If you are getting dents in your brass after firing then you either have crudd in your chamber or you have neglected it for far too long and the chamber has rusted.

If it is just dirt then clean it very thoroughly and you should be ok.

If you have enough rust to dent your brass then you need to take it to a gunsmith and have him clean it all out and polish the chamber for you. If the rust is bad enough it may have pitted the chamber walls and once polished may reveal pitting which may not be able to be removed with out creating a dangerous situation. Your only option there would be to have a thread cut off the barrel extension and rechambered for the existing cartridge, convert the gun to another cartridge but the same caliber (300 win mag from a 30-06), a new barrel, or a new gun.