Which caliber is the closest to 6mm?

.243 Win
6mm Rem
6mm BR Rem
6mm PPC (Sako)
244 Rem
243 WSSM
240 Weatherby
And a host of benchrest & wildcats.

All use the 6mm/.243" bullet.

rcmodel

I don't understand what you mean, 6mm as in actual bullet size, or 6mm as in 6mm remington, the cartridge? If the cartridge, the .243 WSSM is ratically identical in ballistics, about 10% higher than the .243 winchester. for bullet diameter, there is no actual 6mm equiv.

RCModel is correct. However....

... the .243 is the only one for which rifles, action types, and ammo are abundantly common. Fortunately, the .243 is better than the others (except for the .240 Weratherby).

He may be splitting hairs, i.e. .243" = 6.1722 mm.
Still, to his question, .243 is "closest" to 6 mm, but he said exactly, or just over. .243 is just over.

6 BR, 6PPC, 6XTC, 6 BR Norma, 6 Improved, 6 AI etc etc... hell even a 6-.338 is close.
Depends on what you want.

The 6.5's are not bad either...

244 Remington IS a 6mm Remington, just the original name for the 6mm Rem.

I think the OP is looking for an economical version on the 6mm Rem, which would be the 243 Winchester. The 6mm is slightly faster, but the 243 has stood the test of time and does what most hunters need to do.

I thought the OP was asking an abstract question, what is 6mm in inches. I think the answer is about .237.

millimeters to caliber: .x (where x is the millimeter size) divided by 2.54

So .6 / 2.54 = .236

Note that bullet sizes often differ from their official designation, so be sure to know the actual size of the bullet.

So, really, the .236" bullets Ackley was messing with was the closest thing to actual 6mm.....wait, i ckecked on my unit converter, and it is .2362, so .236 is (pretty much) exaclty 6mm.

EDIT: dang it, mike beat me to it.

Some other surprising findings when playing with the converter: The .257 Roberts is actually a tad over 6.5mm, yet we call it a .25. The .260 Rem at .264 is actually 6.7mm. The .270 at .277 is a tad over 7mm, and the 7mm-08 at .284 is 7.2mm. And isn't the 6.8spc a .277 bullet? If so, like the .270 it is just over 7mm, not 6.8.

And 7.62 is actually .300", it is because the bullet gets crushed (only by the the rifling, not acutally crushed) to the stated bullet diameter.

As a handloader, the 6mm Remington gives you more to work with than the .243.

Some calibers are named for bore diameter (.30-06 or .300 Weatherby Magnum) and others are named for groove to groove diameter (.308 Winchester or .264 Winchester Magnum). Jacketed bullets are generally manufactured to groove diameter. Most modern rifles are designed so that bore diameter is .008" less than diameter from groove to groove.

You can really give yourself a headache trying to make sense out of caliber designations. It is not that there is no system, it is that there are several and you have to know which you are looking at. Not helped that Internet Usage is kind of sloppy and many posts say "bore" where they should have "groove." Plus the best and worst efforts of the advertising agencies. Best to just learn most of them by rote.

Horrible example, .257 Roberts has that groove/bullet diameter but the .256 Newton has that bore diameter and shoots .264" bullets like the 6.5mms.

As to the OP, if he wants a true 6mm bullet, there are some out there. The first I read of was known as the "23 and a half" = .235, but I think most of the ones left are .234"; which is not quite 6mm but is as close as you are going to get. Not inexpensive, though. Cheap requires a .243 Win.

What, nobody mentioned the 6mm Lee Navy?
The opposite of common.
Back during the late 1890s the newspapers could never figure out how to refer to it in inch size since their American readers would not have known much about the metric system back then.

Of course, Mr. Watson, you are quite dead on. What are the .234 cartridges? And bullet sources?

Schroeder makes (or made, the last mention I could find) .234" bullets and Pac-Nor makes .234" barrels. I do not recall what case the .23 1/2 was based on.
Pac-Nor has a reamer for .234x.270 which sounds WAY overbore. I think a .234x.243 would be well balanced.

25-06 isn't too far off.

Yes it is.

rcmodel