I just purchased a new Remington Sendero SF II in 7mm Remington Magnum. I also bought 100 new Winchester 7mm Remington Magnum cases. I ran the cases through a RCBS full length die to straighten the necks out. I then ran the cases through the action to make sure they would function fine. Out of the 100 I had 8 that would not chamber. I adjusted the die to size the cases a little more a couple of times, and to no avail, the 8 case still would not chamber. I checked case overall length and it is same as the cases that chamber. I also measure the belt diameter and it is the same as the cases that would chamber fine. What might the problem be ?

Thanks

Check the case rim diameter to see if its fitting under the extractor and flush with the bolt face.

Case neck length may be at fault not to be confused with over all cartridge length !.

A sure fire method to see where the problem lies is to smoke a cartridge case with soot from a candle or match or even a magic marker will work . Concentrate on the shoulder neck and belt area .

Just a suggestion but next time purchase Remington or RWS Lapua cases . Winchesters suck as far as quality any more ...

Another one, that no one mentioned, measure the LENGTH of the belt on those that don't chamber and on a couple of those that do.
It would be a rare occurrence, but you may have a belt recess that's a tad on the short side combined with a few pieces of brass that are on the long side of the belt dimension. If so, DON'T "fix" the chamber as short case belts are the rule and your brass will last longer if the recess is cut short.

the 7rem mag headspaces on the belt, not the shoulder.....in reality you could cut the case off below the shoulder and if the belt was wrong, you could not chamber the case....look for another problem with the brass...

I have that same problem with my 7mm mag.
I suspect my FL sizing die is oversize w/respect to the chamber.

Don

I've also run into case head thickness problems , where the rim if you will is to thick !. Bolt doesn't want to close for head space .

I know the case head spaces on the belt , but I found a few pieces of brass where it was the thickness of the head that prevented the bolt from locking into battery and not the belt.

I've never seen necks shoulders that were to " Long " on new brass but suppose anything is possible with manufacturing being what it isn't today !. ...

The only problem I have had with Winchester brass is the extractor grove being to big on a few so they will not go into shell holder for reloading