Finding Jam - I thought I knew how

[User4302021]

A man with one watch KNOWS what time of day it is...the man with three watches is never sure.

 

[25WSM]

If you take the extractor out you can not use the wheeler method. Your using the extractor to pull on the case to feel the click. If you took the extractor out the bolt will open and the case won't move. I use the wheeler method and I don't remove the extractor or the ejector. It just works great to get you close to the touch length. The wheeler method doesn't give you jam length it tells you when your bullet is just off the rifling. Off the rifling and jam are 2 different things. Crushing the bullet into the rifling on a sized case will give you max jam length. This varies from case to case and depends on lube in the neck or carbon left in or coated bullets. Lots of variables to max jam. But the wheeler method will give you a zero point to start from that is consistent. Find this spot and anything longer has jam and anything shorter has jump. It's the first reference number I write down for any seating depth adjusting.
Shep

 

[david g ranes]

I used Eric Cortina's method, which is what I always used to do. He said it doesn't matter if you get different readings, but if I have a large difference from different shells, how do I know if I my load is going to be .010 off jam or .020 into the rifling?

I used 3 different de-primed cartridges and they were all different. So I did them over and over again. It looks like each cartridge comes out the same each time - but each are different. Hmmmm. So I checked cartridge length. Each are different. In my pea brain, it seems to me it would not matter what the cartridge length is, because we are going for base of cartridge to ogive. That figure should be the same and the bullet jump should therefore be the same when you measure the OAL. Am I right?

It looks like my fired cases measure differently. I have a Lee case hand sizer. It sticks into the primer hole as a pilot and you spin it which cuts to size. But it looks like the length is different.

So, it could be that I am going to reveal some ignorance here, but I am here to learn. Does the actual case length make a difference in how deep the bullet sits from the lands when you measure CBTO?

Secondly, does cartridge length make a difference in accuracy if they are all seated the same CBTO? If so, what would the tolerance be? within .001? .002? or .009? etc. I will appreciate any input. Thanks

Do what dog rocket said but add a little glue to inside of neck let it set up in the action then bullet won't move when ejected. David

 

[Baylor243]

My wife makes jam every year just before Christmas. If I open the fridge I can usually find it. If its not in the fridge then its in the basement-- peach, apple,, plum, and strawberry ---always in Mason jars

Send me some sample jars and I'll measure the neck tension on the lids for ya.

 

[Les in Wyoming]

OK, this stuff is driving me crazy. I had a fairly good load with Hornady ELD-M. But I was running out of bullets, so I figured I needed to try others. In so doing, I have learned more and changed how I am finding jam for seating depth. When I did my Hornady 208 ELD-M load, I pushed a bullet into the lands with a small dowel and ran the cleaning rod up to it. I bought a simple cartridge length kit where you put a clamp on it where it meets the bolt face and then another where it meets the bullet at the lands. The measurement you get is supposed to be your COAL at jam. I developed a load from that figure, starting at .014 jump and went up in .003 increments. As it turned out, my best groups were at .020 to .026. So I have been loading at .021. (Erik Cortiz method).

Well, as I was developing a new seating depth for these different bullets, I went back to using the method we have discussed here. I set a bullet long in a case and closed the bolt. 210 gr Noslers got 4.004. Interestingly enough, 210 Bergers got the exact same figure. So I decided to see how 208 gr Hornady's do. It got 4.000. Hmmmm. they are all about the same. But when doing others, I got anywhere from 4.0020 to 3.965. Each got the same reading when pulling the bullet and doing it again. In testing 10 different ones, 4 got between 3.965 and 3.975. So I "called it" at the shortest one. (And this is supposed to be exact?) So that is my new "jam." I am measuring at the ogive.

So now, I am using the Berger method and am going to test loads with .010 jump, then .050 jump and then .090 jump. According to them, a node is about a .040 wide band. So this method is to quickly find it, and then you tune it from there.

As I shoot these, with bullets ranging as much as .055 where the ogive hits the lands, how in the world can these increments make a difference? I have some labeled at .010 jump, .050 jump and .090 jump. But honestly, they all could vary far more than that.

I have read that when finding jam, you should use the same bullet and same cartridge. I guess so. This keeps you from being confused at how different they all are. So what I am wondering is, why even bother if they all hit the lands at such different measurements. Each bullet has marks where they meet the lands and each is a radical different place, though all the bullets measure within .001 or so.

 

[25WSM]

Well first of all seating a bullet long and jamming it in and closing the bolt is going to give you max jam length. This can be as much as .025 thou from zero jam. And it won't be consistent. That's why so many have said to use the wheeler method. It will be the same number each time you do it. Very repeatable. All that max jam tells you is how long your cartridge is when the bullet jams so hard it quits moving forward and the case slides up the bullet. The reason this measurement is so vastly different each time is the bullet is stuck in the barrel and sometimes it slips in the case before letting go and sometimes depending on neck tension will stay in the barrel and the case will come out with out it. There is absolutely no reason to not do the wheeler method. You don't need any special tools. Except to take the firing pin unit out of the bolt. You probably already have that tool. Look up the wheeler video and try it. It's easy and works. Will get you your zero point number to work from.
Shep

 

[User4302021]

No one has ever been able to explain to me why one datum point is any more relevant than another datum point. The difference in load developement between starting from some amount of "jam" and starting from "touch" is absolutely zero.

All this hand wringing about getting the exact same measurement every single time is wasted when the seating depth "node" you are looking for is supposed to be as wide as possible anyway (something like .030" - .040" according to Berger). That is a pretty big target to hit.

If I cut a slot in the neck of a case and soft seat (jam) a bullet, then deep seat enough to back out of the lands...then my starting point is just as valid as any other starting point.

It makes absolutely no difference if I am actually .010" off "touch", or .015" off touch. The repeatability of the CBTO is only relevant once I start adjusting (either shorter or longer) from that original point.

This is the point Erik Cortina has been trying to make for a while now.

Unless you are sorting your bullets by ogive measurement, then your bullets themselves incorporate some variation. Unless all your brass is a crush fit in the chamber, the brass you size can exhibit some variation which will change the distance from the lands on any given loaded round because the ejector is exerting force on the base of the round as it sits in the chamber. Slight differences in neck tension can affect seating depth for rounds seated at the same die setting.

Wherever you start...it doesn't even need a number designation, you can call it "X". You can then work from "X" and find out if your rifle shoots at "X-.010", "X-.020"...etc.

Even if you need to chase the lands, you are STILL working from "X". If you are trying to go back and find "touch" again and work from there, then you are doing it wrong because the profile of your throat and lead has changed anyway. So .010" off of "touch" isn't the same as .010" off of "touch" when you started.

 

[QuietTexan]

What seating die are you using?

 

[Equalizer]

Neck tension. It's different neck tension between the pieces of brass you are using.
Personally I only use a case fired in my chamber and one bullet. I use the Sinclair tool. You only need to get the measurement once. It's a reference point.

 

[Muddyboots]

+1 to Dog Rocket IMHO. The measurement you take is ONLY a snapshot in the life timeline of the barrel. Which is why I buy into Eric Cortina's explanation on the overall relevance of the measurement. We all shoot far more than most so the throat leade measurement is "moving target" and how fast is dependent on the load itself. Max hot loads accelerates this along with the type of powder. What is only relevant is accuracy which goes back to Eric's points in his video.

 

[orkan]

Taking an accurate measurement the first go round is still a part of Erik's method, which I believe is what this thread is about. What you do with the information after doing it properly once on a new barrel, that's where the opinions seem to divide. Personally, I've always done it like Erik describes.

 

[antelopedundee]

+1 to Dog Rocket IMHO. The measurement you take is ONLY a snapshot in the life timeline of the barrel. Which is why I buy into Eric Cortina's explanation on the overall relevance of the measurement. We all shoot far more than most so the throat leade measurement is "moving target" and how fast is dependent on the load itself. Max hot loads accelerates this along with the type of powder. What is only relevant is accuracy which goes back to Eric's points in his video.

If one uses one of those micrometer seater stems maybe just back it out by .001 for every 50 rounds fired to allow for throat erosion.

 

[orkan]

If one uses one of those micrometer seater stems maybe just back it out by .001 for every 50 rounds fired to allow for throat erosion.

This could be a dangerous assumption to make I think. A great many cartridges will not move at that rate. More importantly, making any arbitrary adjustment of any kind that doesn't directly correlate to what is being seen on target via live fire testing is generally not advisable.

 

[25WSM]

What most people don't realize is seating depth tuning is mainly a function of timing the length of time the bullet is in the barrel. Once this is established that seating depth will work for a long time. Even after your barrel throat starts to erode there will be a time when the erosion slows your load down because of less pressure and you can fix your accuracy by moving the bullet forward" chasing the lands" or by putting in a few 10ths more powder. Pretty much all tuning is just getting the bullet to release at the right time of the barrel swing. Doing a powder charge ladder will give you a course adjustment of your node and then the seating can fine tune the timing even more. So I go through a lot of barrels shooting BR and once you find a good time that is wide I don't change my load till it stops shooting. I do use a tuner on my BR guns but not to find the most accurate load. I use the tuner to keep my load in tune through different temperature shifts. Moving your seating depth and moving your tuner are accomplishing the same thing but in a different way. If you move your bullet out longer it will have a shorter amount of time in the barrel. So if your barrel swing is past TDC or BDC moving your bullet forward will get the bullet out in less time and get you closer to your tune. Moving your tuner out makes the barrel swing slower and your bullet will get closer to your node point. Tuning is simple and is just for the most part a timing of bullet release. The other factors of tuning is how consistent your cartridge combustion is. If it's inconsistent then it affects the timing. And of course some barrels just hate certain bullets and no amount of timing is going to make them shoot. That sucks when the bullet you want to use doesn't play well with your new barrel. PRS and F-class guys do have to chase rifling because the shear volume of shooting they do over the course of a match. Most hunting rifles won't get the amount of shots a PRS gun gets in one weekend in it's entire life.
Shep