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bore dimensions

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Unread 08-24-2007, 01:33 PM
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Join Date: Jul 2001
Location: Walla Walla, WA
Posts: 174
bore dimensions

I have heard that for best accuracy a bore should be smaller at the muzzle than at the chamber end by a bit. Like for a 30 caliber maybe the bore diameter would be 0.3000 at the chamber end and 0.2998 at the muzzle end. If it is reversed tapered supposedly accuracy suffers. I would like to know more about this. What are the premium barrels supposed to be measuring and how big of a deal is it if the bore diameter increases a bit instead of decreases? Thanks, Rufous.
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Unread 08-24-2007, 01:53 PM
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Join Date: Aug 2003
Location: NC, oceanfront
Posts: 4,227
I think everyone agrees that an opening bore at the muzzle kills accuracy potential. It would be fantastic if someone with a 'Hummer' barrel could have it carefully examined in this regard.

But I have never seen credible values for inside bore dimensions throughout a barrel's length. You'll get "consistant from end to end, with the muzzle end tightest". "Our barrels are super-Lupdy-Duper at .0001 from end to end". But that is not a measurement at all. That's comparative guessing with an air gauge type BS.

I personally wish I knew of an instrument capable of actually measuring groove/bore dimensions.
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Unread 08-24-2007, 02:33 PM
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Join Date: Jun 2007
Location: Wilmington NC
Posts: 4,787
For those that take the Precision Shooting magazine, Bill Calfee, 22LR BR gunsmith of the GURU level has been writing a series of articles on that subject over the last year or so.

He talks about busting old myths on barrel dimensions and what makes a truly shooter barrel. Bottom line is that he is a strong advocate (understatment) of the requirement to "slug" barrels with lead slugs to determine high and low spots and that the muzzle absolutely must be tighter than the rest of the bore. With the slugs he determines the exact spot for the muzzle regardless of blank length for pure accuracy.

As for a reverse taper, think of the blunderbuss. Not exactly known for precision accuracy. Now that is an extreme example, but perfectly appropriate.

That is also the major reason after mftr lapping rarely works and often hurts the barrel accuracy. The lead lap normally causes an increase in muzzle bore diameter, requiring you to cut off a minimum of 1" to remove that enlargement.

Also what has become more prevalent lately is the fact that different bullet require different bores dimensions for best accuracy.

for example, it has always been known that Hornady bullets are a little fatter and if nothing else shoots, they tend to shoot in loose bores. top 1k shooter recently built a couple new guns with Broughton barrels (bore .30795) and the new 208 AMAXs would not shoot in them at all. After measuring the bullets and talking with Tim North at Broughton, they discovered that they were just too tight in a Broughton. Rebarreled the guns with Hart barrels (true .308) and now shooting like a house a fire. Not bad barrels but bad bullet/barrel mix.

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