Don't dismantle rare fusion rifles. Just don't. Tuck them away safely in your vault for a later date, for your own sake. The exotic bounty "Shattered Memory Fragment" for the weapon Pocket Infinity requires you to dismantle 10 rare or higher quality fusion rifles. I've had this bounty for 3 days now and I'm about 1/10 of the way there, simply hoping that the gunsmith stocks one sometime soon. It may be months before you ever get this bounty, but when you do you'll be glad you saved the fusion rifles. Otherwise, you'll basically just be left with only 4 open bounty slots for an indefinite amount of time, seeing as the acquisition of rare fusion rifles is completely RNG-dependent.