BFTA Main shoot rules
iii. Fully adjustable Rifle Stocks are permitted to accommodate various shooting styles and positions with no limitation as to design. Additional attachments are not permitted to be added or removed during the course of the event. (Except in cases related to i.b. above).
a. A single rifle sling, separate or otherwise, and/or a butt hook are permitted as a means of steadying the aim.
This rule clearly states that any attachments must remain remain fixed for the whole of the course.