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bullet jam

 
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  #1  
Old 05-11-2006, 09:36 AM
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Location: Nottingham, UK
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bullet jam

Can someone give me a precise explanation of what you are doing when seating a bullet with jam.

Are you forcing the bullet x thou in to the land,Or are you just making sure the bullet is perfectly in line with x amount of jam.
Or have i got it completely around my neck. [img]/ubbthreads/images/graemlins/confused.gif[/img]
As i thought i new until i had a discussion with a gun smith yesterday.
thanks
Colin
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  #2  
Old 05-11-2006, 10:18 AM
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Re: bullet jam

Bullet jam usually means the bullet's seated out far enough to be slightly engraved by the rifling or even set back a bit when chambered. How much the bullet's engraved or set back depends on how tight it's held by the case neck.

When the bullet's seated out just enough to barely touch the lands when chambered, it's sometimes called a "contact" seat.
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  #3  
Old 05-11-2006, 10:35 AM
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Location: Nottingham, UK
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Re: bullet jam

[ QUOTE ]
Bullet jam usually means the bullet's seated out far enough to be slightly engraved by the rifling or even set back a bit when chambered. How much the bullet's engraved or set back depends on how tight it's held by the case neck.

When the bullet's seated out just enough to barely touch the lands when chambered, it's sometimes called a "contact" seat.

[/ QUOTE ]

HI BART B
so lets say i have 2 thou of jam then i go to 4 thou and then 6 thou and so on
so i take it you are not shoving the bullet in to the lands
more like shoving the lands in to the bullet???

so y dose it matter if you have 2 tho or 10 thou of jam [img]/ubbthreads/images/graemlins/confused.gif[/img]
thanks
colin
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  #4  
Old 05-11-2006, 10:45 AM
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Re: bullet jam

Your max amount of jam depends on the amount of neck tension that you have. When using a bushing .004" smaller than ND, it is difficult to acieve much jam because the lands will seat the bullet as the bolt closes. If a smaller bushing is used or, in the case of a crimp being applied to the case neck, then neck tension is increased and pressures can rise sharply.
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  #5  
Old 05-11-2006, 01:17 PM
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Location: Long Island, New York
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Re: bullet jam

[ QUOTE ]
so y dose it matter if you have 2 tho or 10 thou of jam

[/ QUOTE ]

Yes it does. Typically peak pressure rises as "jam" is increased. Also different rifle/bullet combinations perform better with more, less or no rifling engagement. It is a trial and error proposition.

I have a 6.5x284 that wouldn't shoot the 142gr SMK's very well until I tried .015" of crush fit. At .015" the groups tightened considerably.

If you are loading for a hunting rifle, I wouldn't use the "jam" fit at all. You will most likely find a suitable load with bullets loaded off the rifling as they should be for a hunting rifle.
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  #6  
Old 05-11-2006, 01:18 PM
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Re: bullet jam

Any time you are touching the rifling the pressure will rise. Once allowed for it is okay to seat long and then let the rifling finish seating the bullet when you close the bolt. To me that is jam seating or jam depth. That is the figure I always use when I am backing off of the rifling. I measure 3 that were jam seated and then back up from there. That way any king of throat movement has no affect on my loads.
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  #7  
Old 05-11-2006, 02:39 PM
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Location: Nottingham, UK
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Re: bullet jam

So if your bullet will only go in so far,With x amount of neck tension.Say 3 thou how much jam can you apply before all you end up with is the same IE 10 thou 20 thou of jam.
For at some point you must end up just seating the bullet to a set amount of jam,And the rest of the bullet that is seated long no longer dose anything.(i hope you can follow what i mean [img]/ubbthreads/images/graemlins/confused.gif[/img])

And i understand that the pressure will rise as you make it harder for the bullet to start to move (IE the jam)

This all started as i am using berger bullets, And was told to seat them with 10 thou of jam,and off you go.Full stop no need to try any different seating depths,just sort out the powder charge.
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