Fogducker
If they were small base They would have a SB after the caliber stamp.
First I am going to explain something you all probably know.In lever,pump and semi auto rifles they usually have a slightly shorter chamber , There are some exceptions to this old winchester such as 30-30 and 25-35, all the savages, I am not sure about Browning as I have only loaded for a223 in a BLR.The point is cartridges loaded in ordinary full length dies tend to either jam tight or not chamber fully.This is what Small Base dies are for.
Someone suggested the possability that Dons rifle fired before being closed all the way, sort of a slam fire without being fully chambered.It would explain the bulged base and blown primer. This is just a guess based on a certain BSA CF2 with the same problem that cost a little money to replace an extractor that had to be hand made. In any case have the bolt head checked by a smith,it usually cracks that outer ring.
Rich