lost on jump/jam from ogive..... please help

K

Kory87

Guest
I've never understood how to measure jump and jam of a bullet in relation to o-give. I use the bullet jam method where I barely seat a bullet in an unprimed case and then put it in the chamber, close, and lock the bolt. From here I take out my comparator and measure the o-give. I write this as max on my dummy round, and right down my o-give measurement on it also and keep this in my dies case. Now, I shoot 107 grain SMK's and know they shoot best jammed .020" or so, understanding jammed as PAST the o-give. My question is how do I jam past my max o-give? Please help if u can. anything is appreciated.

Thanks,
Kory
 
Everything you need to know about precision reloading is in this pic

BRreloadingcasegaugeandjamfinder.jpg
 
crb said:
Everything you need to know about precision reloading is in this pic

BRreloadingcasegaugeandjamfinder.jpg
Click to expand...

I have the exact setup as the top one, but what's the bottom one do? And how does that answer my question? I am wondering how u find ogive.. and is that like finding a zero where u can jam so many thousandths and jump back so many thousandths?

Kory
 
What ever the number is that comes up when you put the case and bullet in the chamber is your starting point. If you want .020 jam you set the seater up so that the loaded round measurement is .020 longer. If you want .020 jump you set the seater to give loaded rounds that are .020 shorter. Ogive is a number on the bullet box.

The lower case is being measured to set the seater die to give the desired case clearance in the chamber. I can tell from the measurement that is a fired case from my 30BR. I will set the sizer to bump the shoulder back about .002".
 
Kory87 said:
I've never understood how to measure jump and jam of a bullet in relation to o-give. I use the bullet jam method where I barely seat a bullet in an unprimed case and then put it in the chamber, close, and lock the bolt. From here I take out my comparator and measure the o-give. I write this as max on my dummy round, and right down my o-give measurement on it also and keep this in my dies case. Now, I shoot 107 grain SMK's and know they shoot best jammed .020" or so, understanding jammed as PAST the o-give. My question is how do I jam past my max o-give? Please help if u can. anything is appreciated.

Thanks,
Kory
Click to expand...


It's ogive :)

And "jam" or bullet jam or "jamset" is purely a subjective number......

I use a very light neck tension for my initial measurement which results in a "jamset" measurement which is juuust touching the lands. You can get even finer than this using a silver sharpie which will show the tiniest contact....

So you find "touching"....... and you find maybe a "deep jam" using heavier neck tension.... and maybe a REALLY deep jam using wicked heavy neck tension.....

All subjective measurements.

It useta' be you could talk about "square marks" and "oval" and rectangular marks and compare notes but with the proliferation of leade angles and ogive shapes it's become essentially meaningless to compare.

"Maximum jam" or deepest possible seating for YOU will be wherever you end up when you let the rifle seat the bullet. The only way you can go deeper is to increase neck tension. Again, no hard and fast rule.

Look up "comparator" in the dictionary. Comparators don't MEASURE anything, they are used for subjective comparison.

"The ogive" is the curved portion of the bullet ahead of the parallel shank. ALL of it, clear to the tip, the hole in which is the "meplat."

There is no such thing as jamming "past the ogive," it takes 5-600 lb of force to engrave the bullet.

hth

al
 
I think that we have a little terminology issue to resolve. A bullet's ogive is the part in front of where the sides are more or less parallel to the tip. The entire length of this curve is the ogive, not just where your tool is touching. What you are doing is measuring the overall length of the loaded round from a point on the bullet that is more consistent than the tip, somewhere on the ogive. You are not measuring the ogive. Now as to the rest of your problem. When you seat a bullet in an unprimed case is the neck of the case sized the same way as it would be for a loaded round? If it is, the length that it is pushed back to is the jam length for that bullet and neck tension. By definition you cannot get any farther into the rifling, because if you seat longer than that the bullet will simply be pushed back. Now, as to the seating depth that you desire, you can refer to it as so much shorter than jam, or at jam, or so much longer than touch, at touch, or so much shorter than touch. This is not to say that some do not load longer than jam. It is sometimes done, it is sometimes referred to as soft seating. In effect the barreled action finishes seating the bullet. There has been a recent spate of what to me is misuse of terminology. What I think that you are trying to say when you say .020 jammed, is that you are .020 longer than touch. If this is the case then you need a way to measure at what length the bullet just touches the rifling. The method that you described will not give you that length. Whatever neck tension the unprimed case has will virtually guarantee that there will be some engraving of the rifling on the bullet. The length that the bullet is pushed back to will be longer than it would be if just touching the rifling. There are several methods of accurately measuring touch. If you are interested, I can describe them.
 
Thanks for the responses everyone. I'm in my first year of reloading, and am wanting to get into benchrest shooting, so am trying to get my foot in the door by reloading. And yes, I would love for u to go into detail. If this makes any sense I'm trying to find the ZERO. In my mind I see it as a ZERO and then you can either go longer "jam" or shorter "jump". I'm sure I am flawed in this terminology, so feel free to make me better. Also, what books to read would u guys recommend?

Thanks,
Kory
 
Kory,

see, here's the thing...... THERE IS NO ZERO!!! :)

The spot you're finding is an intersection of angles in an imprecise environment.

If you want to try find a "zero" then I suggest you set up a case like the top one in the pic, (You said you've already got that...) now grab up some bullets. Mark them 1-2-3-4-5 etc.

Now jamset #1, measure and write it down.

Now #2....

And 3...

etc.

Write down all your answers.

Now, do them again,

And again.

You see the problem? They will vary. Maybe only 2-3 thou, but they'll vary.

Only in theory and in books are things "zero" in terms of variance, real world there's runout. Things get in the way. Ogives vary, reloads are crooked, the lands themselves are crooked, sometimes the lands grab the bullet and pull it out a bit...... So you find a place to start....you average your measurements and come up with a number, call it "zero" if you want. Now you can work forward and backward off this point.

I recommend the reloading books here http://www.precisionshooting.com/books.html

hth

al
 
Hi, Kory. Here's the method I use to establish the 'zero' or 'just touching' point:
-Remove firing pin. assy from bolt.
-Full length size a case that's been fired in your chamber. If you are using a bushing die, use a bushing that's .003-.004 under what a loaded round measures. You want a good amount of neck tension for this process.
-Seat the bullet 'way long.
-Polish the bullet with some 0000 steel wool.
-Make sure the chamber/barrel are clean and dry. Lube the locking lugs.
-Chamber your dummy round. It will close hard, as the long seated bullet meets the rifling.
-Open the bolt and remove the round. If the bullet stays in the bore, just tap it out with a cleaning rod or wooden dowel.
-Examine the bullet and you'll see the marks from the rifling. If the bullet stayed in the bore, just resize the case, reseat it and repolish with 0000.
-Seat the bullet deeper by .010 and repeat. You'll see the marks starting to fade as you move the bullet further into the neck each time you change the seating depth. When the marks start to get noticably lighter, change the seating depth change to .005, as you're getting closer to 'zero'.
-When the marks just disappear, you're at the 'zero' or 'just touching' point for that bullet.

Record the seating stem length or base-to-ogive dimension and record that. Repeat for each bullet you're going to use.

Once you've done it a time or two, it will take you about 5 minutes to find your 'zero' with any new bullet.

Hope this helps. :) -Al
 
Last edited:
Boyd Please describe your process.

I split a case neck and place the bullet out as far as i can then close the bolt absent the firing pin assembly. take an avg of the three attempts and this is what i use as a refernce point.

Trevor
 
This is what I do also

Al Nyhus said:
Hi, Kory. HJere's the method I use to establish the 'zero' or 'just touching' point:
-Remove firing pin. assy from bolt.
-Full length size a case that's been fired in your chamber. If you are using a bushing die, use a bushing that's .003-.004 under loaded round measures. You want a good amount of neck tension for this process.
-Seat the bullet 'way long.
-Polish the bullet with some 0000 steel wool.
-Make sure the chamber/barrel are clean and dry. Lube the locking lugs.
-Chamber your dummy round. It will close hard, as the long seated bullet meets the rifling.
-Open the bolt and remove the round. If the bullet stays in the bore, just tap it out with a cleaning rod or wooden dowel.
-Examine the bullet and you'll see the marks from the rifling. If the bullet stayed in the bore, just resize the case, reseat it and repolish with 0000.
-Seat the bullet deeper by .010 and repeat. You'll see the marks starting to fade as you move the bullet further into the neck each time you change the seating depth. When the marks start to get noticably lighter, change the seating depth change to .005, as you're getting closer to 'zero'.
-When the marks just disappear, you're at the 'zero' or 'just touching' point for that bullet.

Record the seating stem length or base-to-ogive dimension and record that. Repeat for each bullet you're going to use.

Once you've done it a time or two, it will take you about 5 minutes to find your 'zero' with any new bullet.

Hope this helps. :) -Al
Click to expand...



Depending on neck tension but what most people call JAM is really .010 or more into the lands. I figure with .004 neck tension a person can pull a bullet from the lands that is at least .012 in.

Truly, people who consider jam as simply jamming a started bullet and closing the bolt a time or three have no idea with any precision where they are or where they are going. :eek: :D :p
 
Ditto on what Pete wrote. For my PPC I record and work back from jam. I find that different ogive styles like different seating depths and I usually look at the marks on the bullet to finalize my first trial seating depth. I spin the seated bullet in a wad of 0000 steel wool to make seeing the marks much easier. For simple tangent ogive shapes, I like to start with marks that are about half as long as wide. For double radius ogives, I like long marks, with the bullet seated just far enough (.003) back from jam so that they will not be pulled if I have to remove a loaded round. From there I experiment, once I have a promising powder charge. For my varmint rifles, I use the Sinclair tool with firm thumb pressure on the rod, for what I call hard touch. For my Savage .220 Swift, I usually load .010 longer than that. IMO any method that gives repeatable results that work is OK. It's a hobby. You get to do it your way. My only request is that we standardize a few definitions like jam.
 
Thanks for the advice guys. I've got a great plan now based on boyd and al's method. My brother vin aw will be glad to hear. I've been stuck on this for a while, glad I decided to ask. And pete I totally agree. Whifh is why we have this site. Us younger guys can gain knowledge from ppl who have been doing it for years. I love browseing the forums. Last night I saw bench plans from all over the usa and sweden, and a lot of good shooting, scope, rifle information. Hopefully one day ill be able to help the next generation and get them as interested as I am. Now to get a high quality benchrest rifle.. for the next generation of course ;) thanks guys. Now to figure out how to remove that fireing pin.
 
Kory,

I am certainally no expert in this field, however, just read this post and am more confused now than I was before.:confused:

I have done a lot of trial and error testing in this area and feel I can answer your concerns, simply.:)

The reason I write is, your last post "Now, to figure how to remove firing pin" :confused: says to me that you still are in a quandry over your issue.

If you would like speak via phone, Email your phone number to shootershinny@pa.metrocast.net and I will give you a call.

Please include your time zone and best time to call.

Shinny
 
Hey. I am removing the fireing pin because it was said that thas how they do it according to boyd and al's post. But I would love to chat about rifles as it is by far my biggest hobby. Also, I live in ohio right next to you so same time zone. Ill pm u my house number and u can call anytime before 1015. Talk to you later

Kory
 
Great news.. I did the above mentioned technique and found touching the lands to ne at 1.805". Before I was using the max length which I found to be 1.838". So now I hqve a good place to work from. I'm now wondering if I can use this measurement on other bullets too? And if so, how far back does speer 70 grain tnt like to be set in reference of touching?

Kory
Thanks for clearing the goggles guys!!
 
Those measurements are only for that bullet in that rifle. For most of my varmint rifles I have had good luck setting the seating depth .006 to .010 longer than touch and then working up the charge a few tenths at a time. Start on the low side and watching for pressure signs. When the bolt gets a little snug, stop and back up a little.
 
Thanks again boyd. I'm on my way back to the reloading room now.. the 107 smk doesn't do much on groundhogs... I hate it when they make it to the hole.. thanks for the depth advice.

Kory
 
aaaaahhhh

Remember it is your measurements, with your tools and your rifle.

I means nothing to anyone else.

I bet four different guys will get four different measurements with the same gun and same bullets.

It's a starting point to move in either direction for best accuracy...............................
 
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