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Thread: Bullet jump distance

  1. #1

    Default Bullet jump distance

    Just wondering how big of an accuracy difference others have found by changing seating depth. I have seen some improvements with changing it but not huge. Also on cals like 7mm-30cal what distance do some of you finally settle on assuming the magizine allows it.

  2. #2

    Default

    In some rifles it can make a huge difference for me. My 7mm Rem Mag (mod 700), 7x57 (Ruger #1) and 284 Win (Savage 99) all want really minimum jump for best performance.

    One 30-06 (Mod 700) wants it close, while another (Ruger #1) doesn't seem to care. My 308 Win (Savage) doesn't care, but both my son-in-law's 06 (Ruger M77) and 300 Win (Ruger M77) want minimum jumps.

    I messed around some with both the Weatherby 257's I had years ago, seeing how much effect closing the gap would have, in the process defeating their built-in lead and disregarding magazine restrictions. Both actually shot better when loaded to normal COL, allowing for the typical long jump. Made me think Roy knew what he was doing on that one.

    As a matter of course, I start with all new handloads and rifles having a minimum jump within magazine limits, then only go looking for alternatives if I'm not happy with the results.

    My notes aren't good enough to speculate, but bullet shape and orgive may have some influence on the deal, but I simply can't prove anything one way or the other.

    Good question.

  3. #3
    New member George's Avatar
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    Default seating off the lands

    Yep... what BrownBear said. It's hard or impossible to predict for sure. Some rifles like very little jump and with some it doesn't seem to matter much, within reason. I have one rifle that is best at about .007 jump and another that does fine with bullets seated anywhere from .010 to .030-.040 off. For reliability reasons in hunting rifles I like to start at about .015-.020. In the case of match .22rfs most are reamed where the bullet actually engraves into the lands when chambered. I've found the soft cast or swaged pp pure lead bullets in single shot, BPCR type guns shoot best if the bullet is seated out enough to engrave a little into the lands. In the case of the 22rf or the BPCR loads shooting soft cast or pure lead bullets, pressures won't get squirrely when the bullet is pressed into the lands... pressure can become a concern when this is done with jacketed bullets in high-powered rifle loads.

  4. #4

    Default

    I should have been more clear. What I was wondering was how much of a difference have you measured in group size by changing jump distance?

  5. #5

    Default

    Totally variable from rifle to rifle, bullet to bullet and load to load. It can as much as cut groups in half or more, or make little difference at all. In one of my 22-250's for example, increasing COL .020 over standard shrinks groups from 1.25 down to the .25 I need for long range varminting in the Southwest. The 223 I also carry will digest about anything, giving great results with almost any load, but no advantage for short jumps.

  6. #6

    Default Bullet jump

    Depending on the cartridge, contact distance between the bullet and the rifling can make quite a difference. I had a .257 Roberts Ackley Improved which gave best accuracy with the bullet just touching the rifling, but others, such as the Weatherby family of cartridges need plenty of room, hence the long throats found on most Weatherby barrels. But for most of the cartridges I have loaded for, .030" to .050" seem to be a good average. Because of the variations on bullet shapes, long and short ogives, boattails, etc., it can be quite an individual thing as to just how far a bullet will need to be seated, one of the variables, as you mentioned, being the magazine length, which can really put a crimp on things.

  7. #7

    Default

    Best solution is to simply experiment with your gun and prefered bullets. You needed an excuse to shoot more, didn't you?

  8. #8

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    I have friends who tell me that I will wear my barrels out since I am always trying something. I tell them great, I hope and pray that life will always allow me the opprutinity to shoot enough to actually wear out a barrel.
    I shot about 25, 5 shot groups this wkend. Just tring diff seating depths. The worst part is cleaning the barrel between each change so fouling isn't a factor.
    The gun I tried this wkend had a slight preference for a jump of 25 thousands. I plan on trying another gun this wkend.

  9. #9
    Moderator Paul H's Avatar
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    Default

    I guess you could call me lazy, but unless a barrel shows a tendancy to foul and groups to open up, I put between 50 and 100 rounds down range before cleaning. I think you might see more variances in accuracy from so much cleaning, not to mention excessive cleaning can do as much if not more wear on a barrel than shooting it.

    As far as bullet jump affecting accuracy, as others have said it's a rifle to rifle to deal. I usually start at 0.025" off if the mag allows it, and once I've found the powder charge the gun prefers best, I fine tune w/ seating depth.

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