Identify each firearm with specificity, including serial number, make and model. Make your desires known about what you are doing with it, including who and exactly which specific firearm they get.
For example, to my brother Jon VN I leave my .45 Ted Yost Custom, One Star Elite, Colt Commander, Serial number __________, and my 8 Wilson Combat 46D 8 round magazines.
Different example, I leave my .45 Ted Yost Custom, One Star Elite, Colt Commander, Serial number __________, to Louder than Words to use for charitable purposes as they see fit.
Making such a specific bequest both valid and enforcable is easy for a competent attorney.
(remember, that the law varies from state to state and what may be legal, valid, and enforcable in Texas may not be any of the three in your state) - not a bad disclaimer if I do say so myself.
JeffVN