Replacement No1 mk3 bolthead

 If you want your cases to last more than 8 or 10 reloads, set headspace close to the GO gage limit.  This is the beauty of the Lee action, we can control headspace.   Military brass with tight headspace will get you 40 or more reloads.  PPU is good brass, I’ve loaded mine 30 or more times and still reloading it.  No signs of web thinning yet.  

I use S&B and PPU cases which are fireformed to the rifle and are neck sized only, use reduced loads at 12grn of unique, the split is at the base so have to use a case split tool to remove them , 

That’s why I ask if anyone has spare bolt head with tighter head space, my bolt head is a later type without the grove cut in the thread , thank you all for your reply’s and interest 

Britrifles, headspace on a .303 has nothing to do with the shoulder of the case. The headspace gauge doesn't even have a shoulder on it. I'm afraid the headspace measurement isn't going to help him here.

Zed, those are modern, commercial headspace measurements.

Unique is a pistol and shotgun powder. It burns relatively fast compared to rifle powders. 12 grains of it isn't even a max load for a .357 magnum. That's a small powder charge in a big case. Some powders tend to detonate under those conditions, instead of burning like they're supposed to.  I've even seen it while loading uncompressed loads in a .45colt. It manifests in the form of velocity spikes on the chronograph, which is the result of pressure spikes. That could be what's stressing the brass and causing it to split prematurely. Just a theory. I only load rifles with rifle powders, so I couldn't say for sure.

Case separation, especially in an older No1 has alot to do with with the chamber area that was made oversized to allow for dirt and debris while still allowing the rounds to feed. This rifle was never intended to be used with the reloader in mind. It is and always be a military battle implement and with that in mind,  you can find a rifle with a,"tighter" chamber area but more often than not, it is what it is.  Getting a longer bolthead simply will not achieve the desired affects you are looking for. The rim will still contact the breech face but beyond that, you have that chamber area the case sits in and when the round is fire the case expands to fill that area and if that chamber space is larger than the normal, you get cases that will separate. To help minimize the separations, you can use military or similar thick walled cases for reloading but thats about it...