HI, MLD: The aliuminum barrel in the kit and the one on the spure are both the same length in this kit. I don't have anything handy to measure with but an old tape measure so I'll go by it. The length from the muzzle back to that first angle is 4 3/32 inches long and the length from the muzzle back to that smaller section that goes inside the plastic cannon piece is 4 15/16 inches long. Hope this helps.
I really like this kit. It's something like Trumpeter KV-II, in that there's lots and lots of detail molded into the kit plastic, plus Dragon has added several extras that they have come up with lately--different types of metal tow hooks, wire for tow cable, etc. that really make it outstanding in it's own right.
The only 'bad' thing I can find to say about it is that it's was one of those kits that said "Build me first.'" So I found myself moving, or boxing up the parts from the other kits I've been working on lately, and starting on this one. (Such are the trials and tribulations of this hobby I suppose.) And, like the KV-II was, the more I work on this one the better I like it.
I suppose one of the reasons is that, unlike the Tigers Dragon has been coming out with lately, this is a kit that doesn't take much studying and figuring out just what parts to use and not to use, depending on what version you want to build before you can even start on it. You can pretty well just start dabbing glue to plastic right off the bat. Guess I somewhat lazy, or maybe it's an age-related thing. Ha, ha!
Good luck and take care, Sgirty