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nominal bore size

purchasing a new Kriger barrel 6mm with 1-10 twist chambering for 6br ackley improved. Shooting 88 grain burgers and 95 grain VLD's. Which bore dimension do I need? a .2360 or .2370?
 
Interesting you should ask. Danny Reever recently posted the same question to some top-tier shooters and smiths. Here's what he learned:

Bore Choice .236 or .237 Does it Matter?

Being the inquisitive sort I e-mailed some of the top Shooters, Gunsmiths, in various disciplines, and Barrel manufactures to try and determine what the current thoughts were on the matter in regard to a 6MM 8 twist barrel. Here are some of the responses:

Sid Goodling, gunsmith: I would use the .237, I think pressure would jump up a little slower, just my opinion.

Mikel Shilen, Shilen Barrels: Our tolerances are from .2365 to .2375; six or eight lands and grooves.

Tim North, Broughton Barrels: .237 is the nominal dimension we lap toward. The 8 twist works well .2365 to .2373. If you want tighter it can be done.

Bruce Baer, 1000-yard gunsmith: .236,Bruce is a man of few words!)

Kreiger Barrels: We recommend a .236 for an 8 twist. We feel it is necessary to engrave the bullet deeper to insure the best spin. Although we feel a .237 will work also. Much depends on the shooters wants.

Bruno Shooters Supply: We only stock .237 bores in 6MM Kreigers.

Michelle Sutton, Hart Rifle Barrels: The bores run from .2365 to .2370.

David Tubb: The only advantage would be if the rifling is .001 taller in the smaller diameter bore. If it were so then I would be picking it over the larger.

Daniel Lilja, Lilja Barrels: I personally checked that combination a couple of days ago. It was .2365.

Dave Tooley, 1000 yard Gunsmith: Standard size is .236. Many button-rifled barrels are larger, as big as .238. I think either one would work well.

Henry Childs: Kreiger normally supplies .236 barrels and Hart .237. I specify twist, length, and contour and leave the rest up to the barrel maker.

Bob Green, Gunsmith: This is where it is at when we talk about barrels: there are a lot of different things that make good barrels and this is one important feature of that. I would lean on .237 for my PPC barrels and there is also a range there that goes along with the groove diameter as well. If I were talking about a five groove Broughton, in an 8 twist; I would be ok dropping to a .236 range here again looking at the groove as part of the picture as well. So to say there is one size fits all type of thing; in MHO we need to look at other diameters and the number of lands and so on. Also barrel diameters can affect speed very much. Not sure if this is what you were looking for. It is a big conversation.

Lynn Dragoman: I would go with whatever Kreiger or Hart uses on their barrels.

Craig Kostyshyn, Kostyshyn Barrels: I don’t specify a bore diameter on my barrels, so I assume you are talking about Kreigers. Personally, I don’t think this difference alone is a significant determinant of barrel quality, but all else considered equal I would go with the .237 bore. Reasoning: the .237 bore is more than adequate to impart the requisite spin, but distorts the bullet and bullet jacket less, possibly providing both improved initial alignment of the bullet in the barrel, and in flight ballistics. For this reason, most of my barrels tend to have “Low” lands. Seems to work.

Moderator, Accurate Shooter .com: I asked Kreiger about that and they gave no indication that a .236 would be better, just might last a bit longer.

Jackie Schmidt: I have had great success in my 6PPC 100-200 yard rifles with the .237 bore.

So, what can we glean from all of this information. These are top shooters and gunsmiths so it’s hard to argue with success. However we can come to some conclusions.

Button rifled barrels tend to start out as .236 and are lapped toward, or over .237 depending on the tolerances of each barrel maker. It seems short-range benchrest competitors and gunsmiths favor the .237 diameters to eek out that last bit of accuracy, since barrel life is not a major concern. They feel the .237 offers less bullet distortion, which they feel, translates into tighter groups.

Long range competitors and smiths either choose the .236 diameter bore, or are of the opinion it isn’t that significant a difference to worry about. Barrel makers are concerned in staying within their manufacturing tolerances, but also giving the shooters what they want.

F class and across the course competitors prefer .236 if the other .001 is gained by land height regardless of groove depth. They feel that the extra .0005 per side translates into longer barrel life. Taller lands = better to them.

We haven’t even considered what the mysterious “Hummer” barrels might be, but I suspect it could be either one.

Where might Moly enter into this picture? Who knows?

So what’s the answer to the question? I think Bob Green summed it up pretty well when he said, “It’s a big subject.” My own opinion after all of this is “put your money down, make the best choice you can with the information available to you and find out for yourself if you made the correct choice,for that particular barrel at least.)”
 
Bama Shooter,

FWIW I've a .237 being chambered up by Bob Green right now, so in about eight months I'll be able to tell you if I made a good choice.

A more critical question for you to consider is what freebore to go with.
For the two bullets you mentioned I would not exceed .060 or you won't be able to get enough bullet in the case. The bearing length is only about .325 on the 95 grain so it could even use less freebore possibly .045 even. Once you go into the lands .015-.020,which the VLD's like) you've moved it that much further out.

Hope this helps.

Danny
 

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