting PMC 55 grain 5.56, and was experiencing 5-6" groups at 50 yards. I confirmed my optic was tight (PA GLX 2x prism).
I then confirmed it wasn't me by switching over to my BCM 14.5" and shot a solid 2" group using the same ammo with a red dot.
What kind of accuracy are you getting suppressed and unsuppressed?