Jump v Jam

F

fx77

Member
Please explain the science -physics for why given bullets shoot better at jam, jam +10, jump 20 thousandths, etc.
Reviewing the Euber method of load development , and after one decides groups size on powder charge, one is to start backing off from the lands until the group does not change, gets better, or deteriorates.
Clearly it is an issue. ( Recent post on 108 gr 6BR by J Schmidt found the bullet favored a 20 thousandth jump for ex.) Why would it make a difference? If the bullet, once it leaves the case is ultimately in the rifiling, where else can it go?
What happens between the bullet release from the case, and the rifling engagement that makes a difference with various bullets?
Is it a function of Ogive, and if so why?
I have heard "Hybrid" is less sensitive to jumping dist, or that some calibers are less sensitive to OAL...
Thanks
 
fx77 said:
Please explain the science -physics for why given bullets shoot better at jam, jam +10, jump 20 thousandths, etc.
Reviewing the Euber method of load development , and after one decides groups size on powder charge, one is to start backing off from the lands until the group does not change, gets better, or deteriorates.
Clearly it is an issue. ( Recent post on 108 gr 6BR by J Schmidt found the bullet favored a 20 thousandth jump for ex.) Why would it make a difference? If the bullet, once it leaves the case is ultimately in the rifiling, where else can it go?
What happens between the bullet release from the case, and the rifling engagement that makes a difference with various bullets?
Is it a function of Ogive, and if so why?
I have heard "Hybrid" is less sensitive to jumping dist, or that some calibers are less sensitive to OAL...
Thanks
Click to expand...

There are numerous theories concerning this, but I have resolved myself to just find what I think the optimum powder charge is, and then play with the seating depth until it really tightens up.

I have a lot # of old Bart's HeadHunters from 2008, (double radius ogive, flat base), that will shoot best when you jam them very hard, to where the marks are twice as long as they are wide. This also worked well for that great lot of Bruno Boat Tails I shot through the mid 2000's.

But, I have a nice lot # of 65 grain Barts Boat Tails that liked to be jumped as much as .020. That is the bullet I shot at the Nationals, and did pretty good.

My 30 BR lately seems to like a little jump, maybe .010 from the faintest marks. But it has never liked much jam, either with Barts 112's or BIB 112's.

Keep in mind. I am also playing with the tuner as well. But more and more, I catch myself using the tuner to avoid that dreaded "horizontal tune", especially in Score Shooting.

I also shoot Krieger 4 groove barrels, whether it's a 6mm, 7mm, or 30 caliber. I also set them back on a regular basis, maybe I am a little too paranoid about keeping the throat fresh.
 
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When I started out with my 6BR I shot 80 grain Berger FBs and they liked to be loaded long enough to seat themselves on bolt close. This was my 8 twist Shilen ratchet. When I switched to Shilen 14 twist ratchet, it liked Bart's and JDS 68 grain BTs 9 thou off "touch" (basically what Jackie calls "faintest marks"). This has worked for two barrels from Shilen, both of which shoot pretty well. You just have to find the right powder charge then work on seating, just like Jackie said!

Dennis

ps Jackie, you shooting this month's ARC match?
 
DeltaBravo said:
When I started out with my 6BR I shot 80 grain Berger FBs and they liked to be loaded long enough to seat themselves on bolt close. This was my 8 twist Shilen ratchet. When I switched to Shilen 14 twist ratchet, it liked Bart's and JDS 68 grain BTs 9 thou off "touch" (basically what Jackie calls "faintest marks"). This has worked for two barrels from Shilen, both of which shoot pretty well. You just have to find the right powder charge then work on seating, just like Jackie said!

Dennis

ps Jackie, you shooting this month's ARC match?
Click to expand...

I'm sure going to try and make it. Ed and Geramo said they might try to make it as well.
 
I do this as well but have a suspicion that it's simply the next couple of groups fired are better than the previous setting. Those groups might have been better without changing anything.
 
fx77 said:
Please explain the science -physics for why given bullets shoot better at jam, jam +10, jump 20 thousandths, etc.

What happens between the bullet release from the case, and the rifling engagement that makes a difference with various bullets?

Thanks
Click to expand...


A change in distance the bullet travels before obturation will change the initial burn rate of the powder by changing the start pressure; through - the amount of blow by produced. This changes the location of the peak pressure on the pressure curve and bullet position in the bore / barrel time; with possible recoil and barrel vibration changes.

Ken
 
Tim Oltersdorf said:
Jackie, what is the "dreaded horizontal tune"? Tim
Click to expand...

Tim, we all go to matches and see shooters, in relative light conditions, with .400 "caterpillars" at 100 and .900 "caterpillars" at 200. Not a bullet hole of verticle, just straight accross.

They think the Rifle is tuned, but in fact, it's just stringing shots in the horizontal instead of the verticle. Any little twitch in the conditions seems to push the bullet an exorbant amount in either direction.

I notice this in Score Shooting quite a bit. Shooters will miss the X, or even the 10 ring, to the left or right while seeing hardly any noticeable change in the condition.

If I get caught in this, especially in Score Shooting, I will turn the tuner and attempt to get a little verticle in the Rifle. Sometimes this works, but sometimes I have to admit that on that day, my shooting sucked.
 
i am not sure of your comment, but it is partially in the correct direction.
when the bullet initially moves, it changes the total volume of the combustion chamber( case volume plus volume from the back of the bullet back to the case( as in when the bullet first touches the lands). by increasing volume you slightly decrease pressure, and powder burn responses to pressure. this probably changes vibration/node, and thus shot placement.

with jam you are setting the total combustion volume, or close to it, esp with longer/harder jams.

K Hope said:
A change in distance the bullet travels before obturation will change the initial burn rate of the powder by changing the start pressure; through - the amount of blow by produced. This changes the location of the peak pressure on the pressure curve and bullet position in the bore / barrel time; with possible recoil and barrel vibration changes.

Ken
Click to expand...
 
Burn Rates

CMaier said:
i am not sure of your comment, but it is partially in the correct direction.
when the bullet initially moves, it changes the total volume of the combustion chamber( case volume plus volume from the back of the bullet back to the case( as in when the bullet first touches the lands). by increasing volume you slightly decrease pressure, and powder burn responses to pressure. this probably changes vibration/node, and thus shot placement.

with jam you are setting the total combustion volume, or close to it, esp with longer/harder jams.
Click to expand...

There are numerous ways to characterize powder Burn Rates - Initial, Linear, and Apperent etc. I break it down into two events the initial and the regressive burn that you describe.

Ken
 
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