Zeroing your issue L1A1 SLR (Self Loading Rifle) in the British Army as I remember it from many years ago...
Take your trusty L1A1 and 25 rounds of 7.62x51 NATO ball ammo. A 100 yard range. Fully load a 20 round magazine. Slide the sights to the "300" setting.
Assume the prone supported position. This is either the low prone or UIT (Union International de Tir) prone position as per your preference, with your support forearm resting on a sandbag (no sling). No part of the rifle touches the ground or the sandbag. I always used UIT prone having been a .22 target shooter in my youth.
The Range Officer (RO) takes you through:
You now set the safety to "fire" and fire five (5) rounds, attempting to achieve as small a group as you can.
RO checks everyone has stopped shooting. He orders `Stop,' you apply the safety catch and lay down the rifle. RO would then say something along the lines of `Stand up and dress back.'
You now stand up and step back from the rifle.
At this stage the rifle has 14 rounds in the magazine and one in the chamber and the safety is set to "safe." Nobody goes forward of the firing line while there are live weapons on the line.
RO then says `Take your weapons' which is your cue to readopt the prone supported position. When he's satisfied you're in place, he will say, `5 rounds grouping... In your own time, go on."
You do this, firing five rounds groups prone supported and getting up between each string, until you have emptied the 20 round magazine. Getting up between five rounds strings means you have to be able to adopt the same alignment to the target every time, despite moving (as Jeff Cooper used to say, the sights are there so you can check you're aiming correctly).
When you go forward to examine the targets, it will be obvious whether you can shoot straight or not. A four inch (4") extreme spread 20 rounds group at 100 yards was considered adequate to achieve an acceptable zero. I saw people who could get MOA (Minute Of Angle) groups doing this, even with well worn-out L1A1s... sadly not me. You don't want to see four distinct groups, however small they are. You should have one group of 20 rounds.
You then adjust the front and rearsight to put the group centre one and a half inches (1.5") above POA (Point Of Aim) at 100 yard range and the sights slid to the 300 setting. I don't ever remember figuring out the rationale for that but that's the way we did it. You could hit anything out to 300 by just pointing at the middle of it with the sights set at 300 anyway, seemed like.
You then fire a further 5 rounds group to check your sight adjustment.
That's it. If you didn't get a good zero with the 25 rounds then that was because you couldn't shoot worth a damn.
Ideally, you would also fire a 5 rounds group in various positions, e.g. kneeling, standing, sitting (rice paddy prone was OK instead of kneeling except in shooting competitions) to see how you had to adjust your aim (Kentucky Windage) to account for the different shooting position.
In theory also from the left shoulder but I don't remember ever actually doing that, except shooting round corners with blanks in training exercises.
For standing, we were taught to use the palm of the hand supporting the magazine with the elbow resting on the mag pouch. I don't know if that was "The Way" or just how we did it. When I did a Small Arms School Corps course it was all prone shooting.
We never used the shooting sling. Apart from the fact a properly tied up sling would run the risk of bending the barrel, it made Immediate Action Drills, even changing the magazine, problematic and many shooting competition exercises meant you would need to clear a stoppage quickly or you were out. I did try using a sling a couple of times but it really was more trouble than it was worth with an issue L1A1, even though I was well-used to the shooting sling from my target rifle days. (I think not being able to use a sling is a real drawback of the L1A1 compared to say a Garand/M14 type rifle.)
When I got in the shooting team the armourer fitted a Hythe Sight to the rifle I used. I'm not sure it really made a difference but the theory was that the smaller rear aperture made more precise aiming possible. (An armourer who had to fix the gas tube, that had shot loose, for me at a shooting competition told me the rifle I was using was worn out.)
You generally didn't have to dick around with the gas settings but it was believed that would alter the zero. AFAICR (As Far As I Can Remember) they generally functioned on 5 or 6 or you considered something was wrong with the rifle. Though I may be confusing that with opening the gas port 6 clicks when you start balancing the GPMG (General Purpose Machine Gun)... but that's a whole other story, of course.
We did do dry-fire type training, working in pairs using aim correctors, drill rounds, and having your buddy operate the cocking handle to simulate semi-auto fire, with stoppages thrown in. You should always hear the click of trigger resetting in deliberate fire... if you don't know what I mean by that, you need to work on your trigger control.
Anyhoo, that's as I remember it. E&OE (Errors & Omissions Excepted), your mileage may vary.
Vote from the rooftops. III