View Full Version : 6MM Bore Diameters

07-20-2008, 04:47 PM

A year or so ago I ordered a 6MM button rifled bbl from a manufacturer and specified a .236 bore. When I received the bbl, got out the deltronic's pin and began to gauge the bore. A .2364 pin fit the best and that is what I used for a pilot dia.

Now, a year and a half later, get a second bbl from the same manufacturer, same 6MM, specified a .236 bore and now a .2356 pin is required for a pilot.

Is there really that much variation in the diameter in bbls ordered with the same .236 bore?

Seems like a lot to me but my experience is limited to just a couple of bbls from this manufacturer.


Dennis Sorensen
07-20-2008, 04:57 PM
I have never ordered barrels by bore size, always by caliber, but I think it is common for the bores to vary a little... usually from breech to muzzle as well...

07-20-2008, 05:23 PM

You can specify whether you want a .236 or .237 bore diameter with most bbl makers.

Word on the street is that HP types perfer .236 and benchrest types .237.

Don't know how true that is but I do know about the different bore sizes.


Butch Lambert
07-20-2008, 07:25 PM
Did you measure before you cut it to chamber and crown?

07-20-2008, 09:35 PM

I cut 1/2 inch off the chamber end and 3/4 inch off the muzzle. Chamfered both ends and then started with the pins. For a bbl that was ordered with a .236 bore, the pin that fit was a .2356.

Seems awful small for that bore diameter. but that is what it took.


jackie schmidt
07-20-2008, 10:01 PM
One particular barrel manufacture that I know holds the bore and groove dimensions very close is Krieger. I chamber a lot od .237 4-groove Kriegers, and I have found every one to be no less than .2369 or more than .2371.
But the really important dimension is the groove diameter. Krieger tries to hold that to within .0001 on every barrel, and from what I can tell, they do it.
Bartlein also is doing a very good job at holding consistant dimensions. I have chambered up 4 or 5 for my friends, and I am impressed with the quality.
Of course, what Krieger and Bartlein have in common is both are manufactured with the cut-rifled proccess........jackie

Butch Lambert
07-20-2008, 10:01 PM
If I have fat bullets I like a .237 and with the smaller bullets .236. I have seen very little on the chrono. I think that you tune each barrel and it will be fine.
Just my opinion.

07-21-2008, 02:14 AM

With out much doubt in my mind, besides drastically effecting overall velocity between two barrels, it could have strong effects on bullet performance and reliability in terms of deformation and stress. Due to bore expansion of the barrel and centrifugal force of the bullets (baring in mind the bullet core tensile stress strength).

Happy Shooting
Donovan Moran


You 24 carat imbecile I have strong feeling that you really don't know what you'r talking about.

Shoot better

Bill Leeper
07-21-2008, 01:22 PM
I have frequently noticed a difference in bore diameters in barrels from the same manufacturer over a period of time. I always suspected this was from a change in the button. In other words, a different button was used for the two barrels. I fthe button is made with abore diameter which is a bit smaller, metal may be displaced into the button grooves and result in reduced bore diameter. The point is that the bore dimension is likely to changed after the rifling process. This is in contrast to the cut barrel in which the bore diameter is unaffected by the rifling process. I sometimes wonder if the button riflers don't become a bit lacadaisical about the pre-rifling diameter of their barrels.
I have seen a number of button rifled barrels wherein the button did not touch the bore of the barrel at all and the reamer marks were readily apparent. In these barrels the bore diameter was a bit big. I have seen others where it appeared the maker was varying the twist rate by rotating the barrel at different rates while using the same button. In combination with a starting bore diameter which was a little largish, this produced rifling which was upset to one side of the lands so the the actual bore diameter ended up a bit small on the high side and large on the low side of the lands. Kind of an inadvertent "rachet" rifling form.
Back in the day, Hart and Shilen barrels always varied a bit in bore diameter from barrel to barrel but the variation never amounted to more than acouple of tenths. At this time, Harts were typically .2361-2363. Shilens, with their 8 groove pattern were usually a bit looser in the bore at a bit over .237.
Lapping might account for a very slight variation but it couldn't amount to much. Lapping is usually done to produce the desired surface finish rather than to alter dimensions. Regards, Bill.