Since this is a shotgun forum & it wasn't stated otherwise I assumed we were speaking of shotguns & not rifles.
W W Greener spoke of turning some shotgun chambers quite this & them holding normal loads. When Julian S Hatcher was chief of Ordnance for the US Army he experimented with an ordinary 1903 Springfield in .30-06 by turning the chamber walls down to about .060" thick. These held the service loads but burst when fired with a proof load.
"Most" guns have some amount of safety factor built in, but its still not advisable to exceed what they were designed for.
