Thingiverse user Parkolay has printed two pencil guns and has sent me a video for all to see. It's hard to see the pencil.
These are his prints.
And this one is printed by Thingiverse user Linkreincarnate.
From their experiences it is clear the choice of elastics is very important. The outer two elastics can not be to strong or they will over come the trigger friction and inner elastic. You may be able to use as strong of elastic as you want but the inner elastic will have to be very strong and may make the trigger hard to pull.