If you need to do this, your rifle is not set up properly. Assuming you don't have a chamber neck tight enough to require it, you should be able to run a 25-06 case into your 6mm-06AI die, load and fire. To check if the rifle has excessive headspace, do this to an empty primed case, and fire it. If the primer is sticking up at all above the base of the case, the rifle is set up too loose and should be checked. The bolt should be hard to close the first time, and will be correct after fireforming. If you have a chamber neck tight enough to cause problems, the procedure is totally different.