"8 bore E.M. Reilly on Rock Island auction. SN 18534 (1874). 3 1/4" Chambers. Again don't know if this is original?"

What I don't know about 8-bore guns would fill several volumes, but I do have a few vintage 8-bore Remington Arrow paper shells in my collection. They are roll crimped and in two lengths; 3" and 3.5", so the shorter shell would be very near 3.25" when fired.