To clarify gentlemen, that image is an example of the measurements that need to be taken, from ANOTHER burst.
Craig's observation is correct, and I've discussed the issue with a mechanical engineer. That degree of elastic deformation is unlikely to thin the barrel more than .001 - .002".
For those with the interest, and smarts
Calculation of the tube thickness reduction rate
Wt = ( D-d0 ) - ( H - d1 ) / ( D - d0 ) × 100
d1 = (D-d0)×(1-Wt / 100)
Wt: tube thickness reduction rate (%)
H: sheet hole diameter before tube expansion (mm)
D: outer diameter of tube before tube expansion (mm)
d0: inner diameter of tube before expansion (mm)
d1: inner diameter of tube after tube expansion (mm)
And yes, .020" just past the forend (where the leading hand rests) is too thin.
CIP suggests .030" at 12", for 4140 strength steel.
