Guys,   
I've been using a Fecker scope in BPCR Target rifle competition since this spring.   So long as the scope objective and ocular are under 1" and the tube is 3/4" or smaller and the mount does not have any "clicks". it is legal for use.  Now that narrows down the number of scopes that can be used, but there are still some around.   
I could not make it from 100 yards to 600 yards until I added a .062" shim under the rear base.  Now my zero is 200 yards and I have enough elevation to make 700 yards.
In order to use it at long range 800-1000 yards, I used a shorter block in front.  It is .200" lower then the rear block.  It will reach almost 1100 yards and the zero is about 700.
I have 2 different Feckers and they work fine for midrange.  When you get beyond 600 yards, there is still some parallax (about 9-10 ring across) on the long range targets.  I don't believe they were ever intended to be used at those distances.  Frank Monikowski

JDA,
The way the rules read:

(c) Telescopic
(1) No length or power limitation on the scope. Scope tube body to be 3/4” or less in diameter and any ocular or objective lens, adjusting or assembly rings to be less than 1” in diameter.
(2) No internal adjustments for windage or elevation
(3) Mounts are to be of a traditional style of the period and contain the windage and elevation adjustments for the scope in either, or both, the front or rear mounts. No click adjustments in the mount. Either dovetail mounting or scope block mounting is allowed.
Black Powder Target Rifle Original scope mounts of either the Cataract or Malcom style or variations there of, or replicas or derivative of
either style, are allowed provided the replica or derivative conforms to the criteria of the above.

I take it to mean the outside diameter.  I think the Lyman Jr. Targetspot has a lens under 1" but the OD of the objectve end is 1.125" making it too large.  This rule is copied from the BP Target rifle book.  Other rule books can be found here:   (You need to Login or Register to view media files and links)

Frank M.