Full Length Size New .303 British Cases?


February 5, 2012, 07:48 PM
I purchased some new cases that I will load for my .303 SMLE. I have run each of them through my rifle to ensure they will chamber, and they do.

My question, should I full length size these cases or should I neck size only?

I want to get as many loadings from these cases as possible. What benefit(s) would I get from full length sizing over neck sizing?

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February 5, 2012, 07:56 PM
The usual advice given here is to go ahead and FL resize even new brass, but since you've already chambered the brass and checked the fit I see no harm in neck sizing only.

February 5, 2012, 08:32 PM
I agree, since all chambered. Neck sizing should be just fine.

February 5, 2012, 08:57 PM
If you intend on reloading this brass I suggest you look into fitting each cse with a thin "o" ring at the rim so they are fire-formed to the chamber without stretching in front of the case-head/web. Google or search this site about fireforming .303 british brass

A mild "start" load will do the job and you neck size thereafter. Cases will last 15-20 reloads done this way.

February 6, 2012, 07:57 PM
To ensure functioning with corroded, dirty or dented ammunition, the chambers of military .303's have always been generously larger than the standard maximum dimensions of the cartridges. If you full-length resize your cases after firing, the cases will stretch from the rim forward, and you'll probably start seeing stretch marks just ahead of the rim in four or five reloadings.
If you only have one gun your ammunition needs to fit, neck-size only.

February 6, 2012, 08:26 PM
With full power loads the cases will streach just forward of the rim on the first shot. Neck size only after that as you have an exact fit for your chamber.

I never tried the o ring thing, but maybe there is something to it.

February 7, 2012, 12:05 AM
In 1914 the military .303 chamber was enlarged to make room for the mud of Flanders to ensure function under harsh conditions. This means the chamber is not even close to American commercial SAAMI standards in length and diameter.

Below is a Wilson case gauge and a unfired surplus South African .303 cartridge, please notice the case is even with the top of the gauge.


Below is the same cartridge after it has been fired, the amount the case is sticking above the gauge is how much longer the military chamber is than a commercial chamber.


If you full length resize your .303 cases you will be pushing the shoulder of the case back over 1/8 of an inch too far and will be "OVER RESIZING" your cases and cause case head separations.

Please remember this, your resizing dies are much smaller than the military chamber in length and diameter so you MUST compensate for the difference for the smaller resizing dies.

Below is a animated image of a commercial .303 British cartridge case being fired in a military chamber. Please note the words "headspace" and "head clearance" and how the case will stretch to meet the bolt face.


I place a small rubber o-ring around the case for fire forming the cases, the o-ring holds the case against the bolt face and keeps the case from stretching. Also when the o-ring is compressed it centers the case in the rear of the chamber and helps promote accuracy.


Below the o-ring holding the case against the bolt face.


After fire forming your cases the shoulder of the case will hold the case against the bolt face just like a standard rimless case. WARNING, neck size only and if you must bump the shoulder back buy a .303 case forming and trim die and use it as a shoulder bump die.


Most of my cases are fire formed using light loads and 100 grain .312 Hornady pistol bullets.


Using this fire forming method I have gotten over 30 reloads from my .303 British cases. ;)

February 7, 2012, 08:03 AM
Thank you very much for the input and information, guys. I am aware of the "o-ring fire forming" process, but have never attempted it. I only neck size my cases but the last batch of cases (reloaded from factory ammunition) only lasted for 3 additional reloadings (4 firings total) before 80% showed signs of imminent case head separation. I'm beginning with brand new brass, and my reloading manuals state to full length size all new brass, it didn't make sense to me to size cases that will chamber in your rifle, however I did neck size to ensure all necks were round and proper size to seat the bullets (bonus, I bought a package of 50 cases and wound up with 52). I loaded up 50 reduced charges last night and need to acquire some o-rings before my next trip to the range.

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