My opinion is that anyone competent to rebarrel M1Aicon/M14s would have no trouble with No.4 barrels. Both have inclined locking surfaces, variable receiver face lengths (the commercial M1a are almost certainly worse than any variation in No.4s), indexed sight attachment points, and bolts that have their own variables. The good thing is that Critereon is quite familiar with the M14 headaches, so it's unlikely they'll be much surprised by No.4s.