I'm not all that convinced that the diameter being 311 or 312 or larger is all that important.
When I went from a 312 Hornady to a 311 Speer because I couldn't find anything else, I was a bit surprised to see my groups get half as small. I didn't record it, but the groove diameter was more than 311. I was told that in my two groove 308 bullets would work just as well. I tried it and I don't see the issue.
Cast bullets are a whole different thing, but jacketed bullets just ride on the lands and a bit contacts the bottom of the groove.
I've no experience with plated, hopefully someone has been able to run comparison tests, and hopefully they will chime in.
I'm no expert on the 303. I only have one load:
Calibre 303 British
Powder Varget Weight in Grains 41
Bullet Speer HC Weight in Grains 150
Primer WLR Loaded OAL 2.89
Case W-W
My control AMMO is 1981 South Africa FMJ Military. Fairly accurate stuff at near max.
Nitro
When jacketed bullets first came out, the manufacturers of those bullets were quite used to the problems with ''obturation''
They were also familiar with the issues of "leading'' from the cast bullets they had been using over black powder, even though the bullets were lubed.
They tried to get around this with different lubes and also with paper patching the cast lead bullets, which was quite effective.
When they went to jacketed bullets, they were very careful to make sure the bullets would obturate into the rifling sufficiently to keep the bullets within specified accuracy parameters.
They found a very simple solution.
Make the jacket in the form of a cup and force in a lead center, leaving the lead "exposed'' at the base to assist the resulting pressures in forcing the lead into the cup and expanding the jacket enough to fill the bore grooves, which won't happen if the bullets are to small.
Early rifles are notorious for having bore diameters that can vary from one barrel to the next by as much as +or- .005 inches. They had to have a reliable method of making sure the bullets matched those varying bore diameters and exposed lead bases was the reliable method of choice.
This was a pretty common way to make military as well as commercial ammunition right through WWII and into the mid seventies.
I have a bunch of .312 diameter boat tail fmj ammunition made in spain, dated 1965 on the boxes, with exposed lead bases.
This ammo was made to shoot through the M93 conversions and their CETME rifles. The bores on those rifles were anything but consistent in diameter.
