Tight then loose bore

Discussion in 'Gunsmithing' started by drbill, Aug 24, 2015.

  1. drbill

    drbill Well-Known Member

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    I'm bore slugging a barrel for a gentleman. M70 stw. First 4-5 inches past chamber tight and then loose to muzzle. I dont see any significant erosion or cracking. I normally hand lap the tight spots but this is a whole area and at the beginning of the bore after the chamber. Could the bore be too big the entire length after the tight area? Can I "bump up" the lead slug farther down the bore and mic the slug to measure the bore?
     
  2. J E Custom

    J E Custom Well-Known Member

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    When I hand lap a barrel I like to cast the lap in the muzzle because that Is supposed to be the tightest part of the barrel, (When barrels are made and measured, the tightest part of the bore
    is placed at the muzzle before contouring) on quality barrels it may be less than 1/10,000 but
    it is normal to do this.

    The worst barrel I have ever seen was a mod 70 Coyote. It had 5 tight places up and down the barrel. (Most factory barrels have none and a few have one tight area).

    If a barrel is subjected to over pressure, it can bulge about 2/3rds down the barrel so it is very unusual for it to be larger in front of the chamber.

    I would recommend replacing the barrel rather than trying to lap it true. at best you may improve the groups/accuracy to 1 to 1&1/2 MOA.

    J E CUSTOM
     
  3. jfseaman

    jfseaman Well-Known Member

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    I am no gunsmith but I pay attention to details.

    First of course I would agree with J E.

    Second, is this a factory barrel?
    If factory, surprised but not much. I have enough rifles to understand the unexpected.
    If not, surprised and wondering if it was in house profiled and someone had an accident and reversed muzzle to breach
     
  4. J E Custom

    J E Custom Well-Known Member

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    After more thought about your bore problem, I would recommend that the barrel be replaced.

    There should be hardly any way you should be able to measure/feel .00001 to .00003 (+.00001 is normally the rejection point for the best custom barrel)s. and .00003 for the cheaper barrels.

    Factory barrels will fall well above this normally. (with a few exceptions).

    I would guess he is having you slug the barrel because it wont shoot.

    So If it is bad enough for you to feel the difference, IT IS BAD.

    Just my opinion
     
  5. drbill

    drbill Well-Known Member

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    Yes its a factory barrel. Winchester model 70. He had me do a OCW test on it before slugging it and the best groups were 1.5" and most alot worse. He was never able to produce groups any better. The bore is actually tight in just the first 2 inches past the throat. I bumped up a slug in the middle of the barrel and at the tight spot. The "measured" difference between the slugs is 0.00025. When you pass the slug muzzle to chamber I cant physically get the slug through those first 2" without the use of a brass hammer...
     
  6. LoneTraveler

    LoneTraveler Well-Known Member

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    Sounds to me if this is a factory barrel. I am wondering if it is a hammer forged barrel. With hammer forging it is beating the barrel blank to a mandrel with the rifling cut on it. Because of the heating from the hammering, Machine operation, Mal ability of the blank of steel it would be possible to come up with loose and tight spots all along the barrel. Would get down to money wise. Spend a lot of time on this barrel and hope it works or just replace it....... I lapped a 22 RF barrel for a friend of mine last summer (It was a labor of love to get the rifle to shoot, not a paying job.) It took about 2 weeks off and on and 3 or 4 trips to the range before I got it shooting one hole groups at 50 yards. If I had been charging for the work it would not have been worth it to him.
     
  7. Rustystud

    Rustystud Well-Known Member

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    A few years back(prior to 2006) I was almost exclusively using one Button rifle barrel maker. Had customers breaking national and international records every month. The velocities were 100-200 fps faster and the extreme spreads were single digit. Then all of a sudden, I had five barrels in a row, that were slow, shot fliers and the extreme spreads were in the 50-60 fps range. I contacted the barrel maker and he said nothing had changed and his barrels were still the best on the market. Between my customers and myself we spent thousands of dollars in ammunition and loading time trying to find a load. I set all five barrels back and re-crowned them and still poor results. The barrel maker denied having any problems, with materials and his process. I changed barrel makers and replaced all five barrels at my expense. All five rifles went to shooting one ragged hole (sub .25 MOA). I spoke to several other gunsmiths and they were experiencing the same issues. After talking to another great gunsmith about the problem he recommended that I slug the barrels since nothing was visible with a bore scope. Boy were my eyes opened. All five barrels had multiple tight and loose spots between the throat and muzzle. I know there were a couple of years barrel makers were having trouble getting good steel. But I also found out the barrel maker I was formerly using had lost his three best employees, who were previously hand lapping all his barrels. I was told he was going to the labor pool and getting day help to lap barrels. Lapping barrels is a hands on learned skill I also had noticed that the diameters marked on the barrels were never what I was pin gauging them at. I purchased 20 pins in .0001" increments (10 above and 10 below normal bore diameters) for .219", .220", .221",.222",.223",.224" .236", .237", .260", .277", .284", .297",.298", .299",.300". I found between .0002" and .0005" differences in what was marked on the barrels. Years after the problems I ended up meeting several of the employees who had worked for the previous barrel maker. They told me the barrel maker did not even have a way to accurately measure barrels, that he just put on them what he thought the bores would be based on the reamed hole diameter and the button dimensions and what would be normal spring back.
    I now use barrels made by a very reputable barrel maker. He knows what the bores are based on measurements after the bores are final lapped. He encouraged me to slug my barrels to verify what I was getting. I had a second barrel maker that I was using that had some problems, they realized they had a problem and replaced the defective barrels without question. The defect was easily identified by slugging their barrels. I will be the first to say America is blessed with some great barrel makers. We also have some that are not so great. My recommendation is for people looking for a new barrel to do their homework and look at what the top shooters are using. Talk with your gunsmith, does he offer any guarantee. With quality gunsmith work and good barrels it is not hard to get a rifle to shoot .5 MOA.

    With a good bore scope you can see some anomalies, ie: carbon, copper, fire cracking, tool marks, etc. Slugging a barrel works by feel, things you can't generally see.

    Nat Lambeth
     
  8. BountyHunter

    BountyHunter Well-Known Member

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    What is interesting out of this thread is two things.

    1. Slugging barrels works. Bill Calfee a world renownd 22 LR BR smith wrote a series of articles on this several years ago and was noted for cutting the crown of his barrels at the tight spot regardless of lengths most of the time. It really started the BR community and gunsmiths talking and looking at this with great results.

    2. I have never met or talked to a barrel manufacturer that recommended or would warranty a barrel IF someone else had hand lapped it. Barrel mftrs have guys that are artisans that do this and even they know it takes a lot of barrels to learn it. Also, almost all barrel mftrs mark/cut the blank about 1" back at the muzzle, because the best guys in the world that do this know that lapping will normally bell the muzzle a little bit and this is where you lose accuracy. A rookie lapping will run more barrels than they fix.

    There was a run of bad steel in the mid 2000s that got a lot of barrel mftrs, and I knew several competitors that bought up several blanks to store when they finally hit some quality runs of steel.