The thumbstuds on my AR/GB sometimes hurt my thumbs when opening, what can be done?
Goto your local hardware store and in the pipes & washers secion find a #60 o-ring. Place the o-rings on your thumbstuds.
Some think that the #60 o-ring is to big and prefer the #36 o-ring.

Or just get tougher thumbs.
(Photo by earthangel on USN)