How to find the best bullet jump?

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ghbucky

New member
My rifle reloading is going along well, and I've got a load that my rifle seems to like just dandy: 55gr Barnes TSX FB over 27gr of TAC. (of course it would like Barnes!)

I have them at 2.190 OAL now.

I read a series of blog posts by a benchrest shooter where he started seating the bullets .004 deeper successively to find the sweet spot for jump for his rifle.

Is that the way to go about finding that spot, or is another approach to finding the bullet jump the rifle likes?
 
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cdoc42

New member
ghbucky, is your 2.190 measurement from the base of the cartridge to the tip of the bullet or from the base of the cartridge to the ogive of the bullet?

Before you adjust for the jump you need to know how far the particular bullet you're using is from the rifling (i.e., lands). Your starting point is to make a dummy cartridge so the bullet engages the rifling. Measure that with a comparator (Hornady/Stoney Point...although S.Point may have sold out to someone else) and that will be the overall length with the bullet seated at the rifling. The measurement would be recorded as 0.0" from the rifling. I label the length at this point as OAL-OG.

From there you seat the bullet deeper in increments of 0.01, 0.015, and 0.02 inches. It has been my experience with 17 different calibers that the "sweet spot" has been either 0.015 or 0.020". You may find distances greater than these more satisfactory by increasing the seating depth by 0.005" each time.

So if your baseline cartridge is 2.190 OAL-OG, your next seating depth would give a OAL-OG of 2.200, then 2.205, then 2.210.

If you change bullets, you start all over because the shape of each bullet places the ogive in a different spot such that the OAL-OG will be different even though the distance from the rifling will be the same.
 
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hounddawg

New member
Is that the way to go about finding that spot, or is another approach to finding the bullet jump the rifle likes?
Click to expand...

the only way is to do range testing

there are several way to determine the start position

I use the Berger method to get in the ballpark, then fine tune from there

Berger Method Link
 
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ghbucky

New member
ghbucky, is your 2.190 measurement from the base of the cartridge to the tip of the bullet or from the base of the cartridge to the ogive of the bullet?
Click to expand...

That is from base of the case to tip of the round.

Thanks for the details.

Yes, this is for .223
 
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cdoc42

New member
You can't determine the seating depth from the rifling by measuring the base to tip. It has to be base to ogive.
 
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kell

New member
Year ago, I read a great aticle from Hornady about headespace, jump and geneeral internal ballistics. I can't find it anymore - I wish I'd printed it out. If ayone has it, I'd sure like to see it again.
 
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cdoc42

New member
ghbuckey, here's an example of bullet difference. My AR-15 shoots a "Surplus Bullet Co." 55gr FMJ at a OAL-OG of 1.873". I did not determine the jump, but my measurement from base to ogive is that number. I bought bullets again, but from Everglades in Fla. The OAl of both bullets is close 0.740 and 0.742. But the Ogive differs by 0.24". So I seated the new ones at 1.8.49" and I shot 15 of each today. Both hit in the same place at 100 yards. I put 3 balloons at 100 yards and 4 at 200 yards and hit 7 in 8 shots.
 
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Bart B.

New member
Cartridge base to ogive measuring is meaningless unless the gauge diameter is that where the chamber leade (throat) diameter touches the bullet. It's typically about .002 to .003 inch smaller than bullet diameter.

What's your stuff's diameters?
 
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cdoc42

New member
From the Barnes link:

"When selecting the cartridge overall length (COAL) we recommend starting with a minimum “jump” of .050” off of the lands. You can test different seating depths and find a “sweet spot” that your particular firearm prefers. We suggest working in at least .025” increments as follows seating the bullet deeper to allow a further jump. Your test plan could look something like this:

1st group- .050” jump
2nd group- .075” jump
3rd group- .100” jump
4th group- .125” jump
5th group- .150“ jump
6th group- * see below"

If Bench Rest shooters represent the start of this phenomenon at the leade and back off in increments of 0.01 to 0.005 inches, why would Barnes suggest STARTING at 0.05?"

From the first time I was exposed to this suggestion, years ago, it was my otherwise uneducated opinion that the production of total copper bullets prevented a problem in assuring the ogive was reasonably in the same place per production cycle. Therefore, starting at 0.05" would prevent potential lawsuits if damage of any sort occurred by following traditional exercises.
 
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ghbucky

New member
The Berger article explains that the window is measured in 1/100 of inches, and that if you try to find the window by making 1/1000 adjustments, it will take forever and you are likely to shoot out the barrel trying to find it.

Make 1/100 adjustments to find the sweet spot area, and then go to making 1/1000 adjustments from there if you want to keep dialing it in.

I'm probably wasting my time with doing this, but it is entertaining, since this is for a magazine fed AR. I got the Hornady ogive and chamber gauge and made the measurements today.

The jam is .02 longer than max COAL for .223, so that is my min jump, since I'm not really interested in loading 1 at a time... maybe...
 
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Bart B.

New member
Bullet jump distance to the leade (throat) is controlled by distance from the case shoulder to bullet and rifling diameter at contact. It varies with case headspace, freebore length, case shoulder setback from firing pin impact and rifling angle.

Measuring from the case head is meaningless because it's not against the bolt face when the round fires. That head clearance dimension varies several thousandths when the primer fires.
 
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cdoc42

New member
Bart, that makes sense, but how does one measure headspace in one's rifle? As well, which comes first - the shoulder forward movement coming to a halt at primer ignition or the jump of the bullet into the leade? If it's the former, that results in a fixed event that
removes any variation in that regard. If anything, it gets the bullet closer to the leade which would argue against seating at the leade to start with.
 
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ghbucky

New member
Measure the shoulder to case base with a headspace gauge like the one Hornady makes using a fire formed case fired in that chamber will give you that measure.

Bart's point, which is valid and one I hadn't considered, is that using the standard SAAMI case provided with the Hornady gauge, doesn't consider that the case will have expanded to fill the chamber before the bullet leaves the case and makes the jump.

So, I'll need to do a bit 'o math on this.
 
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Bart B.

New member
cdoc42 said:
Bart, that makes sense, but how does one measure headspace in one's rifle? As well, which comes first - the shoulder forward movement coming to a halt at primer ignition or the jump of the bullet into the leade? If it's the former, that results in a fixed event that
removes any variation in that regard. If anything, it gets the bullet closer to the leade which would argue against seating at the leade to start with.
Click to expand...
Most precise or accurate way is to have a gunsmith use one of these gauge sets.....

http://pacifictoolandgauge.com/incr...-/11035-223556-rem-11-pc-incremental-set.html

....... in your rifle. He may charge $15 but that's less than buying a set for $450.

The round doesn't fire until the primer's dented about .020" after the case shoulder slams into the chamber shoulder. Case shoulders set back a thousandth or more and can easily be measured before and after firing an empty primed case.

Beware of gauges that don't have a full circle flat for the case head to mate with. Otherwise an out of square case head won't have the same reading on the gauge as its twisted around.

If your rifle's bolt face is not squared up, fired cases from it will have a thousandth inch or more spread in measurements as its orientation on a clock face in the chamber changes.

22 caliber bullets start moving forward in the case neck when pressure is around a couple hundred psi depending on how tight the case neck grips the bullet and amount of friction. If the .224" diameter bullet needs 10 pounds of force to push it in the case neck, it'll start moving when pressure is about 250 psi.

Several times as much pressure to push it full into the rifling. Peak pressure occurs when the bullet's a few inches down the bore.
 
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cdoc42

New member
Bart: "If your rifle's bolt face is not squared up, fired cases from it will have a thousandth inch or more spread in measurements as its orientation on a clock face in the chamber changes."

I understand your position to be if one seeks accuracy perfection, measuring base to ogive (or some point on the ogive) omits other considerations that fly in the face of that pursuit, even though most written works that I have read direct us to use the base to ogive measurement.

But using that measurement as a baseline is useful on a practical basis. If I find seating the bullet 0.015" from the lede produces a tighter group than seating 0.02", what real significance is there to that number being 0.0151 or 0.0152?

Or am I missing something due to the limitations imposed by this form of communication?
 
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