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If you shoot and reload for a .303 chambered rifle you probably know all about case head separation. This is where the case stretches and thins just in front of the rim and eventually fails leaving a cartridge case minus the rim stuck in the chamber. The good news is the remains of the case is usually easily removed, either with a phosphor bronze brush or a broken case extractor. I have used both methods in the past however if you can avoid the issue in the first place life is a lot easier.

The thing about military rifles chambered in .303 is the chambers can be rather generous, after all they were designed to be used in war where a shot was fired and the spent cartridge case discarded. The military certainly were not interested in preserving fired cases and a generous chamber meant the round could be chambered in muddy or dusty conditions which it did admirably. The downside is the cartridge case which head spaces off the rim tended to rattle around inside the chamber when ignited, the shoulders would go forward however the rim would go back and this would lead to stretching/thinning of the brass just forward of the rim, this can be evident on even once fired cases where the stretched area of material is clearly evident.

This leads to case failure in time so if I have a new box of brass to work with I add an O’ring to the rear of the case just in front of the rim. This means when the bolt is closed the O’ring compresses and holds the case head tight to the bolt face and as the internal ballistics take place the case head stays where it is and the shoulder moves forward. Yes the case has stretched but in a far more manageable manner.
You can clearly see the difference in the shoulder position on this Before and After image.

Look carefully and you will spot a ring on the bullet just behind the meplat, this is where the seater stem has been cutting into the jacket as it is designed for a 174 SMK as opposed to a 150 grain soft nosed bullet such as the Pro Hunter here. I will machine a new seater stem to suit the profile of the 150 at some stage however as today was a combination of zeroing and blowing the cases neck forward it was not incredibly important.

Regarding fire forming .303 brass (First shot) with an O’Ring seated tight to the cartridge rim, this image shows from top to bottom:

No1 New and unfired .303 brass.

No2 Once fired with O’Ring .303 brass.

No3 Once fired without O’Ring .303 brass.

No4 Shows what can eventually happen as the case is repeatedly stretched and full length re-sized between shots.
No2 has been shot once and the overall length is now +.009 thou/0,02mm to factory length which gives an indication of how far the shoulder and neck has moved forward.

No3 is from a P14 with a reasonable chamber however after just one shot you can see indication of stretching as a shiny section just in front of the rim. The actual case length is now +.007.5 thou/0,02mm so shorter than that fired in the Lee Speed.

The difference in shoulder profiles is interesting, No2 being a commercial BSA Lee Speed probably built around 1900 and No3 being a Remington built P14 from around 1917 so it is worth remembering not all rifle chambers are equal and especially not the .303 service rifles from the first quarter of the last century.

No4 is from a 1915 BSA No1 Mk3 (Star) that was well past its sell by date and sadly had to be retired for a number of reasons.

So there you are, fire formed 303 brass that should last for ages with neck sizing only and an occasional annealing. Before you ask, the O’rings I use are actually those used on our .308 Cleaning Rod Guides and I leave the ordering of such things to the Viking and have long since forgotten the details, however if you would really like to know give her a shout and I am sure she will be able to help.

The O ring is only necessary when you have poor head space, the 303 headspaces on the rim, get that right and the brass will fire form just fine without the need for the O ring.

Good write up Ovenpaa. My Lithgow really stretches cases. My No4's and my No5 less so, the fireformed brass from these can be chambered across all three rifles. Would seem that chamber specs were tightened after the SMLE.

One of my Ross rifles has an enlarged chamber, and the case when fired has only half the neck length of a standard cartridge the shoulder has moved that far forward to fit the chamber. The headspace is sound thought and after neck sizing the case work just fine even if the the look more like a 300 Savage than a 303. All my brass is batched to specific rifles regardless of the calibre.

I agree that head space is important and the difference between Go and No-Go for a military chamber is some 0.006" so you have an instant potential for the case head to rattle back some 0.005" and still be within specification, add the variance in shoulder to bolt face dimension to this and you can have a very short brass life which is why I always give it a fighting chance by using the O'Ring method. I will machine myself a shoulder bump gauge to suit the .303 when I get a moment and see if I can determine the real world variance in case head to shoulder dimensions of my .303 chambered rifles versus the chamber drawing.

What is apparent is commercially built .303 Sporter rifles seems to have have a tighter chamber *sometimes* which would suggest they were not always chambered with the generous military chamber we have all come to know.

Most of my 303 brass has had at least 12 firings with no issues, everything from PPU and S&B to HXP and Norma all fireformed, neck sized and batched to specific rifles, mainly lead loads at just over 2000 fps which is getting up near maximum pressure with N120.

I have been shooting and loading 303s for decades and have owned scores of them and the only time I have had a case issue was when the head spacing was off or been FL resized for shooting in different guns then I am usually still getting at least half dozen firings, I only binned the cases in the past when the necks started to split, in those days 303 brass was cheap/free and plentiful. I am now a bit more circumspect with my 303 brass and do anneal every half dozen firings depending on the rifle as some loads need a more brittle neck. Still I have well over 5,000 once fired HXP so that should see me out.