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Adjusting free bore

Discussion in 'Maintenance & Gunsmithing' started by SJX426, Sep 29, 2011.

  1. SJX426

    SJX426 Aloha - Or New Member

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    I have a Mauser action with a custom barrel in 338. I measured the free bore and found it to be nearly .125! No wonder it won't shoot consistently! Well, that may be one of the reasons anyway.
    Is there a possibility of removing the barrel, turning the face, running a chamber ream and remounting? Of course the head space would need to be checked.
    No sights on the barrel.
     
  2. Straight Shooter

    Straight Shooter North Bend OR Active Member

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    Setting it back one turn and rechambering is a simple task. Setting the head space is part of the chamber job.

    But, does it need it? The SAAMI specification for the 338 Win Mag throat lead or free bore is .2147". I can't think of a single center fire cartridge that truly specifies zero free bore. A true zero free bore reamer is going to be a custom reamer. Is the caliber stamp on the barrel indicate "zero free bore"? If the .125" is just the distance from your seated depth to when the bullet your using makes contact that number varies by each different bullet due to the different ogive profile for that bullet. You could try a fatter looking bullet. One of my most accurate factory rifles I own has about .450" for the bullet I use. All a zero free bore chamber is going to give you is the ability to touch the lands with light bullets. Most people specify longer free bore or throat in order to seat VLD style bullets farther out of the case to increase case capacity. Berger and Barnes are but a few bullet makers than recommend a certain amount of "jump" for max accuracy. In a sporting rifle it is inadvisable to touch the lands as this can cause unsafe pressure spikes or stick a bullet in the barrel if you try to extract a live round spilling unused powder all through your chamber and action. This could end a hunt right there. Seating to just touch the lands requires a specific seater that registers on the ogive in the same place as your ever changing lands begin. Then a tool to measure each bullets ogive length much like measuring head space by the datum line in a chamber with a head space gauge to sort the bullets by ogive length. On guns I can reach the rifling I set my seater 1/4 turn down from my jam length and on guns I can't I usually seat as long as will feed reliably or deep enough to hold the bullet.

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    Last edited: Sep 30, 2011
  3. Jamie6.5

    Jamie6.5 Western OR Well-Known Member

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    Listen to Straight Shooter SJX. He knows whereof he speaks.
    If there's room in your magazine, seat the bullets out farther to reduce jump. But I wouldn't reduce it to zero either.
     
  4. SJX426

    SJX426 Aloha - Or New Member

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    Thanks for the education and "calibration", Straight Shooter! I should do a little more reading! My intent was not to have 0 free bore for all the reasons listed. I read that the optimum free bore was .010 to .015, .2147 is a huge difference. I would guess it scales with caliber size. The accuracy issue must be related to some other contributors.
    BTW: How do you determine free bore from the above drawing? Is it the 2.7515-2.5368=0.2147? My approach would be to take the max dimension of the cartridge of 2.75 (distance from the end of the shell to the bore diam. of the bullet, .330) subtracted from the chamber length to the same bore diam. .330 of 2.715 which equals 0.035. Have I just defined the free bore range recommended by SAAMI of 0.035 to 0.2147?

    Oh boy, another question! Do the dimensions of the part in the die equal the diam. of the bore? In this case, does the part in the die equal .330 in order to represent the chamber specs? Maybe it just doesn't matter.
     
  5. Straight Shooter

    Straight Shooter North Bend OR Active Member

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    The quick and simple approach to getting to the magical .010 to .015 jump is as follows. Resize 3 brass. No primers or powder. Now just seat your bullets about .100" into the neck of the brass. Now chamber and extract all 3. This will take some pressure depending on your brass neck tension. A VLD type bullet may lodge in the barrel. If it does just have either a 1/4 brass rod to drop down the barrel to get the bullet out or a wood dowell. If the bullets aer sticking let me know. Usually they don't. After we close up the gun on 3 dummies this will give you what I call the "jam length". Take a measurement of all 3 and get an average jam length. Now back your seater plug all the way up in your seating die. Carefully close up your press on the most average of the 3 dummies. This is the master dummy and should be painted up with a sharpie and set aside for future reference. One the ram is all the way up thread your seater down on top of the bullet. Snug it pretty hard with just two finers. Now go ahead and load up say 5 rounds uing this seating depth. Now measure and compare to your master dummy to the 5 you just loaded. Most dies are 16 threads per inch on the seater. This means your seater will travel down .062 per revolution. To get .015" turn your seater down 1/4 turn. 1/16=.062 .062"/4=.0156" or about .015" You can see where this is going. If you want .005" clearance only go 1/3 of a 1/4 turn. Now reseat all 5 to your target depth. Measure and adjust as needed to get what you are looking for. You might even do a ladder test with different seating depths. I wouldn't fool around with real minute changes. Try .010" Then .030". Make big moves untill you get out to .150" You should find at least two settings that stand out as an improvement over the rest. Then you can go back in between those two and rerun the test in smaller increments. That said this is the very last step for me when looking for accuracy. Actual bullet choice and powders will make much larger impacts to your group sizes. For Weatherby's I seat max oal to feed. All others are either 1/4 turn from the jam length or deep enough to feed. This has worked well for me for 35 years or so. After a couple hundred rounds make up a new master dummy using the same brand and weight bullet. Remeasure and compare to your old master. It's interesting to what the throat grow over time.

    I have a 22-250 that I used to seat 1/4 turn down. 6000 rounds later I can't get anywhere near the lands with the bullet in the case so now I just seat to the magazine. It still shoots sub 1/4 moa groups so I don't worry about it.
     
  6. SJX426

    SJX426 Aloha - Or New Member

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    Thanks for the explaination. I did a "jam Length" but for only one. That is how I determind the free bore dimension. I should keep better record of activities. I was planning on doing what you suggested in terms of different seating depths. It would be a challenge in a number of ways, first is the punishment of the recoil!

    I checked the bedding yesterday and determinded that it is an unlikely contributor to the accuracy issue. It is fully bedded, which is contoiversial, but appears to be well done. It has Leupold or Redfield low mounts which apear to be solid. The scope is an Weaver V7A. I might try using one of my Leupold scopes to see if it impacts groups.
     
  7. Straight Shooter

    Straight Shooter North Bend OR Active Member

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    Recoil in itself is a major factor in accuracy. There is a lot that must be done behind the wheel of a rifle to consistantly shoot small groups. If you can't concentrate beyond the *** kicking you are receiving then you are wasting ammo shooting at paper. A 2 MOA rifle will kill all the deer you point it at to 500 yards. What are you trying to do with it?
     
  8. SJX426

    SJX426 Aloha - Or New Member

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    Any time I think I start flinching, I quite or do a dry fire to check. I would like to know if it will shoot straight or if it needs attention somewhere. I have a number of long guns and I believe I am consistant with the givens I have. This one is way out of line compared to the others. The others include 243, 30-06, 7mm Mag, so there is a variety of calibers that I use a reference to my abilty to shoot.

    So I checked a couple of features. I used a round that measures 3.307 OAL for these pictures. Clearly there is some room for increasing the length and still get it to fit in the magazine.

    I will set up some rounds based on your suggestions for my next outing with it. In addition, I will determine if I have a 1 pc base I can use in stead of the 2 pc in place now. I will also identify a scope which I know is good and can handle the recoil.

    Since I have two 338's, I would like one that shoots well. The second one is not currenty availalbe. It is a pre 64 M70. If they both shoot well, I will sell the Mauser and keep the M70. How many 338's does a guy need? OK, "need" is not the right term.

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