There's a total of 11 weapons in Atomic Heart to collect, however only 10 of them have blueprints you can craft. You only get 1 Shotgun in the entire game and no blueprints exist to recreate it. So if you happen to dismantle the Shotgun and have your achievement tracking set back to 0% you won't be able to get this achievement. Do not dismantle the Shotgun!
Blueprints are found in the grey/yellow chests throughout Atomic Heart. The individual weapon blueprints appear to be in different locations for different people. Where I found the Railgun blueprint won't be the place you'll find it. However I did see messages on the side of the screen stating I already had certain blueprints, so it's possible you might be able to find multiples of them.
No Shotgun? Tracking bug workaround:
Collect all 10 available blueprints, gather crafting materials so you can create each of them at least once. When you have enough materials, create each weapon ensuring it goes into your Inventory and not the storage (this wont count). If done correctly and you've had all 10 weapons in your inventory/hands, the achievement tracking should sit around 91%. Now start a "New Game" and proceed through to where you first meet Granny Zina and enter Vavilov. Soon as you get "gifted" the Shotgun you'll also be gifted the achievement - All this must be done in one session! Closing/reopening the game will reset progress.