NBRSA Rule
2.11.2 The rear sandbag. The rear sandbag shall be a bag or combination of bags containing sand only. A vertical spacer under the rear bag will be allowed as long as it incorporates no adjustments for windage or elevation. The vertical spacer shall not contain any protrusions which can be inserted into the bench top or the sandbag. The Dunrud type rear ring spacer for use under the rear bag is legal. The rear bag shall not be contained in any manner. No metallic materials may be used in the construction of the sandbag. Tape on sandbag is legal.
I would have to say no for NBRSA
You sure that's not 100/200/300 group specific?
Here's some of the 600/1K Heavy verbiage..
2.4.3. Neither rest can be attached to the bench, the rifle, or each other. They must be movable in all
directions independently of the other. Any part of the rifle resting thereon must maintain a
minimum of one quarter inch distance from any part of the retainer or container holding the
sandbag on which the rifle rests. Whenever the rifle makes contact on its sides, there must be
minimum one quarter inch thick sand on each side.
2.4.4. Sandbags on front rest must be a minimum of one and one half inches wide.
2.4.5. Rear rest sandbags must be a minimum of one and one half inches wide. No device of any
kind can restrict the upward and/or rearward movement of the rifle.
2.4.6. Any sandbag used as a rest that is not supported by a retainer and that has movement without
restraint need not comply with this rule.
2.4.7. Rifles must rest on sandbags and slide on sandbags only; no multiple recoil ("return to
battery") systems on front or rear rests are allowed. Rests, rest changes, or recoil systems must
comply with the above rules and be shown to and approved by the referees prior to
competing.
2.4.8. Rifles are fired utilizing sandbag front rests, which may be supported on a pedestal that does
not co-act with the sandbag to restrict upward and/or rearward movement.
2.4.9. The rear rest is comprised of a sandbag, which may be attached to a pedestal and that supports
the rifle between the rear of the pistol grip and the toe of the buttstock.
2.4.10. Mechanical rear rests that incorporate windage and elevation adjustments are legal.
I did have my rear Farley OK'd.....