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Old August 17, 2008, 02:43 PM   #1
Wildebees
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Barrel rebore

I am not a gunsmith. It does however appear to me that a slight rebore, say 6mm to 6.5mm may be more challenging than 6mm to 7.62.

If I am wrong, please tell me why. It shall ease my mind as I plan to do a 6.5/303 Epps, possibly from a 6mm Musgrave.
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Old August 17, 2008, 03:33 PM   #2
wjkuleck
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In order to rebore, the major diameter ("groove diameter") of the original bore must be smaller than the minor diameter ("bore diameter") of the new bore.

For example. In the USA, a .300" bore diameter is accompanied by customary groove diameter of .308". Therefore, any rebore must be to a bore diameter of at least .308" in order to "clean out" all of the original .308" grooves.

In practice, one generally must go to a larger than .308+" bore diameter, because of the tolerances of one's boring equipment and the original groove diameter. For example, one should be able readily to rebore a .300" bore to, say, a .323" bore (8mm).

0.5mm equates to about .020", so it would appear that a 6.5mm bore would clean up the grooves found in your 6mm bore. However, please confirm that the new bore diameter proposed will "clean up" the old bore in any specific case, and that your boring process can pilot straight down the old bore. Any offset would of course bring these calculations under suspicion.

I hope this helps.

Regards,

Walt
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Old August 17, 2008, 09:59 PM   #3
James K
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The common measurements for a 6mm are .236" (5,994mm) bore, .243" (6,172mm) groove.

For the 6.5mm, they are .256" (6,502mm) bore and .264" (6,705mm) groove.

Since the 6mm groove is smaller than the 6.5 mm bore, a 6mm barrel should clean up to 6.5 mm.

Jim
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Old August 17, 2008, 11:34 PM   #4
Scorch
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In reboring a rifle, it is generally assumed that .015" greater than the original groove diameter will clean up a bore. This removes any erosion, heat checking, etc, and allows for a rifle's bore not being truly straight. You are looking at having your rifle barrel bored .013" greater than the existing groove diameter. It should clean up adequately for a rebore to 6.5mm. I have seen 220 Swift rebored to 6mm Rem and 250 Savage rebored to 308, and in both cases they cleaned up beautifully.
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Old August 19, 2008, 04:17 PM   #5
Wildebees
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All the good advice

Thank you gentlemen,

All your kind postings were the exact technical considerations I needed. I think the local barrel manufacturer seems also to agree that he can safely rebore a 6mm (his stock) to perfect 6.5 for me.

I appreciate your inputs - great pleasure to be part of this forum, real good folks here.
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