Before you bring said 300 WM rifle to a gunsmith please do the following steps.
1) Remove the bolt from the rifle.
2) Examine the bolt face itself for excessive carbon or brass debris.
3) If debris is found then break down your bolt and remove the firing pin and extractor claw.
4) Now give your bolt face a thorough cleaning removing all carbon and brass build-up
5) If all looks good with your bolt lightly oil with say Rem oil or Ballistol wipe dry proceed as follows.
6) Look down your bore with a flashlight if you do not have a borescope.
7) If you see anything out of the ordinary that may be your culprit.
8) Now staring with your rifles chamber and throat area using a pistol cleaning rod (a short one usually less than 8 ")
attach a .30 caliber bronze cleaning brush dipped in bore cleaning solvent and work your barrels throat and chamber area vigorously and set your bore brush aside. ( Do not forget to scrub the area where the bolt face makes contact with your barrel )
9) Follow up with a patch that has bore cleaner on it and cleaner on it and work your bore and throat area thoroughly
10) Proceed with more patches until areas seem to be spotless.
11) Examine your bore carefully with a light, starting from the muzzle end.
12) If your bore is clean you should see nothing but your lands and grooves.
Now hopefully your rounds will chamber, but if not because you are still getting dents in your shoulder area I would certainly locate then isolate that area (look at your Sheels for reference) and repeat your cleaning until your cartridges chamber correctly.
Conclusion:
I honestly believe from what you are saying that you have a foreign body (ie brass or carbon) that has become lodged in your chambers throat area
In that light with a little diligence and time, you can bring your rifle back to working status.
I know that some of this may be redundant to you, however, in order to achieve success you must become be methodical and proceed one step at a time until you have isolated the problem area and repaired your rifle.
Best of luck!