Buford's right. The encore slug barrels, because of the wall thickness & the way the mounting holes are drilled, require the use a special scope base. You can't use just anybody's bases, as you could w/ the rifle barrels. When I ordered my slug barrel, it was shipped with a 1 piece aluminum Weaver base & a note from T/C explaining what I just relayed above. Check with T/C directly...
Thompson/Center Arms Co., Inc.
P.O. Box 5002
Rochester, NH 03866
Or you can call them at: 603-332-2333, or fax them at: 603-332-5133