Easy enough to accomplish
One of the weapon choices in the opening mission is a pistol - pistols will also drop as free items from the Lotus as part of anniversary gifts or special events rewards. Or else can be claimed as with any other by finding the blueprint and suitable resources (check the item description to find these), or by playing Void missions, or if you are really desperate by using Platinum to buy a pistol or akimbo pistols set from the store.
To note as well, you don't need to actually use the pistol for it to rank up but it does need to be equipped on your frame to accrue rank (being in your inventory doesn't count)