If the guy really is a mentor to you, why not write him a card??? And no, writing a card is not gay or lame...
Detail how he has helped you become a better, stronger person. List examples of tough times you have had that were made better by his guidance/advice. Mentoring is a labor of love. You can pay your friend back by telling him what a good job he has done helping you and others.
You can find lots of cool looking cards and quotes, and that might be OK for general purpose use, but it sounds like he's a more than just a "general purpose" friend.
Despite the hard outer shell, real tough guys have big hearts with lots of emotion inside. He'd probably really appreciate something unique and heartfelt. You can close with "can't wait to get back on the range with you" or something "macho" like that. You could attach the card to a box of ammo to show the ultimate in class.
Just my .02...