No4 & No5 Bolt Head Sizes (Update)

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 serrock wrote:

I own a 1942 Savage manufactured No4Mk1. 
On a recent range trip I experienced a split case, the base completely shearing off approximately 12mm, 1/2" from the base as well as several fired cases showing a distinct line at roughly the same spot combined with some primers protruding slightly. 

The cases were S&B which according to local anecdotal information is rubbish brass but, also possibly a head space issue as well.

 (Loads were well within published data so I don't believe my issues are pressure related)

I have examined my bolt head & has approximate dimensions of 0.633"

Are you FULL LENGTH resizing your brass?
If you answered yes, discontinue the FL and NECKSIZE only.
How many times has the brass been resized?
Invest in a Forster FIELD gauge and see if the bolt closes on it.
If it does close, find another bolt head that is between 0.635" and 0.638"
If you are still experiencing a case rupture after a larger sized head bolt replacement and you are neck sizing only, you could possibly have a worn chamber. Remember, you have a 80 year old rifle with an unknown amount of ammunition pushed through it prior to you claiming ownership...