help understanding jamming the bulet into the lands

[rebs]

I have read where some bullets are jammed into the lands, is it safe ? How do you go about doing it and know how far you can jam ?
This is for a 223 rem cal in a Tikka T3x bolt rifle with Berger 75 and 80 VLD bullets shooting 300 yd F class.

 

[Walt Krafft]

I do it all the time for long range loads. Run a light neck tension. Allows the same relative bullet position as the throat wears. Biggest drawback imo, is you can leave a bullet stuck in the barrel and dump powder in the action if you need to eject a loaded round for some reason.

 

[rsmithsr50]

i think I would skip the 80 for just 300 yards.
start load development 'AT THE LANDS" ,marks so small you need a magnifying glass to see.
once you have a load, back the bullet off and see what happens. little steps like .005
I would only look for a jam if the above did not work

 

[dcali]

It’s safe. You might see a bit of a pressure increase, so take that into consideration, but it’s not huge. How far you can jam depends on a lot of things - neck tension, case friction, jacket hardness, land conditions, etc. At some point you will start pushing the bullet back into the case.

I’ve had very good luck seating bullets about 5-10 thous longer than the touch dimension (where the bullet just touches the lands).

Bullets tend to shoot well jammed in my experience. I suspect that has something to do with keeping the bullet centered in the bore, but who knows. The only downside to the practice is that you risk getting a bullet stuck in the lands if you try to unload a loaded round. You will dump powder all over the inside of the action and have to knock the bullet out (preferably with a tight fitting brass rod dropped in from the muzzle).

 

[Ccrider]

There are several different meanings to jammed. I use the term to mean a bullet sized long and then pushed into the sized brass by the bolt. The measure met will change depending on the neck size.

I have seen 50 -75 fps difference between jam and jumped using the same load. On a hot load this could be a problem.

 

[ReedG]

I believe the practice of seating bullets into the lands, or jamming, originated in the early benchrest days. In order to get decent brass, factory brass had to be manipulated with tender-loving care into something consistent. The process usually required necks to be turned to .0075" or even .0065" thickness to clean them up and uniform them. With necks that thin, they didn't have sufficient or uniform neck tension. To overcome the light neck tension, the bullet was seated into the lands. The purpose of the neck tension is to get a consistent powder ignition and burn and the "jamming" worked quite well. I think that modern brass has alleviated the need to seat into the lands, but the process doesn't have any real drawback except the above-mentioned dumping of powder into the action sometimes when trying to extract a loaded round. Modern Lapua brass with a .0085" to .0125" thickness usually more than enough neck tension to get proper ignition. My current loads in 6PPC and 30BR, using new Lapua brass, all show best accuracy when seated .010" to .020" off the lands. I'm sure other people with different components and loads will have different results.

 

[Texas10]

I have read where some bullets are jammed into the lands, is it safe ? How do you go about doing it and know how far you can jam ?
This is for a 223 rem cal in a Tikka T3x bolt rifle with Berger 75 and 80 VLD bullets shooting 300 yd F class.

To set and measure "JAM" distance, you deliberately seat your bullet long in an empty case, chamber it and force the bolt closed. Note: it's best to remove the extractor for this exercise. Open the bolt and gently tap the round out using a rod from the muzzle end.

This will give you the maximum length you can load that bullet to, in YOUR chamber. As noted, you may have a problem removing a live round when called upon to do so.

You've probably noted some describing loading .005 off jam. In this scenario you establish the jam dimension, then seat .005 deeper into the case.

Others will refer to "jam" when actually describing touching the lands. And then go on to describe .005 past jam as meaning .005 INTO the lands after touch.

However you choose to measure touching lands, it's just a starting point you work from. As long as you can get a consistent measurement bullet to bullet, brand to brand, you're accomplishing what you intend.

We're all talking about the same subject, just using different languages.

 

[zipollini]

I usually jam my 6mm DTAC 115 RBTs between .010 and .020". Nice thing about jamming is that you don't need to anneal the necks and your SD and ES will usually be lower

 

[BoydAllen]

In the past I have read that seating to touch the rifling or longer increases pressure by 5,000 to 6,000 psi. If you do your initial pressure testing, starting low and going up in small increments, with the bullet seated longer than touch, there is absolutely no danger (as long as you are properly vigilant for pressure signs and stop when you see them), but you should not change to that seating depth from one that has the bullet jumping to the rifling, without dropping your powder charge and working back up at the new seating depth.

Without going into a long discussion, the simple reason for seating longer than touch is that testing has proven that for that rifle, bullet, powder combination, that it gives the best accuracy. This would require testing with your rifle.

The term jam originated a long time back, in short range benchrest, and properly used is the longest that you can seat a bullet, with the neck tension at which is will be shot, without it being set back into the case as it is chambered. The usual way to determine this measurement was, and still is, seating the bullet as long as one can and still close the bolt, and chambering a round or dummy round, intentionally having the bullet set back in the case. By measuring before and after chambering the round, one can determine that the bullet was indeed pushed back and the resultant seating depth is jam for that barrel, brass, and neck tension. The word is a reference to a specific dimension that is experimentally determined. Shooters will then refer to the seating depth that they are using a so many thousandths off jam, or shorter than jam. It is a good idea to also know the seating depth at which a bullet is just touching the rifling, so that you will know at what point backing off of jam takes you into jumping the bullet. The distance from touch to jam varies with the amount of friction between the neck and the bullet, the shape of the bullet, and the configuration of the chamber throat. It can vary quite a bit, and because of this the only way to know is to take the measurements. Alex Wheeler has made an excellent video on finding the lands, and there are other methods that work as well. The important thing is to have a method that gives repeatable results.

 

[BartsBullets]

To set and measure "JAM" distance, you deliberately seat your bullet long in an empty case, chamber it and force the bolt closed. Note: it's best to remove the extractor for this exercise. Open the bolt and gently tap the round out using a rod from the muzzle end.

This will give you the maximum length you can load that bullet to, in YOUR chamber. As noted, you may have a problem removing a live round when called upon to do so.

You've probably noted some describing loading .005 off jam. In this scenario you establish the jam dimension, then seat .005 deeper into the case.

Others will refer to "jam" when actually describing touching the lands. And then go on to describe .005 past jam as meaning .005 INTO the lands after touch.

However you choose to measure touching lands, it's just a starting point you work from. As long as you can get a consistent measurement bullet to bullet, brand to brand, you're accomplishing what you intend.

We're all talking about the same subject, just using different languages.

Texas10 nailed it!

edit
So did Boyd

 

[dmoran]

In the past I have read that seating to touch the rifling or longer increases pressure by 5,000 to 6,000 psi......

From actual pressure traces, I have never seen pressure increased near those amounts (5,000 to 6,000 psi) from off to in.
More like 1000-psi from a little off to a little in, and more like 2000 to 2500-psi from say -20 to +20.
5,000 to 6,000 would be a substantial indifference - IME

 

[BoydAllen]

From actual pressure traces, I have never seen pressure increased near those amounts (5,000 to 6,000 psi) from off to in.
More like 1000-psi from a little off to a little in, and more like 2000 to 2500-psi from say -20 to +20.
5,000 to 6,000 would be a substantial indifference - IME

Thanks, My source was an old memory of an old manual, and the units were probably CUP. I am sure that your data is much better. For those that are not familiar, could you put up a link to the Pressure Trace unit?

 

[dmoran]

.... For those that are not familiar, could you put up a link to the Pressure Trace unit?

Mine is an older model (Pressure Trace 1) but here is the Link: https://www.shootingsoftware.com/pressure.htm

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[BoydAllen]

Here's a classic good load for 6PPC. The only difference between all these traces is the first 3 were seated right on the lands. The last 4 were 30 thousands of an inch off the lands. Note the substantially reduced pressure and overall reduction in energy.

As the bullet broke its neck tension and jumped to the lands it provides more expansion room for the gases reducing both the energy and velocity.

 

[BoydAllen]

I got this from the Pressure Trace page.

 

[Bindi2]

Shows the best place to be for the most consistent pressure.

 

[dmoran]

Here's a classic good load for 6PPC. The only difference between all these traces is the first 3 were seated right on the lands. The last 4 were 30 thousands of an inch off the lands. Note the substantially reduced pressure and overall reduction in energy.

As the bullet broke its neck tension and jumped to the lands it provides more expansion room for the gases reducing both the energy and velocity.

To me, besides being drastically extreme, those are some unusually low pressures all together. I've never pressure tested any rifle caliber that would have produced that low of pressures, even from lower charge levels. Can't speak at all to such low pressure results, and what I would call a "classic good load for a 6PPC" from H322 and a 70gr bullet would be substantially higher in pressure. Also, the velocities (which directly co-act with pressure) would also have changed several hundred feet per second for that drastic of variation in pressure.

From those traces you posted, there is a pressure variation of 10,071-psi (23.6%) which there would also have to be nearly the same rate of change in velocity. Have you ever seen a 23.6% change in velocity from a 30-thousandths change in seating?

PS: even without using pressure testing equipment, one can estimate pressure variation based from velocity data.

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[Brians356]

To me, besides being drastically extreme, those are some unusually low pressures all together. I've never pressure tested any rifle caliber that would have produced that low of pressures, even from lower charge levels. Can't speak at all to such low pressure results, and what I would call a "classic good load for a 6PPC" from H322 and a 70gr bullet would be substantially higher in pressure. Also, the velocities (which directly co-act with pressure) would also have changed several hundred feet per second for that drastic of variation in pressure.

From those traces you posted, there is a pressure variation of 10,071-psi (23.6%) which there would also have to be nearly the same rate of change in velocity. Have you ever seen a 23.6% change in velocity from a 30-thousandths change in seating?

PS: even without using pressure testing equipment, one can estimate pressure variation based from velocity data.

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The low pressures jumped out at me as well, but thanks for spotlighting the 10k psi peak pressure drop. I too would love to understand more about these data and the associated velocities, and what Paul Harvey called "the rest of the story."

PS it occurs to me the pressure figure annotations are for peak instantaneous pressure, whereas load data usually lists Maximum Average Pressure (MAP). But the associated MAPs for these curves would be even more vanishingly low for real world 6 PPC loads.

PPS ok I checked and MAP actually might better be termed "AMP" as it refers to the average maximum pressure. So the figures in the graph are MAP as published in certain load data. Several peak pressures captures for a particular load are averaged.
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