I have never chambered anything in a 6mm caliber hence, never bought a 6mm barrel blank. In looking at many barrel websites 6mm come in either .236" or .237" bores. No other barrel diameter gives a choice. I am assuming that a typical factory rifle has .237" but what's up with the .236"?

I believe it's usually .236

I also believe the .237 is some more snake oil.

This is a new issue to me.So I checked it out with what I have. I'm looking at a new, unchambered Bartlein #4 SS fluted barrel right now marked .237.

.236" is the standard. Bench rest guys started buying the .237". Most believe you buy .236 for fast twist and .237" for slower. It's the same as number of lands, one has no advantage over the other.

GAP built me a 6 creedmoor a couple years ago - George said they use a .237 instead of a .236 because they had better success with the .237. And that was a 7.7 twist bartlein 5r.

I wonder why this is only an issue with 6mm?? The other calibers don't give you a choice.

6mm barrels carry a lot of clout because they are the undenied kings of BR. And those guys don't order one barrel at a time they order a quiver.

30 cal barrels have a similar option. Slight changes in both bore and groove diameter. The Bartlein Calibers page lists particulars.

Obermeyer also several options for more calibers. He has a .234 bore diameter for 6mm barrels, and a .298 bore diameter for 30 cal barrels, among other choices for other calibers.

From Obermeyer's page:
What are your typical bore diameters, and what are your installation recommendations
regarding reamers?

The installer needs to be able to pilot his reamer with respect to the dimensions of each barrel. I recommend a removable-pilot reamer so the installer can also work with conventional diameters standard to other rifling processes. (In addition, some other barrels are simply manufactured large, and the removable pilot allows the installer to correct his reamer to work on them.) It should also be noted that pressure does not jump simply because the land diameter is tighter; it is the cross-section area of the entire bore that counts. My 5R system often results in a larger cross-section
area than many shallow-rifled, wide-land conventional barrels. Special bore sizes associated with various typical calibers are as follows: .217" (.22 caliber center-fire), .234" (6mm), .254" (6.5mm), .274" (7mm), .298" (.30 caliber), .300" (.303), .311" (8mm), and .328" (.338). Pilot sizes are calculated by subtracting .001" from the bore diameter.

Great post. That's what I find interesting.

No wonder all those oversized groove barrels we built back in the 80's and 90's were so easy to work with.

Dennis,

John Krieger talks about the two 6mm .236 or .237 land dimensions here. He suggests it has to do with the diameter of the bullet being fired. according to John Bergers are supposed to work better with the .237.

I have limited data. I used two Bartlein 5R .237 barrels. One barrel is 1 in 7.5" twist 6mm-284 the other a 6 BR 1 in 8 twist. Both shoot the 105 hybrid with excellent accuracy. The 6-284 shot the 107 Sierra as well.

https://www.youtube.com/watch?v=_-5kPxE4gSY

I watched it for 2 minutes and then I got a headache from all the horrendous background noise.

With the exception of the palma barrels, I don't find any barrel maker's website giving straight up options on bore size other than 6mm.

Kinda like the 5R stuff. Barrelmakers produce what people ask for and not necessarily any better. I've shot 236 and 237 and can't tell a difference. Same with 5R, ratchet, 3,4,5,6, and 8 groove barrels. I've also shot polygonal barrels. If they are good, they are good.

If you would have stopped there you would be 100% correct.

Show me proof.

Exactly

I ordered a Shilen in 8 ratchet and a Schneider in 8 5P as he calls it. Neither one have an option for bore diameter. Looks like I am going to have to buy a bunch of pilots for the reamer to make sure I have it covered. I will see which barrel shoots the best too!

Have used several of the Shilen 6 ratchet rifled barrels, they were closer to .237" than .236" but several pilots are necessary, never tried a Schneider in 6mm. keep us posted

btw: why the 8 over 6 ???? what cartridge ???

Just did a quick check of some of my Shilen barrels, my ratchet barrels are 4 groove and my standard Shilens are 6 groove, did'nt know they made a 8 ratchet