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Cocentric bullet seating

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  #1  
Unread 01-07-2005, 02:01 PM
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Join Date: Oct 2002
Location: Alberta
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Cocentric bullet seating

Sometimes I wonder if buying a cocentricity gauge was such a great idea, being blissfully unaware has its merits! My question relates to a problem I'm having seating light (therefore short)flat base bullets straight in my 6mm Rem. Using normal RCBS FL dies I can get fairly straight cases (less than .003 runout). The problem comes when I seat the 68 gr. hollowpoints, which need to be seated less than one caliber deep to get anywhere even remotely close to the lands. (I'm 60/1000 off at this depth)I seem to get decent accuracy anyway but it bugs the you know what out of me! About 50% of the loaded rounds have 5 - 12/1000 runout. I've tried rotating the shell while seating, I polish & clean my brass, I chamfer the cases carefully etc.

Is it the shallow seating that's causing the trouble (short bearing surface)? Would benchrest dies such as Forester's help?

ANY advice would be most appreciated!!
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  •   #2  
    Unread 01-07-2005, 04:19 PM
    dcb dcb is offline
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    Join Date: Jun 2004
    Location: Ohio
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    Re: Cocentric bullet seating

    I think it might be the bullet seater does not match the tangent shape of the bullet
    the bullet could be bottoming out and being pushed to one side
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      #3  
    Unread 01-07-2005, 05:05 PM
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    Join Date: Jan 2005
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    Re: Cocentric bullet seating

    Been in the same shoes with trying the nosler 55 bt's in my .243. They just don't seem to get straight when seated shallow. Mine also shoot great--as a matter of fact really great--my average runnout with the combo is about .007". One day I was at a friends house and my friend took my ruger varmiter (w/6-20 leo) and fired 2 shots. They almost touched each other--another buddy says let me try and he shoots 2 shots almost touching the first two. Everyone is excited now (me the most)--I grab the gun and shoot a shot for a 5 shot group fired by 3 different people--the result is we covered them with a nickel at 100yds. It's my opinion that runnout is way overrated--especially on short stubby bullets like we are talking about. I have nothing to back up my "feelings" but it just seems to me that it would be easier for a barrel to straighten out a short bullet than possibly a long "matchking" type bullet. I guess if it's bothering you I'd say load a batch of 20 rounds and segregate by runnout--I'm thinking you gotta get some that just by luck turn out to be under .003"--try some groups with some different types of runnout--I'm thinking you won't see much difference--at least out to about 300 yds.
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      #4  
    Unread 01-07-2005, 09:31 PM
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    Join Date: May 2002
    Location: Washington
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    Re: Cocentric bullet seating

    The "runout" thing has been tested for many years and I remember reading in the Precision Shooting magazine many years ago about it. It was found that "runout" does play an important part in accuracy and that contary to what a lot of people think...<font color="red">the barrel will not straighten out the bullet if it is launched crooked! </font>

    The closer to having the bullet in line with barrel, the better chances that you will have in getting your weapon to shoot smaller groups.
    -------------


    Zod
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      #5  
    Unread 01-10-2005, 03:22 AM
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    Re: Cocentric bullet seating

    I agree that runout is an issue. But, I also think neck tension plays a bigger role in accuracy than runout..my .02...sakofan..I may be wrong, of course! [img]/ubbthreads/images/graemlins/tongue.gif[/img]
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      #6  
    Unread 01-10-2005, 06:28 AM
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    Join Date: Jun 2007
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    Re: Cocentric bullet seating

    trick to cure that problem with flat base bullets.

    Neck turn about .050-.075 down the neck and about .002 cut.
    When fired this will expand out and leave the cut on the inside of the case and will allow you to start the bullets straight and keep them straight.

    Try on 1-2 cases and see what cut thickness works best for your gun before cutting all of them.

    BH
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      #7  
    Unread 01-10-2005, 12:28 PM
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    Re: Cocentric bullet seating

    Just for the experimenting, seat the bullets way deeper into the case. Is the runout the same? If so, change to another seating die, polish or change the shape of the seating plug (any marks on the nose of the bullet?).

    You may also want to reduce the amount of runout in the sized case. Use a bushing or collet neck die. That way you have a little more room for runout in the seating stage.

    Jerry
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