The higher pressure of the .30-06 caused stretching problems with the steel that was used in the 1895s at that point in time. (Not a problem with the new 1895s of today). Re-barreling to .35 Whelen very well could have the same problems?
You should check with someone who really knows about .35 Whelen pressures in relation to the pressures of a .30-06.


Ole Cowboy