Jump or jam, how does it affect the results?

[SteveOak]

Here is the essence of my question. Varying the OAL of a cartridge, either pushing the bullet into the rifling (jam) or holding it short (so that the bullet jumps to the rifling) is commonly used to fine tune accuracy. Can anyone tell me what is being affected by the change and how does that effect translate into a change in accuracy?

I can think of two general categories of effects;

Powder combustion
Bullet alignment

Some possibilities I have pondered;

Causing the rate of powder burn to increase or decrease
More complete ignition earlier in the combustion cycle
Higher pressure before bullet moves
Higher pressure before bullet moves significantly
Bullet is more centered in the bore
Bullet is free to self center

It is interesting too that the testing to determine optimum OAL is commonly performed after a primer/power/charge weight is determined. That would seem to indicate bullet alignment rather than powder charge as the mechanism.

Secondarily there is the question of why is it not universal? Sometimes, seemingly with everything else being the same, cartridge, powder, bullet, primer, charge etc. jamming or jamming harder improves accuracy while other times jumping is the answer. If I remember correctly, which happens less and less often, certain bullets are more likely to respond positively to jamming and some are more sensitive than others to jumping or jamming.

 

[JEFFPPC]

Here is the essence of my question. Varying the OAL of a cartridge, either pushing the bullet into the rifling (jam) or holding it short (so that the bullet jumps to the rifling) is commonly used to fine tune accuracy. Can anyone tell me what is being affected by the change and how does that effect translate into a change in accuracy?

I can think of two general categories of effects;

Powder combustion
Bullet alignment

Some possibilities I have pondered;

Causing the rate of powder burn to increase or decrease
More complete ignition earlier in the combustion cycle
Higher pressure before bullet moves
Higher pressure before bullet moves significantly
Bullet is more centered in the bore
Bullet is free to self center

It is interesting too that the testing to determine optimum OAL is commonly performed after a primer/power/charge weight is determined. That would seem to indicate bullet alignment rather than powder charge as the mechanism.

Secondarily there is the question of why is it not universal? Sometimes, seemingly with everything else being the same, cartridge, powder, bullet, primer, charge etc. jamming or jamming harder improves accuracy while other times jumping is the answer. If I remember correctly, which happens less and less often, certain bullets are more likely to respond positively to jamming and some are more sensitive than others to jumping or jamming.

Here is the essence of my question. Varying the OAL of a cartridge, either pushing the bullet into the rifling (jam) or holding it short (so that the bullet jumps to the rifling) is commonly used to fine tune accuracy. Can anyone tell me what is being affected by the change and how does that effect translate into a change in accuracy?

I can think of two general categories of effects;

Powder combustion
Bullet alignment

Some possibilities I have pondered;

Causing the rate of powder burn to increase or decrease
More complete ignition earlier in the combustion cycle
Higher pressure before bullet moves
Higher pressure before bullet moves significantly
Bullet is more centered in the bore
Bullet is free to self center

It is interesting too that the testing to determine optimum OAL is commonly performed after a primer/power/charge weight is determined. That would seem to indicate bullet alignment rather than powder charge as the mechanism.

Secondarily there is the question of why is it not universal? Sometimes, seemingly with everything else being the same, cartridge, powder, bullet, primer, charge etc. jamming or jamming harder improves accuracy while other times jumping is the answer. If I remember correctly, which happens less and less often, certain bullets are more likely to respond positively to jamming and some are more sensitive than others to jumping or jamming.

There is not a one for all, black and white answer because every barrel and bedding job is different.Just my opinion.

 

[urbanrifleman]

I will tell you this, it will get me flamed I am sure, but years ago the Sierra tech told me that most "standard" designed bullets will shoot best at 20 to 25 thou jump.

With a exceptions being these bullet designs with the pointy noses, I have found that still works.

 

[NorCalMikie]

I know from my own experience, if you work up a load with the bullet seated at a jam into the lands, you can go higher in powder charge than if the bullet was seated to a jump. Not sure of the whys but it works. Something to do with "spike"?
All my loads (except my gas guns) are set to jam into the lands.
I have at times had to dump powder from the the action when extracting a live round with the bullet stuck into the lands but I knew ahead of time to expect that. The larger the caliber, the more a jump to the lands works in your favor. Some of the BIG guns (30 caliber on up) are built/chambered with up to .100 freebore so there's a BIG jump and they seem to shoot bug holes with no problems.

 

[urbanrifleman]

I know from my own experience, if you work up a load with the bullet seated at a jam into the lands, you can go higher in powder charge than if the bullet was seated to a jump. Not sure of the whys but it works. Something to do with "spike"?

I have heard all kinds of explanations, but from experience, the shorter the OAL the high the pressure at the same charge.

 

[jamesdmock]

I have heard all kinds of explanations, but from experience, the shorter the OAL the high the pressure at the same charge.

This idea seems backward to me. Weatherby cartridges were designed with a long free bore to REDUCE pressure. The free bore of the Weatherbys was designed to keep the bullets from being jammed. I guess one could do a test with a chronograph and get some idea....dunno. Good shooting...James

 

[urbanrifleman]

This idea seems backward to me. Weatherby cartridges were designed with a long free bore to REDUCE pressure. The free bore of the Weatherbys was designed to keep the bullets from being jammed. I guess one could do a test with a chronograph and get some idea....dunno. Good shooting...James

Sure at a given charge for a given case capacity. Increasing the freebore increases the case capacity.

In my experience, if you work up a max load at 20 off, you will get pressure going shorter, not longer. Even into the lands.

 

[jamesdmock]

Sure at a given charge for a given case capacity. Increasing the freebore increases the case capacity.

In my experience, if you work up a max load at 20 off, you will get pressure going shorter, not longer. Even into the lands.

Case capacity is increased by seating the bullet farther out....BUT, this length is limited to an overall length that will feed through a magazine.....thus Weatherby avoids the jam. Maybe some on here has one of the Oehlers that are capable of measuring pressure and can determine this theory by testing. James

 

[jamesdmock]

Here is the essence of my question. Varying the OAL of a cartridge, either pushing the bullet into the rifling (jam) or holding it short (so that the bullet jumps to the rifling) is commonly used to fine tune accuracy. Can anyone tell me what is being affected by the change and how does that effect translate into a change in accuracy?

I can think of two general categories of effects;

Powder combustion
Bullet alignment

Some possibilities I have pondered;

Causing the rate of powder burn to increase or decrease
More complete ignition earlier in the combustion cycle
Higher pressure before bullet moves
Higher pressure before bullet moves significantly
Bullet is more centered in the bore
Bullet is free to self center

It is interesting too that the testing to determine optimum OAL is commonly performed after a primer/power/charge weight is determined. That would seem to indicate bullet alignment rather than powder charge as the mechanism.

Secondarily there is the question of why is it not universal? Sometimes, seemingly with everything else being the same, cartridge, powder, bullet, primer, charge etc. jamming or jamming harder improves accuracy while other times jumping is the answer. If I remember correctly, which happens less and less often, certain bullets are more likely to respond positively to jamming and some are more sensitive than others to jumping or jamming.

Hall of Fame shooter, Don Geraci once told me to let the barrel tell you what it likes. Start testing at jam and shorten the cartridge until you find the ideal seating depth. Don't start with preconceived ideas like, "I am not going to jump bullets because I don't believe in jumping'. Just test with an open mind. James

 

[barefooter56]

Here is the essence of my question. Varying the OAL of a cartridge, either pushing the bullet into the rifling (jam) or holding it short (so that the bullet jumps to the rifling) is commonly used to fine tune accuracy. Can anyone tell me what is being affected by the change and how does that effect translate into a change in accuracy?

I can think of two general categories of effects;

Powder combustion
Bullet alignment

Some possibilities I have pondered;

Causing the rate of powder burn to increase or decrease
More complete ignition earlier in the combustion cycle
Higher pressure before bullet moves
Higher pressure before bullet moves significantly
Bullet is more centered in the bore
Bullet is free to self center

It is interesting too that the testing to determine optimum OAL is commonly performed after a primer/power/charge weight is determined. That would seem to indicate bullet alignment rather than powder charge as the mechanism.

Secondarily there is the question of why is it not universal? Sometimes, seemingly with everything else being the same, cartridge, powder, bullet, primer, charge etc. jamming or jamming harder improves accuracy while other times jumping is the answer. If I remember correctly, which happens less and less often, certain bullets are more likely to respond positively to jamming and some are more sensitive than others to jumping or jamming.

Steveoak,
This may help:http://www.bergerbullets.com/effect...coal-and-cartridge-base-to-ogive-cbto-part-1/
and
http://www.bergerbullets.com/effect...coal-and-cartridge-base-to-ogive-cbto-part-2/
For what its worth I always find my bullet seating depth accuracy node using the lowest powder charge listed for the bullet/powder/cartridge combination I am testing. Then work the powder charge up to find my highest velocity accuracy node.

 

[Joe Salt]

If you start into the Lands you only have one way to go!

Joe Salt

 

[lpreddick]

You can usually find an accurate load by starting at jam, then seating the bullet deeper by .005. If you find a one holer at .025 off lands and load some for later...months later, you will possibly be disappointed...no one HOLER! The bullet and brass have bonded...cold bonding. Higher pressure is required to break this bond and higher pressure changes velocity and barrel time., and groups. Relax everybody...this topic has been discused/argued ad infinitum. The bullet in these cases can be seated .005 deeper and you can hear AND feel a "pop". Some who load today to shoot next month will leave the bullet long, then seat at last minute. Therefore, I find my accurate load at .020 into the lands(1° 30 slope) and can shoot same hole next year as I do today. I surmise the pressure to break the bond is less than the pressure to move into the rest of the lands.

 

[kris bassett]

You can usually find an accurate load by starting at jam, then seating the bullet deeper by .005. If you find a one holer at .025 off lands and load some for later...months later, you will possibly be disappointed...no one HOLER! The bullet and brass have bonded...cold bonding. Higher pressure is required to break this bond and higher pressure changes velocity and barrel time., and groups. Relax everybody...this topic has been discused/argued ad infinitum. The bullet in these cases can be seated .005 deeper and you can hear AND feel a "pop". Some who load today to shoot next month will leave the bullet long, then seat at last minute. Therefore, I find my accurate load at .020 into the lands(1° 30 slope) and can shoot same hole next year as I do today. I surmise the pressure to break the bond is less than the pressure to move into the rest of the lands.

Do coated bullets resist the cold bond?

 

[dminn1]

Here is the essence of my question. Varying the OAL of a cartridge, either pushing the bullet into the rifling (jam) or holding it short (so that the bullet jumps to the rifling) is commonly used to fine tune accuracy. Can anyone tell me what is being affected by the change and how does that effect translate into a change in accuracy?

I can think of two general categories of effects;

Powder combustion
Bullet alignment

Some possibilities I have pondered;

Causing the rate of powder burn to increase or decrease
More complete ignition earlier in the combustion cycle
Higher pressure before bullet moves
Higher pressure before bullet moves significantly
Bullet is more centered in the bore
Bullet is free to self center

It is interesting too that the testing to determine optimum OAL is commonly performed after a primer/power/charge weight is determined. That would seem to indicate bullet alignment rather than powder charge as the mechanism.

Secondarily there is the question of why is it not universal? Sometimes, seemingly with everything else being the same, cartridge, powder, bullet, primer, charge etc. jamming or jamming harder improves accuracy while other times jumping is the answer. If I remember correctly, which happens less and less often, certain bullets are more likely to respond positively to jamming and some are more sensitive than others to jumping or jamming.

I have suspected at times that accuracy tuning through seating depth changes has more to do with the combustion side of the cartridge than with the relationship of the bullet to the lands. One reason for my suspicion is my .243 Win shooting 105g Berger Hybrids will continue to maintain tune and 600 yard accuracy through .018 of throat growth without my changing the seating depth of the bullets. Why this is I have not a clue. I would be interested in hearing others thoughts on the matter.

 

[Alex Wheeler]

Seating depth is not fine tuning, its a critical part of the load. Just as critical as powder charge. While .005" in seating depth can have a huge effect on accuracy, it has an almost unmeasurable effect on pressure.

 

[Fred Bohl]

The following is offered as information only and applies to my experience in my barrels only - YMMV.

Tangent and hybrid ogive bullets produce the best groups at 0.010 to 0.020 jump relative to touch point. This combo also produces very long competitive barrel life, moderate MV ES and POI change.

Secant ogive bullets produce the best groups at soft seat jam. This combo also produces short competitive barrel life, low MV ES and POI change.

 

[spclark]

...but years ago the Sierra tech told me that most "standard" designed bullets will shoot best at 20 to 25 thou jump.

Which is and has been my practice for several years now... though admitting I haven't been using Sierra's products much for the past ten or so.

I'd heard they've been getting better at shipping bullets with consistent dimensions batch to batch (the opposite of which is why I stopped using them) so I bought 1,000 2156's late last season.

Glad to find they support what I'd heard.

 

[Capt. Oblivious]

The following is offered as information only and applies to my experience in my barrels only - YMMV.

Tangent and hybrid ogive bullets produce the best groups at 0.010 to 0.020 jump relative to touch point. This combo also produces very long competitive barrel life, moderate MV ES and POI change.

Secant ogive bullets produce the best groups at soft seat jam. This combo also produces short competitive barrel life, low MV ES and POI change.

How did you get to the competative barrel life difference ?

 

[Fred Bohl]

How did you get to the competative barrel life difference ?

The simple answer is shooting a lot with both barrels. Over the course of three seasons (here in Wisconsin at 68 years old when this trial started a season is April thru October) the two barrels were shot at a private range at both 100 and 200 yards. The criterion I use for barrel life is aggregate group size that would have been competitive back when I was still competing.

Tangent and hybrid ogive bullets produce <0.300 groups at 0.010 to 0.020 jump relative to touch point for 1800 rounds. Secant ogive bullets produce the <0.300 groups at soft seat jam for 1200 rounds.

The two barrels used were Krieger 4 grove SS 26 inch HV using my reamers.

 

[Capt. Oblivious]

Thanks allot