Seems to read pretty clearly to me:
Originally Posted by WFTF Core Rules 2012
The surface of the hamster (and knee pad) must be flat and not shaped to provide lateral support to the rifle.
"Astrid" - Steyr LG110FT / Falcon T35S
"The Bangstick" - Falcon FN19 .25 FAC / BSA 8-32
Too many tinker projects, not enough working rifles.