benelli m2 tactical low with slugs

[cousins55]

I have a new to me M2 Tactical that puts slugs about 7" low at 25 yards, it has rifle sight so I have no elevation adjustment that im aware of. Anyone have any thoughts? Im definetly no ballistics wiz and ive only made one trip to the range with the shotgun so I have no idea where it shoots at 50 or 100 yards but 7" low seems a little wacky to me. I tried walmart bulk 2 3/4 slugs and brennekke reduced recoil 2 3/4, they seemed to impact about the same spot at that distance. I guess its worth mentioning im running the IC choke that came with the shotgun. For some reason i was thinking ide be a little high at 25 to be close to on at 100yds. Educate me guys! Is this abnormal? Do I just need to compensate and go with it? Im also a lefty shooting a right handed shotgun, and ive read briefly about shimming benelli stocks but im not sure if that applies to me with a pistol grip stock or if thats just on the comfortecs. I dont meen to ramble i just wanted to give you as much info as I could. Any thoughts? Thanks!