Pete, you are really on to it and have done what I always advise when having problems with firearms, slow things down have a good look at what is happening and a good think about sequences of events. As with any firearm,double, single or bolt, every chamber is different and it is absolutely certain that each chamber in a double cam be slightly different in dimensions than the other. Normally this will not be noticeable to the owner and cause no issues. With brittle brass as in your case the case neck splitting can manifest itself in one chamber and not necessarily the other, although now that you have taken special note, you are experiencing it in both chambers.

The good news, your gun is fine, the bad is that you will continue to lose cases from that batch of S&B ammo from classic brittleness of brass, maybe age related or could well be a badly annealed batch. All factory brass is annealed, some show the colour change on the brass, others have it polished off before loading.

Are your S&B cases berdan primed, if so probably not worth worrying about keeping to reload. If boxer primed and you really need the cases for reloading then do as I advised earlier, pull your S&B ammo down and anneal the neck and shoulder area and do this (annealing)to all your other cases as well then you will enjoy a very long life from them.

As to shooting a lot of ammo in a gun where the necks or shoulder split, I have not observed any any great gas escape which potentially could gas cut chambers. I suspect that the splitting occurs possibly a fraction of a second after the powder burn and the bullet has left the barrel. I have seen cases that do not obturate properly in the chamber due to low pressure loads and they certainly show signs of gas blow by.