Friend, you will need a proper Action Wench, a Barrel Vise, a Lathe and a Chamber Reamer to do that job. A gunsmith likely will charge $200 or more to do the work. And you don 't need it.
MOST Lee-Enfields expand their casings right at that spot. It comes from the fact that they were expected to operate faultlessly after being dropped into a shell-crater with 8 feet of mud in the bottom. So they BUILT them that way, and even moreso after the middle of 1916, which is when the decision was taken to build all the rifles with chambers even larger than previously. There was a LOT of bad ammo running about right about then; the Ross was not the ONLY rifle to have troubles..... just the only one that got publicity.
The CAUSE is a Large Chamber, not headspace.
Besides, Headspace has been blamed for far too much. It really is not all that much of an issue if you are shooting RIMMED cartridges.... and the .303 has a rim.
With RIMLESS ammo, it is more of a problem. The Americans are all paranoid that anything more than .003" is dangerous (which it isn't if you know what you are doing). Well, friend, you can have an absolutely PERFECT Lee-Enfield, buy the wrong box of shells..... and have EIGHT TIMES that much headspace OVER TOP of the MAX, just from sloppy SAAMI manufacturers using their own rim specs instead of the proper military specs.
So what does a guy do?
Gather up all your brass and get a handloading outfit, set it up. NECK-SIZE all your brass from that rifle. Now head on down to the local Dollar Store or whatever you have, go to the girls' department and get a blister-pack of Pony-tail Ties. They are little rubber elastic bands, just the right size to fit onto a .303 cartridge, just ahead of the rim.
If you have fresh ammo, you fire it always with one of these little elastics in place. This FIREFORMS your brass to YOUR CHAMBER. You do this on the first re-firing of your old brass, too.
In future, you neck-size your brass only and stick with loads about 2250 ft/sec, no higher. Your brass will last a LOT longer. A good and accurate load with a 180 is the Sierra Pro-Hunter 180 with 37 to 38 grains of IMR-4895. A good load with a 150 is the Hornady Spire Point .312" with 39 to 40 grains of 4064, no higher. With BOTH these loads, you seat to the OAL of a military Mark VII BALL round. It might look funny but it works just fine. These are two of the cheapest .303 bullets around, they are both very accurate and they both have FLAT bases; .303s do not like boat-tailed bullets, as a rule.
If you want to do a LOT of shooting, cheap, get a Lee mould for the .312-185 bullet and get a Lee Sizing Die, which just happens to double as a very good gas-check seater. Then you load 13 grains of Red Dot shotgun powder behind a bullet made of old wheelweights. The result is a mild load which will shoot 2 inches at 100 and is okay for Gophers out to 200 or so. It costs less than TEN cents a shot to make these up and your barrel will last halfway to forever.
Yes, I shoot .303s myself. I generally have 10 or a dozen ready to go at any one time and I am working on another half-dozen right now. I like .303s, as you might have guessed.
Have fun!
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