Check the thread you posted last week. I told you precisely how to assemble your rear sight to your rifle. Go ahead and give it a try. Remember, friend, rocket science it ain't: the SMLE was introduced a hundred and ten years ago. You don't need a gunsmith, friend, you need a screwdriver and a hammer and a thin punch (at most). This is NOT a difficult job, really. You can do it. I know that much. And when it's done, you will laugh at your own trepidation. So do I..... every time I do another job that I haven't done before. But ya gotta start somewhere.
As to adjusting your headspace slightly, why even bother? The SMLE uses the .303" cartridge, which is RIMMED. Rimmed cartridges have MUCH less in the way of problems than do rimless ammo. For one, thing, the rim prevents the cartridge from going too far into the chamber, which is the big scary thing with rimless ammo.
So the design of the ammo itself takes away the dangerous part. But commercial ammunition sometimes is so far from military spec that it isn't even funny and, because the .303 headspaces ON THE RIM, it is possible to have a rifle which gauges "perfect" and get 6 times as much headspace as would cause heart attacks to the Americans, just from sloppy ammo. This goes to several problems, the first one being that the Americans, who write all the gun books, really do not understand headspace, nor do they understand the concept of manufacturing tolerances.
There is a truly EXCELLENT discussion of Headspace in HATCHER'S NOTEBOOK, which was written by an American General who was in charge of the Accidents Investigation Division at Springfield Armory during the 15 years when they had all that trouble with the 1903 Springfield. General Julian S. Hatcher identified and solved the problem of the falling-apart Springfields in World War One and later condemned a MILLION of them because of the problem. His book has been available since 1948, but it seems that nobody wants to read the hard parts. And that is lamentable, because Hatcher writes in a brilliant, easy style which is just SO EASY to understand. You can download your own free copy by zipping on over to milsurps dot com and taking out a free membership and then pillaging their wonderful Military Knowledge Library. While you are there, you can also download a MANUAL for your rifle (start with "Rifle - 1942") and then get some of the archival material on the older models. Also get a copy of "Shoot to Live!" which is the single best book on shooting with the Lee-Enfield Number 4 rifles, which yours does resemble to a great degree.
As to any problem with ammunition, you can cure most difficulties for one buck. Head over to your local Dollar Store and go into the Girls' department, spend $1 and get a blister-pack of 100 pony-tail ties. They are little elastic bands, just the right size for the base of a .303" round. Slip one onto the base of each round and fire it like that. The elastic will hold the cartridge back against the bolt-face and the rifle will fire-form the brass casing to fit YOUR chamber and YOUR rifle, perfectly. Now all you do is get one of those Lee Collet Dies and start handloading the most accurate .303" ammunition that it is possible to make...... and do it for less than half the price of store-bought rounds.
Go get em!
And be sure to have fun.
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