Thanks for all of the comments! This is exactly what I was hoping for when I posted my question.
I'm not completely happy with my photos. I want a high-light down the length of the gun. I have a few ideas about how to do this (and realize it isn't a matter of the lens). It turns out that lighting is complicated.
I will try to rejigger my set up. My current set up is vertical: gun on floor, soft box above, camera squeezed in from side of soft box. The main limitation with a vertical set up is our ceiling! I will try to set things up horizontally and try to shoot from further away.
Once I'm happy with the set up, I'll rent a few lenses from Calumet (SF) to see which work best. Then, I'll buy a lottery ticket, win the lottery, and buy a full frame camera!