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How-To How to find jam for your rifle chamber and bullet combination

conditionzero

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Learning how to find jam is a great skill to develop as you begin to learn about advanced reloading techniques. Especially as they relate to barrel harmonics.

To get started you will need a few things materials and tools.

A. Bullet / Projectile
B. Unprimed Case
C. Calipers
D. Ogive / Bullet comparator device
E. Bullet Seating Die
F-G. Some way to record the data


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Procedure Overview
  1. Seat the bullet in the case JUST enough so that it will not fall out on its own.
  2. Gently insert the round in to the rifles chamber.
  3. Thrust your bolt forward and lock it down.
  4. Quickly lift the bolt and eject the round.
  5. Measure jam ogive.
  6. Subtract 0.002" (twenty thousandths) from the measurement, adjust die, reseat.
  7. Test ease of action operation and ejection.

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Step 1 - Make the initial bullet seating.

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Step 2 - Gently chamber the round.

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Step 3 - Thrust the bolt forward and lock it down.
I have experimented with this step and you don't need so much force that you have a running start. But also gently cam'ing over the bolt has not generally been enough to get the bullet seated deep enough. Essentially just throw the bolt forward as if you were doing a fast follow up shot with a bit of authority. You should then be able to lock the bolt down with relative ease.

Before thrusting, you can see the bolt is back a bit.
1694803218017.png

After, it is seated completely. At this point the bullet has jammed in to the rifling of my barrel.
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Step 4 - Quickly lift the bolt and eject the round.

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Step 5 - Measure Ogive.
This is your first measurement. For this specific chamber and this specific bullet. You cannot transfer this data to a projectile of different shape etc.
For my chamber, and the Berger VLD Target 140gr 6.5mm, jam length at the ogive is 2.2760"

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Step 6 - Subtract twenty thousandths from the measurement in Step 5, adjust your die, and reseat.

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Step 7 - Validate your seating depth and test the ease of chambering and ejection of the round.

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And that is it! You now have two very important measurements for your chamber and bullet. Jam & maximum OOAL (Ogive Over All Length, Jam - 0.002" (twenty thousandths). It is important to note that your OOAL is almost certainly too long to magazine feed. That is a choice you will have to make on your own, if you want to start load development here or at your maximum available length of your magazine.

Once you determine your powder charge and have consistent combustion, you can start to tune barrel harmonics by adjusting the seating depth. Armed with this data, you know exactly how far you can go before you hit the lands of your barrel with this bullet. Repeat this process as many times as necessary for new bullet testing.
 
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I've always found with my guns if I do this, the bullet lodges in the rifling. My own choice is the Hornady tool. I can lock the post in place and replace the bullet after I get it out to get my measurement. Have you noticed this with any of your rifles?
 
I've always found with my guns if I do this, the bullet lodges in the rifling. My own choice is the Hornady tool. I can lock the post in place and replace the bullet after I get it out to get my measurement. Have you noticed this with any of your rifles?
I have not run in to that with any of my rifles, are you using your standard neck tension?
 
To find jam your round needs to jam. To find the lans your round needs to touch the lans. How are you setting everything up. Are you setting to stay off jam or are you setting up to stay off the lans.
 
To find jam your round needs to jam. To find the lans your round needs to touch the lans. How are you setting everything up. Are you setting to stay off jam or are you setting up to stay off the lans.
Yes good point. Your bullet will jam in to the rifling here. I took the original reply as it was staying in when attempting to eject the case.
 
Yes good point. Your bullet will jam in to the rifling here. I took the original reply as it was staying in when attempting to eject the case.
Yes. This is why I like your write up on the subject.
 
Amazing how applying KISS to just about everything works so well.
 
You explained it better than anyone else has I need to get a couple bullet comparators
 
Well I fell into the hole today and got my comparator today and have been bugging Richard with stupid questions he is probably thinking I don’t need to be messing with this stuff lol so I done like the thread said and found the measurements and backed it off .002 now do I load 5 at this measurement then continue making them shorter until I find what the rifle likes? I know more dumb questions lol
 
Well I fell into the hole today and got my comparator today and have been bugging Richard with stupid questions he is probably thinking I don’t need to be messing with this stuff lol so I done like the thread said and found the measurements and backed it off .002 now do I load 5 at this measurement then continue making them shorter until I find what the rifle likes? I know more dumb questions lol
Yes, that number you arrived at is the maximum length you should load at for that rifle. Otherwise you risk being in the lands. Your rifle might like that length, but that is usually longer than a magazine will allow. So if you have limits like that, you will have to figure out what the max length your platform will allow.

Things to check:
  • Magazine Max OAL
  • Feed ramp -> magazine binder plate / forward wall interference
Once you have those sorted out, then you know how long you can seat while still reliably running your rifle. Or what you need to do to load longer, like modifying the feed ramp, getting a magazine without a binder plate, etc.

*This is all assuming you are running a rifle that has a detachable magazine, like an AICS pattern bottom metal.
 
Not sure if that was a typo, but back off .020" off jam, not .002" 2 tho off jam isn't enough in my opinion.

No worries at all on the questions. Ask before you waste components, materials or kaboom 😂

CZ is correct. Make sure you load for mag length. I like .003" shorter than mag length to get 100% reliable feeding from the mag.
 
I get the feeling this is going to be expensive lol
In the end, not loading at 2.800" COAL will probably end up being cheaper. Less time and money spent on components to achieve the accuracy for your rifle you want.
 
I checked jam 3 times and got something different every time I’ll mess with it more tomorrow
 
What were your measurements? You may not get an exact ex: 2.222" 2.223" each time. You are forcing the bullet into the case by jamming it into the grooves. It should be close though
 
Welcome to the rabbit hole. I just wish I had more time at home to better my reloading skills. I may get another press and more tooling for my camper.
 
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