You do have to make the barrel at least 16" to mount it on a carbine stock. It's better to make it a little longer than that (16 1/16, 16 1/8, 16 3/16, etc.). The person making and installing the break should be able to accomidate this.
As for the perminitcy fo instalation, it must not be easly removed. This means that you can't do it easly in the field. Anything that can be added can be removed with the right equipment and time.
There are several ways that a break can be installed that I've been told are OK with the ATF. Although this may be subject to interpertation by the on-sight agent but usualy can be explained alright if you don"t get to upset and start yelling at the agent.
The easiest method is simply to properly use red locktight so the break must be heated in order to remove it.
The next method is to have a front sight installed with the front of the sight over the break. This sight should have it's frount mounting screew under the dovetailed sight blade. The blade should have a drop of red locktight used so it can't be easly removed.
Another method that is a bit more perminent is to have a hole drilled and tapped through the break and part way through the barrel for a set screew. Use a set screew longer than needed and afrer setting it, cut it down and finish it off smooth with the break. Inorder to remove the break you have to drill-out the shortened set screew.
For those of you that think welding is the only way, consider how T/C installs their breaks. Several people have talked about how they have had to retrive their break after it went flying down range when the 'glue' that held it on came undone. If the manufacture can legaley simply glue a break on without any threading then the first method should pass any inspection if properly done.
I hope this helpes with your question. If anyone else has any ideas or have any personal contact info with the ATF I'm shure it could help us all.
