Trying to get the fincan/MM Tube to slide into the pre-sloted body tube took a lot of effort. Thankfully I was warned of this by my brother, and I was prepared.
First, as the instructions suggested I took off the 1/32" joint line that is at the middle of the fin can
Second, I sanded down the entire fincan (no, not the fins them selves), with 100 grit.
Saddly that was not enough, because when I dry fitted the assembly into the body tube, it still got stuck about 1/2" from where it needed to be. So out came the heavy artillary. I wraped 120 grit sand paper to a nylon ... something on a stick. I dont know that it really is, I always used it to cleah out RMS tubes. (see pic) . With this tool, I sanded the INSIDE of the body tube where the fincan was going.
This did help but I was still 1/4" short of the goal.
My next trick was to use my dremal and sand down/bevel the first 1/4" of the fincan. Finally, it fit.
Now it sits curing. Don't tighten the zip-tie too much, just make it snug, or else you will bow out the tube, the Fin-ders will not sit flat, and you will have that much more gap to fill.
While I waited I finished a few more steps on the AV-bay and top section. Specificly sanding the avbay so that it slipped in and out with out grabbing, and then marking, drilling, and reinforceing the rivit holes.