I can't speak for the OP, but it is not a design flaw if REAL casings are being used. They have too much 'meat' to allow for the necessary expansion when inserting the transmission. Some extra material must be removed from that area to prevent crushing the transmission.
Stan, when I make casing pens, with a 30-06, the tube is completely inside the casing. But I assemble the nib, tube and transmission before inserting it into the casing so I know the OP is not talking about one I made (pshew)
The only thing that would inhibit inserting the transmission (that I can think of, besides glue in the tube) would be the diameter of the hole drilled in the top of the casing.
But if I handle commercially-made kits, I don't want to redrill each one before I ship it!!!!!!!!!!