bullet jam

[col48]

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. /ubbthreads/images/graemlins/confused.gif
As i thought i new until i had a discussion with a gun smith yesterday.
thanks
Colin

 

[Bart B]

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.

 

[col48]

[ 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 /ubbthreads/images/graemlins/confused.gif
thanks
colin

 

[EddieHarren]

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.

 

[Varmint Hunter]

[ 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.

 

[Hired Gun]

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.

 

[col48]

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 /ubbthreads/images/graemlins/confused.gif)

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.

 

[Bart B]

[ 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 /ubbthreads/images/graemlins/confused.gif


[/ QUOTE ]Colin, all these guys who've responded did such a great job explaining, I'll have to pass any credit that leaks my way on to them.

 

[Hired Gun]

What they are talking about is seat your bullets .010" out farther than the jam length. That way you are sure they are in nice contact with the rifling when you close the bolt. The rifling will finish seating the bullet. This is what they are talking about. It would be the same if you set them .050" long. The bolt would just be a lot harder to close because you would be out past the cam action of the bolt. It's not that you are actually pressing the bullet .050" into the rifling. Neck tension will have a very small affect on how tight the bullet is actually jammed into the rifling due to how hard the bullets are.

You're making this much harder than it is. Just set 3 rounds really long. Force the bolt shut on all of them and then measure the overall length. From that average set your seater to seat your bullets .010" longer than your average. Done deal. Now just find the powder charge that gives you the best result. Seating into the rifling like this eliminates a variable. After you find the best powder / charge combo then you can safely back off the rifling in small increments to further fine tune the load.

 

[Varmint Hunter]

[ QUOTE ]
You're making this much harder than it is. Just set 3 rounds really long. Force the bolt shut on all of them and then measure the overall length. From that average set your seater to seat your bullets .010" longer than your average. Done deal.

[/ QUOTE ]

I have to disagree here. Not saying who is right or who is wrong but:

Crushing bullets into the rifling and then adding .010" more is a techniqe that would leave you with an indiscriminant amount of bullet intrusion into the rifling. It would vary quite a bit depending on the neck tension, neck cleanliness and the amount of bullet shank that is in contact with the case neck.

Typically, .010" of jam (or crush fit) means .010" longer than a loaded round that just touches the rifling. A loaded round that is .010" more than a maximum crush fit could be any length at all.

 

[Hired Gun]

Apparently you have never tried it. It is very precise. To get the bullet into the rifling by any measurable amount beyond jam length about takes a hammer blow to the back of the bullet. The differences in depth due to neck tension would be negligible at most. Other than my Weatherby's I do this every time I load and I keep records to watch for throat movement. The jam seated rounds usually all measure to within .001 of each other. The biggest variances are due to tip deformities if measured with calipers. When measured to the ogive in my seater they are nuts on.

 

[Pete L]

so basically what we are looking at when you have founf the OAL of your loaded bullet add 10 thou to this measurment and with the correct neck tension lets say 2 thou this is enough to sft seat.jam the bullet so they are all correctly positioned to the rifleing ??
one more question I have tried this I have 2 thou of neck tension and when I add 10 thou to my OAL the bolt shuts fine with very little pressure but when I extrcat it the OAL has lengthend as i prsume the lands have held the bullet and with minimum neck tension has allowed the bullet to be pulled out slightly,I presume this is OK when you actually shoot each round topu chamber but will it efeect anything if you keep taking the bullet in and out as over here in the UK you are not guaranteed to make a shot

 

[col48]

so how you have explained it,what is the difference between soft seating and bullet jam.or are we talking about the same thing.

 

[EddieHarren]

I would agree with VarmintHunter. Neck tension does matter. It also affects thr pressure spike.