Bore Size vs Accuracy

Discussion in 'Technical Questions & Information' started by CountryGunsmith, Feb 24, 2003.

  1. CountryGunsmith

    CountryGunsmith New Member

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    Mauser45
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    (1/5/03 7:30:40 pm)
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    ezSupporter
    Bore size vs accuracy
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    I started a post yesterday about barrel lapping and I opened up a can of worms! Been measuring the bores sizes on a couple of my guns and I find that the dimensions ain't what they should be according to generally accepted dimensions. The rifles are an SKS (Norinko),a 6.5 Swede and a 7.7 Jap. All of them are pretty good on the bore diameter size but way oversize on the groove diameters by .002"-.003". The SKS will shoot 3" or less groups with federal ammo. The Swede will shoot 1"-1 1/2" groups with most handloads, or better. I have gotten a couple groups from the 7.7 that were less than an inch at 75 yards ( original sights). I realize that the bullets will "slug up" in the barrel to fill in the groove dia. (Maybe,or maybe not enough?) What is the most important barrel dimension in regard to accuracy? For sure, not all barrels are match grade within a couple of ten-thousands of an inch yet they can shoot awfully good! Does all of this mean that you really only have to have either the bore diameter or the groove diameter good in order to get acceptable accuracy?

    The MORE I learn, the LESS I know !!!

    kdub01
    *TFF Senior Staff*
    Posts: 1702
    (1/5/03 9:55:48 pm)
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    Now, beleive me - I'm far from being an expert on barrel manufacturing and tuning for accuracy!

    Military rifles are assembled enmass by the hundreds of thousands with "minute of man" accuracy being completely acceptable standards. The sights are designed for combat use - sturdy and simplistic. The barrels are chucked up and spit out as fast as possible to drill, broach, turn and thread. To have any military rifle in it's original form shoot 4" groups and less at 100 yds is an accomplishment.

    The first thing on barrel accuracy is, of course, being straight and with the best cut - broached - forged - rifling with the appropiate rate of twist for the bullet weight desired. Then, the whole shebang must be shaped (turned) with uniform taper and leaving a tube of moderate or heavy weight. Chambering must be perfectly aligned with center of bore and carefully reamed and polished to desired dimensions. Some folk prefer long throating (free bore), some want the rifling to be just short of engaging the ogive of the preferred bullet. Barrel threads must be carefully cut square and true. The matchup with the receiver must be perfectly aligned with the center of chamber and bore. Sights, either mounted on barrel, receiver or both, must be perfectly aligned with the centerline of the bore.

    As far as the land or groove diameter being the most important, I would have to say the groove diameter would be the most critical. When measuring bullet diameters, this is the measurement that you go by. Too small diameter of the lands will cause deep engraving of the bullet and probably some increased resistance to travel, but too tight or too loose groove diameter will certainly cause either blowby or greatly increased resistance through swaging action.

    Yes, bullets of a softer metal will "bump up" to fill grooves a few thousands overbore and make a satisfactory seal of propellent gases. Overly hard jackets or metal will not do this. Most premium barrels are air gauged to assure dimensions are held withing +- .0001 to .0002 the length of the barrel. The standard barrel on a factory produced commercial rifle will allow for +- .002 to .003. tolerances.


    "Keep Off The Ridgeline"


    Mauser45
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    Posts: 25
    (1/6/03 4:24:48 pm)
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    ezSupporter
    Re: Bore size vs accuracy
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    Kdub01, Looks like I may have to prove to myself that the bullets in my particular rifle are really filling up the grooves. When I get the time I will have to try to recover some bullets to measure them and find out how much they have expanded. It's tough to do, since the base of the bullet often gets bulged by the impact. So much depends on how tough the bullet construction is, and how much oversize the groove diameter is. I do know that the Barnes X bullets will not upset very good to fill the bore. I shot some of them out of my 6.5 Mauser into a stump. After chopping them out of the stump, you could clearly see "burrs" sticking up beyond the outside diameter of the bullet, on the edges of the rifling grooves. To me, this indicates that the bullet definitely did not "slug up" to the groove diameter. I have to agree with you that the groove diameter is probably the most critical since bullets are made to match this dimention. I don't think that the bore dia./rifling depth is as critical as long as everything is made to a uniform dimension. I think that the best I can do is to lap the barrels until they are as uniform in dimensions as I can get them throughout their length. No tight spots or taper. Should be an interesting project! If I mess up any barrels, I'll have an excuse to buy another gun!

    kdub01
    *TFF Senior Staff*
    Posts: 1717
    (1/6/03 6:26:02 pm)
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    The last time I slugged a barrel, there were 3 loose spots and 2 tight spots. Have no idea how long it would have taken to get a consistent diameter bore from end to end. The story is a long one - short version is a gunsmith installed the wrong barrel to my action. Was supposed to be 6.5 cal and he put a .257 cal on it. Swagged bullets for 2 years before stumbling onto the fact the bore was .007 too small. Wonder what those recovered bullets would look like!

    You can bet I was lapping the bore with each shot fired! Had pushed several hundred rounds downrange before finding the gremlin. The bore STILL had the tight spots.
    "Keep Off The Ridgeline"


    Mauser45
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    Posts: 40
    (1/19/03 7:45:24 pm)
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    ezSupporter
    Re: Bore size vs accuracy
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    Kdub01, I can sympathize with you about the undersize barrel! That would really tick me off!! I imagine you were getting excessive pressure signs even if the loads were mild. Just out of curiosity, how good/bad was the accuracy?

    kdub01
    *TFF Senior Staff*
    Posts: 1843
    (1/19/03 8:56:52 pm)
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    Finally, through manipulaltion of bullet type/ weight and powder type/weight, plus primer brand/type, was able to acquire approximate 1.5" groups. Only problem, the shots in the group would establish their own groups - 2 together here, 2 other over there. I tried free floating, solid bedding, bullets seated to lands, bullets seated to clear the magazine, etc, etc. Was ready to toss the rifle in the trash can when I decided to mike it and found the true culprit.
    "Keep Off The Ridgeline"
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