right on the one on the top has a nice smooth curve but this one is kind of cracked.
the bump handle should be bump maped then try a cylindrical unwrap now and bump map it.
I had a Winchester Defender (I used to be a gun nut )which is pretty much the same gun. Looks fairly accurate.
Here's the very minor things which I think would push it to perfect:
(see attachment)
A - This part is to conical, take the verts that I'm pointing to here and scale them just a bit to make it more rounded.
B - This part doesn't look exactly right, it's good though, but I think the seam should be more defined on the bottom. Just a lil nitpick tho.
C - The slide that hooks to the 'pump' is actually two pieces on either side. It's hard to tell from your render but it looks like one. It also has an indentation where it gets smaller towards the reciever.
The 1300 is a great gun, so great that this gun - the FN Police Shotgun is actually a modified 1300 built by Winchester:
Actually after looking at this pic now your pump looks alot like the one in this pic.... so maybe it's good enough after all.
I actually made the whole pump more of an oval shape. That's why the cone part there is a little off. That's easily fixed though. Good call on part B, I think the angle I made it at is wrong.
As for C, do you mean the spacer between the barrel and the pump?