Carl, nice work on the pre-shading and other tone variations. 
Today was one of those model building sessions where you get on a roll and time just flies by...before I knew it, 4 hours had gone by but a lot of progress happened in that period, so on with the update!
Step 13 deals with details for the turret floor/bottom and is a pretty straightforward step. The only thing I did different was to attach the floor support arms to the turret bottom first and then attached the circular tread plate to them vs the other way around...the attachment points to the tread plate aren't very large and it's very easy to install them slightly crooked if you do it this way. The turret bottom had several large semi-circular pour stubs present with matching holes that had to be dealt with...so some Squadron White putty and sanding did the trick. All of this will get painted and detailed prior to assembling the turret later on.
Steps 14 and 15 deal with the assembly of the main gun and the mantlet/recoil housing. The option is provided to use either a turned aluminum one-piece barrel or a two-piece styrene barrel. I opted for the styrene barrel since it a) had rifling molded in while the aluminum did not, b) the two halves were molded in such a way that the seam was hidden away inside the whole assembly and c) I had some slight concerns about added weight (later proved to be non-issue) and the ability of the gun to elevate/position.
Step 14 assembles the barrel and breech with the breech in two halves so some slight sanding is needed to eliminate that join seam. The breech block is a separate part and was left off to make it easier to paint later on.
Step 15 is where there's a major mine field waiting for the unwary and I do not recommend assembling things as the diagram indicates. This is particularly true of the external mantlet, part Y3, as there's really nowhere to attach it to at this stage, so it should be left separate. There's enough clearance for it to slip over the antenna rail under the barrel, so no worries there. The internal mantlet and recoil housing should be assembled (parts F22-F12-F37) using the gun barrel to insure they are all lined up properly but don't glue the gun barrel into position at this stage or you may have major trouble in Step 16 when it comes to installing the recoil guards.
The step also has one of the elevation pin supports (W7) backward in the instructions...they should both face the same way and should not be glued to the pins. They are a tight friction fit and will support the weight of the full gun assembly with no problem. The coaxial MG sleeve (Y1) is supposed to install into the molded opening on F22 but the opening is significantly too small...so I had to carefully enlarge it with a round needle file to get the fit needed. Last but not least, you can ignore part F30, it serves no usable purpose and its detail is completely hidden away inside the recoil housing once the full assembly is done.
That brings us to Step 16, which deals with installing the gun and mantlet into the turret front plate. The space tolerances here are very tight so it is necessary to first install the gun by very carefully placing glue only above and below the openings that the curved side of the elevation pin covers fit into...that way you don't get glue on the pins but still secure the mount into position. After letting that dry thoroughly, then the external mantlet is added along with the armored vision port covers on either side. I also used a drill bit and pin vise an opened up the molded-close port for the gunner's scope. The option is there to pose the ports in the open position using PE parts for the hinges but they don't actually fit due to the presence of molded detail on the inside...so my momentary temptation to pose one open was eliminated due to the effort required to make it work.
The main gun is dry-fit only at this point, still haven't glued it in place yet.
With the outside done and set up, I set to work on the interior details for the gun and plate. The recoil housing has to attach to the front turret plate AND points on the gun breech...so if you didn't get the gun breech aligned properly earlier and glued it into position, it would be very tough to fix at this stage. The recoil guards were added, starting with the left side and working my way around and once everything was set, the gun breech was finally glued into position. I also drilled out the eyepiece on the gunner's scope, installed the coaxial MG, and added the spent shell basket to round things out.
Next up is the turret top!