Well its all about the gearing between the pully on the weapon motor and the pully on the weapon. For me I have a 25mmish pully going to a 40mm pully and i use this calculator to work out mph ect.: http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php
The other thing i would say is that countersunk bolt teeth give a much better bite then caphead teeth would. The profile of the teeth really allow the drum to hook under wedges and bite into plastic much more that the caphead profile!