With regret, I am completely lost.
I will restate the facts as I know them and that will be about as much as I can do.
I own an Ishapore Enfield chambered in (American).308 Winchester (7.62 Nato) whose projectile(bullet) diameter is .308
I understand the British .303 projectile (bullet) is .311 diameter and cannot be fired in my rifle.
That said, in the Speer Reloading Manual for Rifle and Pistol #12 in addition to the .311 diameter projectiles (bullets) to be loaded into the .303 British case, there is also a propriatory projectile(bullet)- the Speer Plinker(tm) in diameter .308 on page 330 also to be loaded into the .303 British brass case.
I am unable to find any rifle projectile (bullet) diameters in .309 or .312
Those diameters (.309 and .312) seem to be considered pistol calibers and are listed under 32 S&W Long, 32 H&R Magnum etc.
I have no explanation for any of this, I state the facts as they exist in my corner of the universe and as they can be found in the Wikipedia Article on Caliber.
Best regards,
There is a small RANGE of acceptable bullet diameters for a given bore diameter. Just because a given bullet manufacturer makes a .308 bullet for a 30-’06 doesn’t mean that the original INTENDED EXACT bullet diameter for a 30-’06 rifle was exactly .308” in diameter.
In fact, it wasn’t.
The 30-40 krag, the 30-’03, and the 30-’06 were originally spec’d to .3085” diameter. The 307 winchester is .309” diameter. The 300 Holland and Holland magnum is .309” diameter. THe 308 winchester is .308” diameter.
And each of these cartridges had a RANGE of acceptable bullet diameters. Also, the bore diameters of one rifle model are going to vary a small amount from one rifle to the next. I really don’t know off hand what these acceptable ranges or variations were for the barrels or the bullets.
To a handloader, all these cartridges are going to be reloaded with the exact same bullet...308” dia.
The other “foreign” thirty calibers in the second list I gave you are all generally reloaded with bullets that are .311” diameter...or at least bullets that are advertized as being .311” diameter. But a 303 british was originally spec’d for a .312” dia bullet. THe 7.7mm jap was a .313” dia bullet(I think). But there could very well be some slight differences in “recommended” bullet diameters from one bullet manufacturer to the next. ie, speer may not agree with hornady.
You can safely put a slightly smaller bullet in a cartridge meant for a larger diameter, but accuracy will suffer. For example, if you use .308 bullets in russian 7.62x54R ammo, you are not risking damage, you are just risking poor accuracy. THe russian round is generally recommended to use the same .311 bullet that is recommended in the 303 british. Even though the russian was originally spec’d for a .310” dia bullet and the 303 british originally spec’d for a .312 bullet.
What you don’t want to do is put a .312” bullet in a 308 winchester case. THat bullet is too big for a 308 and will cause chamber pressures to spike to potentially dangerous levels. And if you try to compensate by using less powder, you could end up with a squib.