This should be a fun game.
If you had to buy one more gun (in addition to the ones you have) and that one gun was the last gun you could ever buy for the rest of your life, what would you pick and why?
Once you buy that one gun you can't sell or trade any of your other guns for the rest of your life. You can keep the guns you currently own, but you can only have one more. That one gun would complete your collection and you are permanently stuck with what you have until you die. That one gun is it.
For me I'd pick the M1 Garand... the reason is obvious