In the continuing 3D-modeled series of transitions this one is the BT-5 to BT-50 transition that will fit Estes BT-5 and BT-50 airframe tubing.

The distance between the two sizes of tubes (transition taper length) is 1.0 inch (25.4mm).

There are four models:
1) Solid,
2) Solid with a 3.2mm hole to allow for screw eyes to be fastened to it,
3) Hollow,
4) Hollow with end caps.

You can download these models for free from Thingiverse and print them on our 3D printer.
https://www.thingiverse.com/thing:4927099