My grandad has a Remington 700 12 ga. He said it's only firing high brass shells, low brass will either put a small dent in the primer or not one at all. He said he used to could shoot high or low brass.
Thoughts?

Some primer brands are harder than others. If a firing pin spring needs to be replaced, or the bolt needs a good cleaning, some brands of ammo will just get a light strike mark while others will shoot. That has been my experience.

Thanks Trapper, will look into that

Remington 700 is a rifle model. Sure it’s not an 870, 1100, or 1187? If semi, the gas ring orientation is probably wrong which won’t allow the bolt to seat all the way forward.

Gas ring wouldn't prevent the high brass from going into batter with the low brass loads. Probably an ammo issue with a particular rim dimension.

Not sure of the model he said it was a 700 i know for sure it's not a semi auto.

I assumed it was an 870. Ask him if it was Fiocci low brass that was getting primer strikes with no ignition. European primers are sometimes an issue with that. If not, try a good cleaning and maybe a different brand of ammo. I got some Winchester 10 gauge stuff several years ago that wouldn't get ignition in two different shotguns. I switched to Federal and the problem was solved.

Me and my brother went and bought him a box of some Federal 7 1\2 high, fired off 3 in a row.
He took it apart and cleaned and oiled it. I'll ask him what brand the ones were that were getting primer strikes.

[quote][/quote]Only shells he's got are Federal and Remington, and the ones that struck and didn't fire are remingtons...

Went and cleaned it, and oiled it good, reassembled it, shoots everything now