I'm product photographer, prime lens will give you sharpness and less CA, but these are important only for large blowups and not for web-images or small press.
I suggest zoom like 24-105L, for photographing firearms you will need something longer if you have your studio large enough, maybe 70-200 (f4 nonIS is enough for studio)
For sharp product photography there is good old 100mm 2.8 macro, old version is good enough and you can get it really cheap these days. Try to avoid wide lens for these kind of product-photography shots.
And yes I agree with Paul, I use 90TS too, but it can be price overkill for beginners, there's amazing lens correction and perspective correction tool in ACR or photoshop, you can always upgrade to TS lens later when you will have more money, I really recommend FF camera first.