Jam length in a bolt rifle?

[Bullet]

Jam length in a bolt rifle?

I’ve been reading about Jam length and it seems that people use this term to mean two things.

1 – The length of a cartridge where the bullet just touches the lands.

2 – The length of a cartridge where the bullet is jammed into the lands by seating the bullet long and having the bullet be seated farther into the case by the lands when the bolt is closed.

I also read that people seem to be interested in finding the first jam listed above (bullet just touching lands) and see that there are tools for sale to try to measure this distance. Also some people use different methods like using a marker or smoking their bullet.

Since all the above is to try to get more accuracy out of a rifle why would knowing the distance to the lands be of any value. I believe that knowing the distance to the lands doesn’t matter and that using the second meaning of jam listed above would be the way to go since you only seat your bullets into the case more to find the OAL that shoots best. Who cares how far into or out of the lands you are if wherever you are shoots best. Am I missing something?

 

[Zak Smith]

You might want to measure the ogive to case head dimension when the bullet contacts the lands so you can start off in a certain offset range. Or you might want to measure it so you can increase your COAL as the throat erodes. Or you might want to measure it just to be sure you have enough clearance.

Also note that some competition shooters "soft seat" their loads LONGER than will fit, and closing the bolt full seats the bullet. This is generally a bad idea if you might have to eject a live round.

-z

 

[MNgoldenbear]

General rules of thumb
(1) Seating into the rifling can elevate pressure. This is bad if you were already near max pressure. Also, I'm pretty sure it would reduce the max velocity you could work up to, as max pressure would be reached at a lower powder charge than otherwise possible. It IS often advised to seat into the rifling with a light to moderate charge when case forming to keep the head against the bolt face and eliminate head separations.
(2) Target loads often shoot well seated near the rifling, say less than 0.050". If you know how deeply to seat the bullets to touch the rifling, you can start from just off the rifling and work back, looking for a "sweet spot" for that gun.

Also mentioned was the issue of compensating for throat erosion.

And there is also a possibility of seating the bullet into the rifling, then pulling the case free while extracting a live round. (Also mentioned)

 

[USSR]

Who cares how far into or out of the lands you are if wherever you are shoots best. Am I missing something?

Bullet,

Target shooters care about the distance from the bullet ogive to the leade. It's not the cartridge OAL that helps determine the accuracy of a particular load, it's the distance a bullet has to jump that is the determinate factor. Assuming you don't have a lot of runout, the longer the distance a bullet has to move forward before engaging the lands, the more chance of it greeting the lands in a less than straight manner. I typically set my cartridge OAL so that the bullet ogive is 0.010" off of the leade. Consistancy is the key to developing accurate ammo.

Don

 

[bogie]

IMHO, "hard jam" is when I have little squares of rifling on the bullet when it comes out. Or when the "lines" don't get any squarer, you know? I back out from there. Generally I think about about 0.003 from "hard jam" is a decent place to be.

 

[Bullet]

USSR

“It's not the cartridge OAL that helps determine the accuracy of a particular load, it's the distance a bullet has to jump that is the determinate factor.”

I agree with you that you should not use the OAL to measure your cartridges. It should be made from the base of the case to the ogive on the bullet (If there is a term for this I don’t know it) when seating longer than the loading manuals listed OAL’s..

“Target shooters care about the distance from the bullet ogive to the leade.”

They shouldn’t, they should care about the best accuracy they can get. I think you may find that the best accuracy attained in some rifles will have no jump and be seated into the lands. But this is the point I am trying to make that measuring to the lands is not necessary to find the best seating depth.

There are other factors to finding the best accuracy in your rifle but I was only trying to address the issue of finding the lands, which I believe, is a waste of time.

bogie & Zak Smith

Two other terms for Jam - soft seat & hard jam.

I think both of these have to do with neck tension.

 

[Kiwi98J]

I typically set my cartridge OAL so that the bullet ogive is 0.010" off of the leade. Don

Very interesting. How do you determine the "ogive to leade" dimension? Do you cast the chamber? or base off reamer dimension? How does this compare to the case to land dimension?

 

[Zak Smith]

Bullet,

Keep it mind that if you seat with "0" clearance, then you have zero allowance for seating error, or crud buildup. Again, it can make extracting a loaded cartridge problematic. Both of these will reduce reliability.

I can honestly say that this is the first time I've heard COAL vs. lands and soft-seating referred to as "jam".

Here is one reason you might want to know exactly where it is-- you are working up a load and increasing the COAL in the series. At the point the ogive touches the lands, pressure will rise, and may be dramatically. To avoid this condition, you'd have to do a charge work-up series at every COAL increment. If you know you are off the lands 0.010" or whatever, you know pressure increase will remain more or less predictable.

Another reason is that if you KNOW your lands start at datum COAL "X" and you want to seat the bullets such that the ogive has 0.030" or less clearance, then you can start at X - 0.030" or so, NOT at 0.100" or whatever back, and waste all those load increments working towards it.

 

[Kiwi98J]

I think you may find that the best accuracy attained in some rifles will have no jump and be seated into the lands..

I think everyone would agree with the above statement and would also agree that in other rifles some "jump" is needed to find best accuracy.

So how is that distance measured so that "best accuracy" is repeated with regards to seating depth?

you should not use the OAL to measure your cartridges. It should be made from the base of the case to the ogive

But where on the ogive of the bullet if you do not know the point the lands contact the ogive?

 

[Bullet]

Kiwi98J
“So how is that distance measured so that "best accuracy" is repeated with regards to seating depth?”

You do need to measure your cartridge using a bullet comparator & caliper. You can also use this to set up your seating die.

Zak Smith

If you start working up your loads from Jam the neck tension will determine how far your bullet will be into the lands. If you start here and work up your powder there should be no more of a problem with pressure than when working up powder charges from off the lands. Either way you still have to watch for pressure signs. Once your powder charge is determined you start seating your bullet .005 into the case and keep going back until you find the best seating depth. The trouble with starting at the lands and then going back in the case is that you will never know if your rifle would shoot better seated into the lands.

“Here is one reason you might want to know exactly where it is-- you are working up a load and increasing the COAL in the series. At the point the ogive touches the lands, pressure will rise, and may be dramatically. To avoid this condition, you'd have to do a charge work-up series at every COAL increment. If you know you are off the lands 0.010" or whatever, you know pressure increase will remain more or less predictable.”

The above is another reason for starting from Jam and working backwards then you don’t have to worry about pressure. You will not build pressure from Jam by reducing seating depth. Plus Jam is easy to find.

I have not tried this but this is what I've read and I'm going to try this in my rifle.

 

[Zak Smith]

Bullet,

We've named good reasons to want to know the dimension, which was the original question. You can disagree with those reasons, but there they are. No one is forcing you to buy a gauge.

In all but specialized games, I believe that seating the bullets so long that they are fully seated and jammed into the lands upon closing the bolt is a quite bad idea. A person usually realizes this the first time he goes to eject a live round, the bullet stays stuck, powder gets dumped into the action, and the now empty case is ejected. I have in fact seen this happen during a "Sniper" match.

FWIW, here are some other relevant points:

1. It is likely that the magazine will limit OAL before the bullet will hit the lands. If it doesn't when the rifle is new, it will a couple thousand rounds later as the throat erodes.

2. I believe some bullet/chamber/barrel combinations "prefer" a closer dimension, while some "have no preference." In my AI-AWP for example, I have run the 155gr Lapua Scenar from 2.800 to 2.930 without any discernable change in accuracy at 100 yards.

3. Insufficient purchase of the case neck on the bullet shank can harm accuracy.

4. Reliability is worth considering.

 

[Bullet]

Kiwi98J
“But where on the ogive of the bullet if you do not know the point the lands contact the ogive?”

I use this bullet comparator –

http://www.sinclairintl.com/cgi-bin/category.cgi?category=RESDTCO&item=09-600&type=store

My bullet comparator doesn’t measure from where the bullet actually contacts the lands. If you take an empty case no powder no primer that has been sized and seat a bullet long then chamber your dummy cartridge then carefully extract your dummy cartridge you will see exactly where the lands mark your bullet.

Zak Smith

In your last post I agree with 1, 2 & 4. Not so sure about 3.

I also agree that if there is not enough neck tension and you eject a round the bullet could or will remain in the lands (not to good).

I have neck bushings so I can control the neck tension but I do admit that this is not for everyone or every rifle. However this interests me and I’m going to try it. If anyone is interested I’ll post my results but it’s getting cold here so I’m not sure when I’ll be able to shoot.

 

[Zak Smith]

“In your last post I agree with 1, 2 & 4. Not so sure about 3.

In the limit case, there is virtually no neck tension holding the bullet true to the bore. Even feeding from the magazine up the feed ramps into the chamber could deflect the bullet.

I also agree that if there is not enough neck tension and you eject a round the bullet could or will remain in the lands (not to good).
I have neck bushings so I can control the neck tension but I do admit that this is not for everyone or every rifle. However this interests me and I’m going to try it. If anyone is interested I’ll post my results but it’s getting cold here so I’m not sure when I’ll be able to shoot.

In this case, you are overpowering the "jam" friction with the neck tension. It may be enough, or it might not be enough to retain the bullet upon extraction. Will it work with a fouled, or dirty chamber?

Most people who use bushing style dies do so to minimize the work done to the necks, prolonging case life, not to give "extra" tension to retain bullets.

-z

 

[Bullet]

Zak Smith

“In this case, you are overpowering the "jam" friction with the neck tension. It may be enough, or it might not be enough to retain the bullet upon extraction. Will it work with a fouled, or dirty chamber?”

The amount of neck tension does make differences. I guess this is part of what I will find out.

“Most people who use bushing style dies do so to minimize the work done to the necks, prolonging case life, not to give "extra" tension to retain bullets.”

I believe the statement above is right. How much tension would you consider extra? My loaded rounds measure .339 at the neck.

You seem to be very knowledgeable and I appreciate your replies. At least this subject has been discussed here where others can form their own opinion.

 

[Bullet]

bogie
“IMHO, "hard jam" is when I have little squares of rifling on the bullet when it comes out. Or when the "lines" don't get any squarer, you know? I back out from there. Generally I think about about 0.003 from "hard jam" is a decent place to be.”

I think I’ll give it a try. How much neck tension are you using?

 

[bogie]

I turn my necks to 0.00865, and size with a carbide 0.2575 (I think...) button that I got from Skip. With a Bart's Ultra bullet seated, I've got clearance at the base. This is probably heavier neck tension than I need, but it keeps the bullets from pulling out as easily.

 

[USSR]

Bullet,

Aren't you using your comparator in conjunction with a Stoney Point OAL Guage? If not, why not? This precludes the need to "jam" bullets into the lands, and is far more accurate to boot. BTW, I use the very same Sinclair comparator as you, and Yes, while it does not give you an actual base to ogive measurement, it does give you a relative measurement to go by.

Don

 

[Rockstar]

I started using a comparator, in conjuntion with the Stoney Point tool, after hearing all the concerns that bullet tips aren't uniform, etc., etc. Then it occurred to me that, unless the i.d. of the comparator is exactly the same size as the i.d. of the barrel where the lands begin (wonder what the odds are?) then how could the comparator be exactly accurate either?

Maybe it's just that the comparator is more consistent.

 

[Rockstar]

I use the Stoney Point tool.

 

[USSR]

...unless the i.d. of the comparator is exactly the same size as the i.d. of the barrel where the lands begin (wonder what the odds are?) then how could the comparator be exactly accurate either?

Again, the measurements involved are relative measurements. After using the Stoney Point tool and getting the OAL length of your chamber using whatever bullet you will be using, you slip the comparator over the bullet and get the base to the top-of-the-Sinclair-comparator measurement. Then you setup your seating die and adjust it until you get the same base to top-of-the-Sinclair-comparator measurement. It really doesn't matter whether the point at which the bullet makes contact with the leade is the EXACT point at which the bullet makes contact with the comparator, as the relationship remains equal in both instances.

Don

 

[Zak Smith]

In 308, I am using a .334 bushing for Lapua brass. I would go looser but this seems to duplicate the tension of the new brass or factory loads (which is not really much more than a guess on my part, based on a couple things).

 

[Bullet]

USSR
“Aren't you using your comparator in conjunction with a Stoney Point OAL Guage? If not, why not?”

I don’t have a Stoney Point OAL Guage so I’m not sure what it does. If it measures OAL I can do that with just my calipers or to the ogive with my bullet comparator.

“This precludes the need to "jam" bullets into the lands, and is far more accurate to boot.”

I’m not sure what you mean. Jam length is just a place to start. Far more accurate than what?

“BTW, I use the very same Sinclair comparator as you, and Yes, while it does not give you an actual base to ogive measurement, it does give you a relative measurement to go by.”

I agree with this and use it to measure to the ogive of my cartridges. I also use this measurement to set up my seating die.

 

[Bullet]

Zak Smith
“In 308, I am using a .334 bushing for Lapua brass.”

I am also using Lapua brass and my rifle is a 308. I don’t have a bushing that small.

I started this thread because I’m trying to find the most accurate load in my rifle and I thank you for your replies.
Your replies are very well worth considering before seating bullets into the lands.

 

[USSR]

I don’t have a Stoney Point OAL Guage so I’m not sure what it does. If it measures OAL I can do that with just my calipers or to the ogive with my bullet comparator.

Bullet,

The Stoney Point OAL Guage works in conjunction with a modified case of the same cartridge you are using, such as .308 Winchester. What they do is take a .308 case, drill out the primer pocket, and tap the hole to accept the OAL Guage. You then attach the OAL Guage by threading it into the hole. The Guage has a small rod that slides up into the interior of the .308 case. You then place a bullet into the neck (it is large enough so that there is no neck tension on the bullet), adjust the rod upwards so that you can move the bullet up and down, thereby effecting seating depth. You then insert the modified case with the OAL Guage attached, into the chamber with the bullet not out very far from the neck. By pushing the rod forward, you push the bullet forward until it contacts the leade. This is the maximum OAL in your chamber with THAT particle make of bullet. If you want to use a different make of bullet, you will need to do this again with the new bullet. Now, there is a set knob on the rod that will tighten and hold the rod in the position it is now. You set the knob, remove the Guage/modified case and bullet, slip your comparator on the bullet, and measure the base to comparator distance of the modified case with bullet with it still attached to the AOL Guage. Believe me, it's alot easier to do than it is to describe it. Now that you have the base to top-of-comparator dimensions of your chamber, you simply adjust your seating die to duplicate the results, minus 0.010" or whatever you want to have your bullet's ogive off of the leade. Hope that helps.

Don

 

[bogie]

If your bullets are accurately produced, the ogive/base will be the same on all of them.

I don't see a lot of the Stoney Point dealies at benchrest matches. I see a lot of the Sinclair nuts... They take three hands to use, but you don't need to leave them attached to your caliper, etc...

The Sinclair nut also has the advantage of allowing you to use a case which was fired in your rifle, sized (to your preferences) in your die, etc...

But you don't need it to figure jam length. You don't even really need to know it, if you can leave your dies set up.

Take a primerless case, sized.

Seat a bullet long.

Buff up the bullet real good with some 0000 steel wool.

Measure the OAL and write it down.

Chamber the thing.

SLOWLY open the bolt and remove it. If the bullet stays in the lands, seat it a little shorter next time, or use more neck tension.

Measure the OAL, and make sure the bullet actually went into the case some. If it didn't, you weren't seated long enough.

Put the case/bullet...

a) In your seating press

b) In your hand die

With the seating stem way out...

a) Insert the case into the seating die via the press handle. If the press cams over, hold it at the cam point (I _do not_ like seating with a "standard" press). Now screw in the seating stem until it hits the bullet. This is your jam length. Screw it in about 0.003" more, and you should be good to go.

b) With your hand die, hold the die/top in your left/right hand, depending, and with the other hand slowly screw in the seating stem until you feel it contact the bullet. This is your jam length. Screw it in about 0.003" more, lock it down, and you should be good to go.