A quick look at the SAAMI drawing for the 12 gauge shell indicates the maximum rim diameter to be .886", with a remarkable amount of tolerance on the low side (.021").
This is of course the dimension one would expect AA shells to be at or under. The chamber specification is .887".
AA shells seem to measure .882 to .883 here, fired or sized in a Grabber. STS shells here measure .875 to .877. Both meet SAAMI spec.
If you want the gun to chamber ANY SAAMI spec shell, the rim cut will need to be .887".
Only you can decide if this is a acceptable. The gun predates any SAAMI or CIP standards.
Personally, I'd leave the rim cut alone and just use STS cases for my reloading. They are not particularly scarce. Yet.
Grinding on the rim of a AA is not the brightest thing you can do. The rim is not solid. The brass case head is formed from sheet stock in a die. The rim is therefore hollow.
While the rim is not part of the pressure vessel per se, making it thinner invites a partial case head separation which at minimum would be inconvenient.
