I'll echo Greg's sentiment...when building the same structure for the StuG IV build (http://www.bpmodels.net/Model/Album/phpBB2/viewtopic.php?t=46), I found that the following sequence was the only way to make it work:
"Step 14: This is an incredibly tricky step as it installs the gun into the fighting compartment, installs the casemate to the hull, and installs the exterior gun barrel and mantlet to the gun...and also adds the roof (which I haven't finished yet). What makes this tricky is that there's a very specific order that you have to do all these things in or it will not work properly...and there's only ONE way it will all work...so I studied this very carefully and did lots of dry-fitting before committing to glue. So, here goes...this is the order you have to do it in:
1) You must install the mantle to the breech portion of the gun
2) You must then elevate the gun to it's highest possible elevation which in turn depresses the recoil guard at the base to it's lowest point
3) You must then slip the elevated gun, from the bottom, up over the bottom lip of the opening in the casemate. This is a tough thing to do and you have to be patient because the tolerances around the mantlet are tight but if you carefully work at it, you can get it all to fit. Don't force it though and make absolutely sure the mantlet glue has set up properly to avoid an accidental separation of the mantlet from the breech.
4) You must then place the casemate and the gun together onto the lower hull, which means you have to simultaneously glue the base of the gun into the mount plate and glue the casemate to its mount points on the fender. Test the gun elevation carefully as pressure on the mantlet front end can cause the base to lift up from the mount plate which can cause major problems if it comes all the way undone.
5) After you have this all glued in place, you then add part C12 to the top of the casemate gun opening and that essentially secures the gun in place permanently.
If you pull all that off successfully, you get a very nice solid installation of the gun and casemate sans the roof."