Originally Posted by
Ballistics in Scotland
A few thousandths of an inch of expansion in the hollow part of the case are normal, especially in military rifles. A perfect fit of case to chamber, such as target shooters and benchresters use (Oh, and factories which want to tell you how accurately their guns will shoot) becomes an impossible fit with a little dirt, and is liable to seize up when the barrel is really hot. Towards the shoulder area of the case a bigger gap is permissible. People commonly make the Improved version of cases by firing the normal factory rounds in the Improved chamber, and get pretty good accuracy. (An exception is the .25-35, in which the factory cartridge frequently splits at the shoulder, but even this isn't dangerous.
Chambers don't wear to speak of. One might enlarge through frequent rust and its removal, but it would be hard for that to happen without a deteriorated bore as well. Most likely your chamber is fine, but if it worries you a chamber cast, made preferably with Cerrosafe alloy which melts at less than the boiling point of water, is the way to go.