If there is zero intent to make a functioning weapon attachment system, that lets one gun accept different attachments, just give us unlimited copies of attachments to add/remove at will.
Of course, they shouldn’t have the ability to be sold. As for soldier knives being sold for orders, just include it in the sell price of soldiers when you send them to the rear.
Inventory management is already a pain in the ass, and this helps address related issues in the present and then in the future when more stuff gets added down the line.