I've seen pictures and models of 76mm Shermans with and without a ring around the muzzle. I can't make out any pattern to the different sizes among the various models that mounted the 76mm, or any explanation as to what they are. My best guess would be a counter-balance weight for the gun, as the Easy 8 was the only war-time Sherman with a muzzle break.
I have the 1/72 Monogram Revell/Germany M4A1, and it has one on it that's thicker than any I've seen in Squadron's Sherman in Action. Does this need ground down or removed?

That was the muzzle brake. The muzzle brakes were removable. I believe during the war there was not enough of the muzzle brakes to go around which is why you see some tanks fitted with and some without.

BlueBear:

Early 76mm guns had a smooth taper and no "bulge" at the end of the muzzle. Lots of M4A1s recevied this gun and you'll see it on a a few M4A2s and M4A3s as well. These early guns are M1 or M1A1 guns -- they are externally identical.

The "bulge" seen on the end of some 76mm guns is a ring to protect the threads for attaching a muzzle brake. These 76mm guns with a thread protector or muzzle brake are designated M1A1C or M1A2 guns (again, externally identical to each other).

BTW, there were plenty of 76mm gun tanks fitted with muzzle brakes that were not "Easy Eights." For instance, check out photos of Soviet 76mm M4A2s and you'll see plenty of muzzle brakes.