Jim Boatright

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Feb 21, 2018
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Yes, JB.IC, I see that the muzzle speed dy/dt function calculation got garbaged up since I had it correct a few days ago. I have been mostly working with the muzzle position y(t) function. I fixed it again.
At bullet release, 1328 microseconds, the muzzle position is slowing almost to an upward moving stop at 1413 mu-sec. The upward speed is 2.3 mm/sec which is not really going to disturb the pitch of the bullet nose down very much during its exit from the barrel. Here is a pdf of the corrected spreadsheet.
Anyone who wishes to use this tool for their own rifle and loads will need the current "live" Excel workbook. QuickLOAD and Excel software are both also required.
 

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  • Barrel Natural Freq Calculator V04 Sheet 4.pdf
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Jim Boatright

Sergeant of the Hide
PX Member
Minuteman
Feb 21, 2018
197
288
This example load is accurate in the rifle for which it was developed because bullets exit when the muzzle is almost to an upward-moving halt, and because this exit timing provides positive "compensation" for the usual variation in muzzle velocities within a group of shots. Compensation involves lower muzzle pointing angles for faster, earlier exiting bullets and vise-versa.

As far as I know, this compensation tuning was discovered by British rifle tuners about 100 years ago while tuning No.1 Mark III* SMLE's for long-range target accuracy.

If one wished to tune for bullet exit times even closer to the time of upward muzzle halt, one could reduce the powder charge a few tenths of a grain or use a slightly heavier bullet to delay bullet exit slightly without affecting anything else appreciably. Changing barrel length also affects bullet exit times directly, so it is not an independent variable.
Jim Boatright
 
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JB.IC

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Jan 25, 2019
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my QL seems to be off.

All the cartridge dimensions are 100% correct. The only uncertainty is the max case capacity but I’m using the 33XC and Tubb has said it’s 139 gn H2O. The filling/L.R. % seems to be correct however.

The MV it spits out is always about 140 FPS slower. Maybe this has something to do with difference in bore diameter? That’s something I don’t have the tools to check.

id imagine this is an acceptable tolerance all things considered but how does this variation apply to the muzzle position calculation?

Also Jim based off the examples you posted on your excel workbook, the new version that’s posted is missing some of the inputs compared to the examples you posted.

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Jim Boatright

Sergeant of the Hide
PX Member
Minuteman
Feb 21, 2018
197
288
The two things I adjust in QL to get closer agreement with chronographed MV are first Shot-Start pressure in the powder panel (for bullet hardness and throat roughness and angle) and then Weighting Factor in the cartridge panel (mainly for the amount of "necking down" of cross-sectional area in the interior of bottlenecked cartridges. If QL suggests WF of 0.50 for your 33XC, I would try 0.52 instead, for example.
I have continued developing the barrel motion calculations in my spreadsheet, including improvements in the Data Input screen, as I have been using it more myself. My current barrel motion calculations are based on a Gaussian function model approximating the base-pressure curve, which is working quite well. I have also added an exponential vibration damping function to allow for continued "ringing" of the barrel vibrations after the initial peak of base-pressure driving function has passed.
I will update the Excel workbook and accompanying write-up when I am satisfied with them.
Jim Boatright
 
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JB.IC

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Jan 25, 2019
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I adjusted Short Start Pressure. It went from 3626 psi to 12200 psi. I’m loading -0.03 off the lands btw. This gives me the correct MV.

WF was set for 0.55 by default of the program. It says it suggest 0.33 for over bore bottleneck cases.

if I adjust WF to 0.33 then I have to adjust SSP to 8600 psi which seems to be a less drastic adjustment compared to the one previously mentioned. With these adjustments, it gives me the correct MV and then I went to the other powder I tested as well and the numbers it gave for MV were almost exactly what I got. QL gave a 15fps faster result.

appreciate the help Jim!
 
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Jim Boatright

Sergeant of the Hide
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Feb 21, 2018
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Yes, straight-walled cases should have a WF of essentially 1.0 in QL, while the proverbial "50 BMG case necked down to fire phonograph needles" would be close to 0.10.
I have had to use SS pressures of 12 ksi for some of my monolithic copper solids in QL, but usually around 8 ksi, even for them. Hard military or tough thick-jacketed hunting bullets require 5-6 ksi, while really soft alloy lead-cored match bullets having thin (annealed) jackets run about 2.4 ksi with the usual bullet jumps. Bullet seating depth is a big factor, with BR-style jam-seated bullets requiring much larger SS pressures.
Your chronograph distance is also a factor in MV measurements, as is use of an MB or suppressor. I have successfully compared inductive-sensing MagnetoSpeed and optical-sensing Oehler 35P readings, and now use microwave LabRadar readings (for convenience) which tend to be measured farther downrange. An inertia-triggering accessory for the LabRadar unit seems to provide the most reliable triggering for nearest-to-muzzle readings. A red dot alignment sight accessory is also best for LabRadar measurements all the way out to 100-yard targets with larger than 30-caliber bullets. I also use a substantial baffle C-clamped to the shooting bench to protect the LabRadar unit from direct muzzle blast for more consistent measurements. I recommend a piece of 3/8-inch plywood through-bolted along the bottom edge between two pieces of 1.5-inch angle iron for constructing the baffle. The baffle deflects a lot of pressure with each shot.
Jim Boatright
 
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JB.IC

Jackass of the Hide
PX Member
Minuteman
Jan 25, 2019
566
304
Yes, straight-walled cases should have a WF of essentially 1.0, while the proverbial "50 BMG case necked down to fire phonograph needles" would be close to zero.
I have had to use SS pressures of 12 ksi for some of my monolithic copper solids, but usually around 8 ksi, even for them. Hard military or tough thick-jacketed hunting bullets require 5-6 ksi, while really soft lead-cored match bullets having thin (annealed) jackets run about 2.4 ksi. Bullet seating depth is also a big factor, with jam-seated bullets requiring larger SS pressures.
Your chronograph distance is also a factor in MV measurements, as is use of an MB or suppressor. I have successfully compared inductive-sensing MagnetoSpeed and optical-sensing Oehler 35P readings, and now use microwave LabRadar readings (for convenience) which tend to be measured further downrange. An inertia-triggering accessory for the LabRadar unit seems to provide the most reliable triggering. A red dot alignment sight accessory is also best for LabRadar measurements out to 100-yard targets with larger than 30-caliber bullets. I also use a substantial baffle C-clamped to the shooting bench to protect the LabRadar unit from direct muzzle blast for more consistent measurements. I recommend 3/8-inch plywood through-bolted along the bottom edge between two pieces of 1.5-inch angle iron for constructing the baffle.
Jim Boatright

I also use a Labradar and a inertia-trigger. I typically shoot suppressed but I’m waiting on a thread adapter for my 33XC barrel for my suppressor as my barrel is 3/4” and my suppressor is 7/8”.

I typically have the Labradar a few inches in front of the muzzle off to the side about 8”. Between that and my magnetospeed, they’re normally within 12 FPS of each other. It’s easy to true the difference in MV.


I noticed there’s an “#NAME?” error on the Muzzle Motion worksheet. The error is traced to the L column under the SUM(mm). I have only put inputs on the Data Input worksheet.

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