Jump question…

nikonNUT

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Good morning all! So I have been fiddling with a load for my 338LM and I had a question about bullet jump. I’m running 300gr Scenars and found my velocity node at 2805 ft./s with the bullet jammed .010” in the lands. Now it’s time to search for my jump node but I’m a tad confused. Ours like to be close? Or do they like to jump a mile? My gun shoot factory S&B 300gr ammo well (.4 MOA) and based on the CBTO is .120” off the lands. Will the Scenars like a similar jump? If so what jump range would you suggest to start with? I know this is all subjective but I’m just trying to get a range of where to start. Thank you for the help! Just trying to get this joker dialed in. 😊
 

J E Custom

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Testing Is the only way to find out how much jump you rifle likes. the main advantage of more jump Is normally velocity and less chamber pressure or both. Bullets should not be jammed In the lands for consistency, but they can be touching. Most shooters start with .010+ off the lands and increase until the accuracy node is found. Jump can range anywhere from zero to over .100 thousandths with some exceeding .150 thousandths.

I would recommend that any Magnum should start @ .020 and go + from there with more and more seating depth because of chamber pressure.

J E CUSTOM
 

nikonNUT

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Testing Is the only way to find out how much jump you rifle likes. the main advantage of more jump Is normally velocity and less chamber pressure or both. Bullets should not be jammed In the lands for consistency, but they can be touching. Most shooters start with .010+ off the lands and increase until the accuracy node is found. Jump can range anywhere from zero to over .100 thousandths with some exceeding .150 thousandths.

I would recommend that any Magnum should start @ .020 and go + from there with more and more seating depth because of chamber pressure.

J E CUSTOM
Thank you, sir! I only went with .010" jammed because I figured if I don't see pressure at max and jammed I'll be safe-ish when I do start jumping. I will start at .020" (my rifle likes the SMKs at .030 jump) and go from there! Are .010" increments good or are .005" increments better for tuning accuracy?
 

Mikecr

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Be sure to back off your 2805fps charge a couple grains at least. This, is not for safety, but to get away from that powder node -so that you can purely consider which seated depth is communicating best with your bore.
Then start over with powder testing.

When you went to a powder node at jammed, then you gotta know moving seating out of jam will collapse that powder node and the gun will shoot worse. You might not find other seating at that charge shooting as well as you seen at jammed. So how will you evaluate results to know best seating?

The answer is to do seating testing away from any powder node to begin with. Ideally at the worst powder charge.
With this, even while groups are large & ugly, you can see them vastly open and close -due to the seating changes you're making.
You pick the best of that coarse seating, AND THEN move to powder testing.
Then go back to seating, with best coarse seating and best powder, and tweak it in it's window for tightest group shaping.
 

mwkelso

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Be sure to back off your 2805fps charge a couple grains at least. This, is not for safety, but to get away from that powder node -so that you can purely consider which seated depth is communicating best with your bore.
Then start over with powder testing.

When you went to a powder node at jammed, then you gotta know moving seating out of jam will collapse that powder node and the gun will shoot worse. You might not find other seating at that charge shooting as well as you seen at jammed. So how will you evaluate results to know best seating?

The answer is to do seating testing away from any powder node to begin with. Ideally at the worst powder charge.
With this, even while groups are large & ugly, you can see them vastly open and close -due to the seating changes you're making.
You pick the best of that coarse seating, AND THEN move to powder testing.
Then go back to seating, with best coarse seating and best powder, and tweak it in it's window for tightest group shaping.
When you’re saying to choose the worst powder charge, are you implying to run a powder test to find the worst possible? Or would this be simply using the book’s load minimum for that bullet and powder?
 

Mikecr

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I don't know what book minimum is, but good loads are often near SAAMI max, so I'd back well away from there.
With a 260AI capacity I dropped the charge 2 full grains down from max. 1full gr for a 223Rem.
This put me between expected upper/lower powder nodes for the loads & barrel lengths, and there was nothing good about the grouping.
That's fine, I don't want good grouping. I want best seating as clearly seen with whatever grouping I'm at.
I test seating at 200yds, but 100 would work.
 

nikonNUT

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Oops. I goofed on backing off the load but there is no proper load data for what I'm doing aka no load data for the 300gr Scenar with Ramshot LRT so I went with the data for the 300gr SMK and proceed with caution to 101.5 gr or .5gr over max. My sweet spot is 96 gr (2794 FPS) to 97gr (2802FPS) with 96.5 producing the 2805 FPS so I'm right in the middle of the charge weight. Hopefully it doesn't skew my data.
 

Mikecr

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As you move off the lands MV will drop out of your 2805fps node.
But it's a big cartridge so you could also drop a couple grains in powder to get well away from that node.
Otherwise, seating deeper and deeper will raise MV back into or near node.

Coming into/out of powder node with seating changes would make results appear better/worse than the seating itself is really causing. That's not good for the test.
 
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epags

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---The answer is to do seating testing away from any powder node to begin with. Ideally at the worst powder charge.
With this, even while groups are large & ugly, you can see them vastly open and close -due to the seating changes you're making.
You pick the best of that coarse seating, AND THEN move to powder testing.
Then go back to seating, with best coarse seating and best powder, and tweak it in it's window for tightest group shaping.
Now you tell us (me). Will definitely try this process on my 7mm mag Remington.
Thanks.
 

nikonNUT

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So an update... Finally found my jump and it was WAY out there as in .109" off the lands. I can't say I was surprised as my rifle shoots S&B 300gr stuff really well and that runs a .120" jump. Watched the groups close up and at .109" I shot a .43MOA group. Now I just gotta revisit the velocity node and see what moved, what the MV is, and what kind of ES and SD I'm getting!
 

asd9055

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So an update... Finally found my jump and it was WAY out there as in .109" off the lands. I can't say I was surprised as my rifle shoots S&B 300gr stuff really well and that runs a .120" jump. Watched the groups close up and at .109" I shot a .43MOA group. Now I just gotta revisit the velocity node and see what moved, what the MV is, and what kind of ES and SD I'm getting!
Real nice article on bullet jump by Mark Gordon
 

epags

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[
Be sure to back off your 2805fps charge a couple grains at least. This, is not for safety, but to get away from that powder node -so that you can purely consider which seated depth is communicating best with your bore.
Then start over with powder testing.

When you went to a powder node at jammed, then you gotta know moving seating out of jam will collapse that powder node and the gun will shoot worse. You might not find other seating at that charge shooting as well as you seen at jammed. So how will you evaluate results to know best seating?

The answer is to do seating testing away from any powder node to begin with. Ideally at the worst powder charge.
With this, even while groups are large & ugly, you can see them vastly open and close -due to the seating changes you're making.
You pick the best of that coarse seating, AND THEN move to powder testing.
Then go back to seating, with best coarse seating and best powder, and tweak it in it's window for tightest group shaping.
Question: How many shots constitute a group? How many rounds of each seating depth do you shoot? One round @ each seat depth or 3 rounds? Or More?
 
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nikonNUT

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I did three shot groups to test jump and now I will return to find my "micro" node inside the 97 to 98gr (She lost about 50FPS with the increased jump) zone to see where the sweet spot is.
 
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