QuickLOAD uses a default value of 3626 for shot start/initialization pressure. Is there reason to change this (for example for bullets seated into the lands), and, if so, how does one do this? I couldn't find anything on this in the QL User's Guide.
Shot Start/Initialization Pressure Adjustments in QuickLOAD
- Thread starter South Pender
- Start date
If you hold the cursor over that field a popup window will show you what you might use for some different bullet materials, AND. . . it says "WARNING: With bullet seated to touch rifling, add 7200 psi to Start Pressure".
In order to change the field, you do it like you would for the burn rate: click on the box just to the right of the "Ratio of Specific Heats" so that the temperature button pops up just above the "Apply & Calc" button. Then you can make changes.
jepp2
Gold $$ Contributor
Ned Ludd
Silver $$ Contributor
I find the shot start pressure to be a bit of a mystery, myself. How could anyone possibly exactly how far into the lands someone seats a bullet? It would make a big difference in pressure. So I find the notion of simply adding 7000+ psi to the start pressure for any bullets touching or seated into the lands to be a gross over-simplification.
FeMan
Gold $$ Contributor
Since the dynamic coefficient of friction is always less than it is for a static condition, it makes sense that the start pressure will be higher if you are jammed. Adjusting start pressure for different materials and coatings makes sense as well. I wonder how accurate the given parameters are though. Sometimes I get QL to match my actual data extremely well and other times its a fair bit off. It is not perfect but it is a useful tool.
What is the adjustment for different jumps? The QL recommendation for bullets touching the lands (but with no amount of "jam" noted) is to add 7200 to the default value of 3626. However, what should be added for bullets off the lands by varying distances? Presumably, the amount added would be more for a .01" jump than for a .03" jump. Is there some guidance on adjustments needed for bullets off the lands, but by varying distances?
FeMan
Gold $$ Contributor
As long as the bullet is moving when it hits the lands (i.e. not jammed), the start pressure really doesn't vary. So one jump is the same as the other as far as start pressure is concerned.
I adjust the Weighting Factor for the case overbore ratio, then adjust the Shot Start Pressure to match chrono data. After doing that for awhile, you develop a sense of scale for a first guess at a start pressure when setting up the first pressure ladder test with new components.
The start pressure for a given bullet varies significantly from barrel to barrel, as the barrel breaks in, and then ages. With lead core bullets, it also goes up with chamber pressure. Factory barrels can require very low start pressures to get a velocity match. The first 25 or so shots from a new match barrel also tend to give low values. I start load development 10 thou jammed and don't see a large drop in start pressure if I back off to jumping 20 thou. It does drop, but the increase in pressure from the lowered usable case capacity tends to cover it and moderate the velocity change. I don't test many factory barrels and only shoot long heavy for caliber bullets, so my perspective may be skewed.
The Weighting Factor adjusts the relationship between Maximum Chamber Pressure and Muzzle Velocity. Lowering the Weighting factor increases the velocity estimate and reduces the pressure estimate. It's effect on the velocity and pressure estimates is smaller than the start pressure's. If you don't adjust it, the increase in velocity at a given pressure that results from increasing case capacity will be under estimated. I don't adjust the Weighting Factor for a given cartridge for different bullets or powders.
If you adopt this method, you're not going to win any internet speed games and may be startled at what others claim doesn't produce pressure signs.
The start pressure for a given bullet varies significantly from barrel to barrel, as the barrel breaks in, and then ages. With lead core bullets, it also goes up with chamber pressure. Factory barrels can require very low start pressures to get a velocity match. The first 25 or so shots from a new match barrel also tend to give low values. I start load development 10 thou jammed and don't see a large drop in start pressure if I back off to jumping 20 thou. It does drop, but the increase in pressure from the lowered usable case capacity tends to cover it and moderate the velocity change. I don't test many factory barrels and only shoot long heavy for caliber bullets, so my perspective may be skewed.
The Weighting Factor adjusts the relationship between Maximum Chamber Pressure and Muzzle Velocity. Lowering the Weighting factor increases the velocity estimate and reduces the pressure estimate. It's effect on the velocity and pressure estimates is smaller than the start pressure's. If you don't adjust it, the increase in velocity at a given pressure that results from increasing case capacity will be under estimated. I don't adjust the Weighting Factor for a given cartridge for different bullets or powders.
If you adopt this method, you're not going to win any internet speed games and may be startled at what others claim doesn't produce pressure signs.
Internet velocities for the 6 Dasher seem to run in the mid to higher 70 000 psi zone.
The weighting Factor can be done by calculation.
I think the issue with QL for many is they do not have a basic understanding of internal ballistics, mix methods such as ladder tests, chasing bullet seating depths , starting low and working up etc.
QL is a program/system that is stand alone, the two important things are a 'node' (OBT) and optimum charge weight (OCW).
The node corresponds to the length of the barrel and material, carbon or stainless steel.
The OCW is one where the selected components insure that the deterrent chemicals in the powder are all consumed at peak pressure.
With these two criteria looked after the load is usually the best the platform can produce.
Seating depth "opinion only" is varied to mask an issue with bore /cartridge alignment .
According to Quickload, the powder is never completely consumed at peak pressure. I am not sure that any material comsumption has that property.
Yes, I've seen that value (29 psi per .001 off the lands) on another forum. Where does this come from? I didn't see this in the QL User's Guide. The advice I saw on the other forum was to add 7200 psi to the default value of 3625 psi, and then subtract 29 x (the distance from the lands/.001). So in the case I was running (270 Win.) with bullets seated .030" off the lands (hunting load), this meant a shot start/initialization pressure of 9955 psi. This makes a tremendous difference in the resulting velocities and pressures.
As just one example with Alliant Re26 in the 270, with the default value of 3625 psi given as the starting pressure, I'm getting 3217 fps with acceptable pressures (61,308 psi) with 63.4 gr. of powder. On the other hand, with 9955 psi as the starting pressure, I'm getting 3193 fps with acceptable pressure (by the SAAMI, but not the CIP standard) of 64,254 psi with only 61 gr. of powder. The implications for practice are pretty significant.
According to this principal of altering the shot start/initialization pressure, you'd have to have your bullets seated about .25" off the lands for the default value of 3625 to work in the calculation.
I'm having difficulty in getting my head around this QL variable!
I'm having difficulty in getting my head around this QL variable!
It is a problem, as there is very little good data on what this should be. What is (reasonably) clear is that shot-start-pressure is much more important for small cartridges using fast rifle powders, than for magnums and larger cartridges using slow rifle powders. In internal ballistics models, shot-start-pressure and barrel friction (another vaguely understood property) are often adjusted empirically to achieve better agreement with measured pressures and muzzle velocity.
True, the powder is not always, most often not, consumed at peak pressure, the powders deterrent chemicals (makes powder fast or slow) are consumed at peak pressure makes an OCW load along with a broad pressure curve, see post #9.
Barrel is manipulated by bullet weight by a few percentages.
I record the distance from cartridge base to rifling, check base to ogive measurement...same thing. Set seater die up so I will be at magazine length or if single loading 10-15 thousands off. If I am 10 thou off i subtract 290 psi from the jammed pressure, this is my start pressure, 10 535 psi.
At barrel cleaning time, 150-200 rounds, I will check the base to ogive distance, if there is ,lets say, 10 thou erosion the start pressure will be decreased another 290 psi, new start pressure is 10 245 psi.
Now I will adjust powder charge to maintain a velocity and therefore maintain an OBT.
Operating QL, as I do, gets very good results. Velocity predictions are really close , within 30 ft/s of predicted, with a slight Ba change to reach predicted velocities therefore barrel time.
All inputs to the program must be as exact as you can get them. What you are looking for is a barrel time that indicates a node, with powder type and charge weight you will look for a combination exhibits a long broad curve to peak pressure and beyond (see post #9) and have the Pm and Z1 line overlapped.
I use a Labradar to get accurate velocities, with out accurate velocities the program falls apart.
Barrel is manipulated by bullet weight by a few percentages.
I record the distance from cartridge base to rifling, check base to ogive measurement...same thing. Set seater die up so I will be at magazine length or if single loading 10-15 thousands off. If I am 10 thou off i subtract 290 psi from the jammed pressure, this is my start pressure, 10 535 psi.
At barrel cleaning time, 150-200 rounds, I will check the base to ogive distance, if there is ,lets say, 10 thou erosion the start pressure will be decreased another 290 psi, new start pressure is 10 245 psi.
Now I will adjust powder charge to maintain a velocity and therefore maintain an OBT.
Operating QL, as I do, gets very good results. Velocity predictions are really close , within 30 ft/s of predicted, with a slight Ba change to reach predicted velocities therefore barrel time.
All inputs to the program must be as exact as you can get them. What you are looking for is a barrel time that indicates a node, with powder type and charge weight you will look for a combination exhibits a long broad curve to peak pressure and beyond (see post #9) and have the Pm and Z1 line overlapped.
I use a Labradar to get accurate velocities, with out accurate velocities the program falls apart.
One would have to assume that the pressure trace system is accurate as it has been set up.
With QL, everything starts with velocity and the acceleration rate of the bullet from case mouth to muzzle.
The 'math' takes the variables into account.
No. I can say that with absolute certainty. Not only do you need the pressure as a function of time, but you need to know when in that pressure-time trace the bullet sets off down the barrel and that is the tricky bit.
Aberdeen Proving Ground have microwave interferometers which they use to track the position of the bullet in the barrel in real time - which is not something you will find down at the ACME Reloading store, or on the Internet. They might have done reports on shot-start-pressure in small arms, but I have not seen any. There are other reports they have done (on primer force in the 308 Win and the 223 Rem) where the reported data shows bullet position and pressure as a function of time and where you can extract the shot-start-pressure, but they do not actually make any comment on this in the reports(!)
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