I believe that the barrels are thinnest up 8-10 inches behind the muzzles is for two reasons. 1) it is safe to have the barrels thinner up there and 2) if you have ever filed on a set of barrels the middle of the barrels is where it is easiest to grind steel off! After trying to even up a set of Ruger barrels I have profound respect for craftsman who make barrels perfectly smooth without taking too much metal off.
Regards,
Jeff

"We are men of action. Lies do not become us."
Wesley