The Tiger had a fixed sleeve protecting a portion of its gun. Hidden inside this sleeve was a rubber ring that prevented dust from working its way back along the barrel. Therefore, the mouth of the sleeve didn't need to clamp the gun tightly.
This image compares the mouth of Takom's gun sleeve (on the left) to a real one (on the right). I have scaled them to the same size, as shown by the dashed lines.
It's obvious that the lip of Takom's sleeve does not have quite the same proportions as the real one. The gap between sleeve and gun is smaller on the kit than on the real Tiger.
I don't yet know if this means Takom's gun is too wide, or their sleeve is too narrow.