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Bullet JUMP Distance for My RAR Predator Loads

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Bullet JUMP Distance for My RAR Predator Loads

Old 08-19-2017, 09:00 AM
  #1  
Spike
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Default Bullet JUMP Distance for My RAR Predator Loads

Hi Folks,

High velocity .243 Winchester cartridges have a reputation of being “barrel burners”. I hope to extend my RAR Predator’s barrel life of by using a slower burning powder like IMR-4350 and by reducing the recommended starting load. I have been hand loading .243 Winchester brass (more off than on) for about a year. I have been exploring reduced velocity loads in the 2500 feet per second neighborhood assembled with CCI LR #200 Primers, IMR-4350 powder, and Sierra #1520 85 grain bullets seated to a Cartridge Overall Length (COAL) of 2.650 inches.

Recently, I found an article called “Determining Bullet Seating Depth”, by the Sinclair folks. It was very interesting. I decided to order Hornady’s “Lock-N-Load Overall Length Gauge”, a “Modified .243 Winchester Case”, and their “Bullet Comparator Basic Set with 6 Inserts”. These tools along with my digital caliper are supposed to allow me to estimate my rifle’s JAM distance. The JAM distance is the distance between my rifle’s closed bolt face and the start of my rifle’s lands and grooves. In addition, the bullet comparator along with its 0.240" insert and anvil attached to my caliper should let me estimate the distance from a cartridge’s case head to the approximate start of the bullet’s ogive (CHTO). The CHTO distance is similar to the COAL, but shorter. I should be able to estimate a bullet’s JUMP distance for my rifle by subtracting the cartridge's observed CHTO distance from my rifle’s JAM distance. For a chambered round, the JUMP distance is the distance between a bullet’s ogive to the start of a rifle’s lands and grooves. I believe this summary explanation is correct. Is it correct?

By using the bullet comparator, I should be able to adjust my seating die so it will produce cartridges with a desired bullet JUMP distance. I think that I more or less understand the basics. Now it becomes a matter of practice and developing a good technique for using these tools so that eventually I will be able to make reliable and repeatable estimates. So far, the estimates of my rifle’s JAM distance are not very repeatable. My estimates vary as much as 0.005 of an inch. Is this typical?

Your comments and suggestions will be deeply appreciated.

Thank You,
Ron
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Old 08-19-2017, 12:42 PM
  #2  
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IMO IMR4350 is a great powder for the .243. I've shot mine using 40.5 gr of this powder over a Hornady 100 gr bullet for years and took a lot of deer with it.
As for bullet jump I like to start at about .050" off the lands and then tinker with it from there.
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Old 08-19-2017, 08:13 PM
  #3  
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Well, you went the expensive route to solve a problem that can be easily solved with 4 very cheap things. 1: Empty unprimed case. 2: Bullet of choice. 3: A sharpie or any wet felt tip marker. 4: Good set of calipers. You can easily find the seating depth where you are in contact with the lands doing it that way. You can then go from there to find how much or how little jump your particular rifle likes. I personally use an almost identical load that Mr.Bronkoski uses, 41.5gr IMR4350 with 100gr Interlocks, in my .243 but that particular rifle likes the bullet to be right at .010 off the lands. And after approximately 2500 rounds sent through that barrel, I have near 0 throat erosion. One of the major causes of throat erosion isn't hot loads, it's people shooting way to fast and getting that barrel cooking eggs hot. Especially with low recoiling rifles such as the little .243. Heat is your enemy with any rifle.
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Old 08-19-2017, 09:08 PM
  #4  
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While the process you described matches up with the Hornady instructions for the tool, it is greatly flawed. You'll note, even in the Hornady instructions, they recognize there is an inherent difference between the headspace length of the Modified Case products and actual headspace lengths appropriate for real-world rifles - this creates an offset between the length you are reading on your tool and the REAL length you need to be using for your loads.

The Hornady system of tools does not tell you your "JAM" distance, at all, as it does not actually dead-length against your bolt face. When you insert the modified case into the chamber, the case dead stops against the chamber shoulder, and when the bullet spindle is used to push the bullet forward, it dead stops the bullet against the leade.

This is critical information, but it is absolutely NOT relevant to the distance between your bolt face and the datum line where the bullet would meet the leade. The system of tools ONLY tells you the distance from the datum line of the case shoulder where it headspaces against the chamber and the datum line where the bullet ogive meets the leade.

A reloader then has to determine the appropriate headspace length between the bolt face and the datum line of the shoulder.

A guy simply has to correct their measurement by comparing their modified case against a once fired case (ideally a twice fired case, after the 2nd firing, before resizing).

All of the Hornady tools have to be used in congress to pull this off, the OAL Gauge spindle, the Modified case, the Bullet Comparator bushing, and the Headspace Gauge bushing. Otherwise, you're not actually getting any real measurements of your actual chamber.
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Old 08-20-2017, 10:42 AM
  #5  
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I just use a ramrod, a sharpie marker, an 8/32 screw with the head cut off, and a pencil, gives me the exact to the lands measurement. also I would not cut the starting load, underloaded is just as bad as overloaded unless you are using H4895
RR

Last edited by Ridge Runner; 08-20-2017 at 10:47 AM.
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Old 08-23-2017, 12:01 PM
  #6  
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I am getting to 5000 rounds through a 243 I have been shooting since 1964. Accuracy is still very good. I would not worry about barrel life, just do not overheat the barrel.

If you still want to download go with a powder in the 4895 range. About 30 gr. of it will be what you are looking for.
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Old 08-23-2017, 03:06 PM
  #7  
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depends on the cartridge capacity, my 7mm allen magnum melts the throat in 600 rounds, but it burns 110 gr of powder every time it goes bang.
RR
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