@mattdm nailed it: I've been shooting long guns for a decade. The single most important factor is light. For a catalog-quality shot you need lots of good diffuse light, which you can get by bouncing a speedlight off a white ceiling or wall, or by shooting in daylight with reflectors. For black guns you need to go between +1 and +2 EV to bring out detail. To maximize sharpness you want to stop down to about f/8. Finally, to avoid perspective distortion (assuming you're not using a tilt-shift lens) you typically want to get the camera perpendicular to the subject.

Depending on the quality of the lens at that point you may find that you need to crop tightly to avoid all the possible lens distortions, although if you have a good lens profile you can correct a lot of that in post.

Examples: Here's one in high sunlight balanced with a speedlight from the front-left quarter:

Here's a black gun done indoors with a speedlight:

Both of these happen to have been done on an APS-C body with my shortest prime: a mid-grade Sigma 28mm EX DG lens. I almost always go to that lens because any longer and I can't fit a long gun in frame at a convenient distance.

So if I were making lens suggestions I would say pick a prime no longer than 28mm for crop sensor, and preferably closer to 20mm; or 28-35mm for full-frame cameras. No need to pay up for extreme apertures: it's better to put more money into lighting for this type of shooting.