QuickLoad vs Chronographed Velocities: Effect of Shot Start Initialization Pressure

South Pender

South Pender

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I've been experimenting with QuickLoad in connection with my .270 Win. and have found big differences in estimated pressures and muzzle velocities when the Shot Start (Initialization) Pressure is set at the default value of 3625 as opposed to when it is set to account for bullets seated close to the lands. The QL instructions say that you should add 7200 to the 3625 (i.e., use the value 10,825) for bullets seated to actually touch the lands. Then subtract 29 for each .001” seated off the lands. So as an example, with my .270 Win with bullets seated with a .030” jump (.030” off the lands), I would subtract 870 (29 x 30) from the 10,825 for a value of 9955 as the Shot Start (Initialization) Pressure. I should note that there are some guys who suggest that you should use the default value of 3625 for any jump. When I run QL for my .270 Win., I get very different pressure values and velocities when I use the 3625 value as opposed to when I use the adjusted value of 9955.

So here's my question: Do you guys who have been using QL and chronographing your loads find that you get the best agreement between what QL says and what your chronograph says

(a) when using the default value (3625) for the Shot Start Pressure or

(b) when you adjust that for closeness to the lands, in which case the Shot Start Pressure you enter will be much greater than 3625 as shown above?
 
I've been using Gordon's Reloading Tool in place of Quickload, but I find that my start pressure tends to fall somewhere in between the default 3625 psi and the quickload suggested value of 9900-10800 psi for pretty much all my rifles. It usually tends to fall in the 5500 to 7000 psi range to provide the best modeled results calibrated against chronographed velocities. Once I've found the start pressure the models results tend to be quite accurate for most powder charges when compared to chronographs velocities for those charges.

I've had some folks rigidly tell me I should use the default and some that will not stray from the higher QL starting pressures. I believe the answer is variable and tends to land somewhere in the middle and that's been working out for me.
 
I do calculate the shot start pressure the same way you do. I also measure the fired case volume in grains of H2O (avg 5 of cases) as it makes a good difference. I also measure the bullets instead of trusting the default values. I find the more is measured instead of using default values the more accurate results you get.
 
Not sure I understanding what you are saying (typing), but if you change the initial input from 3,625 psi to ~10,000 psi, of course you're going to get a higher output pressure & velocity.

But, when everything else has been adjusted (eg. temperature & barrel measurements), you've got to work backwards so that your actual readings match your actual results. You may wish to do this via shot start pressure or by changing powder parameters.

Sort of a corollary to that:
I have noticed, in both QL & GRT, that because there are no adjustments for twist rate in the programmes, there are differences that may not be explained any other way, eg. .308 Winchester with 1:10", 1:12" & 1:14" using the same components & the projected velocities do not jive with actual chronographed velocities, even when the lands/grooves & temperature calculation is done. Then you have to work backward to adjust powder parameters. I wonder if adjusting the shot start pressure might be a way to make them match, rather than adjusting powder parameters. I have noticed that in loading the same bullets in 6.5 barrels with differing twists that the quicker the twist, the faster the pressures max out.

In addition, there is no adjustment for primer "hotness", at least in QL.

Further, I have found that neither programme has the capability to properly assess differences from oversize grooves or undersize bullets, due to "blow-by". I have also noticed that in this case, not all powders are equal in this regard.
 
jamesharrison said:
Not sure I understanding what you are saying (typing), but if you change the initial input from 3,625 psi to ~10,000 psi, of course you're going to get a higher output pressure & velocity.
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Yes, of course. I understand that. My question was whether anyone had noticed which of the two Shot Start Pressure values (default or adjusted) seems to produce QL velocity results that closely match their chronographed results.

You've introduced an interesting point about twist having an effect. Do you think that it's possible that a faster twist can change the velocity (presumably reduce it) from what you'd get from a slower twist with all other factors held constant?

I think cbh560's insights are particularly interesting. They suggest that some value for Shot Start Pressure between the default value (3625) and the value adjusted for bullet jump might better model actual chronographed results. This is definitely worth considering.

I should point out that if you start with the 10,825 shot start pressure (for bullets touching the lands) and work back by the formula noted, when you get to a jump of about .25", you get back to the default value of 3625. So this default value would not seem to be right for, say, a jump of .030", by the logic suggested by QL. Still, as I noted above, some QL users insist on using this default value for any jump.
 
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South Pender said:
Yes, of course. I understand that. My question was whether anyone had noticed which of the two Shot Start Pressure values (default or adjusted) seems to produce QL velocity results that closely match their chronographed results.

You've introduced an interesting point about twist having an effect. Do you think that it's possible that a faster twist can change the velocity (presumably reduce it) from what you'd get from a slower twist with all other factors held constant?

I think cbh560's insights are particularly interesting. They suggest that some value for Shot Start Pressure between the default value (3625) and the value adjusted for bullet jump might better model actual chronographed results. This is definitely worth considering.

I should point out that if you start with the 10,825 shot start pressure (for bullets touching the lands) and work back by the formula noted, when you get to a jump of about .25", you get back to the default value of 3625. So this default value would not seem to be right for, say, a jump of .030", by the logic suggested by QL. Still, as I noted above, some QL users insist on using this default value for any jump.
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It is my opinion, but the high variability in ranges of values for every other rinput parameter suggests to me that the start pressure is not as simple as X or Y. I think it's going to be highly dependent on every other measurement (actual bullet diameter, actual bore diameter, twist, land lead angle, and on and on). I did the same calculation you did for increasing the jump and it just didn't make sense to me.

As jamesharrison said, I do also make minor adjustments to the powder burn characteristics, but GRT calculates that for you.

In response to jamesharrison on the primer issue, GRT allows a standard and magnum option for primer which is a difference of approximately 2200 psi higher for magnum rifle in regards to the start pressure. GRT also has a couple of options for lead vs. Copper jacket vs. Solid bullet for the calculation of the start pressure. Since the designer of the GRT program recognizes that all those parameters may cause differing start pressures I don't think that QL's simplified approach of X or Y start pressure is entirely accurate

I should also note that I've "proven" my theory (at least to my own satisfaction) at the range. Using QL's high starting pressure and work up some loads to max pressure. I then went out and shot them, resulting is slower velocity and no pressure signs, suggesting that QL was modeling on the high side. I also worked up some loads using my lower start pressures, velocities matched and pressure signs started showing up at calculated max pressure loads.
 
cbh560 said:
I should also note that I've "proven" my theory (at least to my own satisfaction) at the range. Using QL's high starting pressure and work up some loads to max pressure. I then went out and shot them, resulting is slower velocity and no pressure signs, suggesting that QL was modeling on the high side. I also worked up some loads using my lower start pressures, velocities matched and pressure signs started showing up at calculated max pressure loads.
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That's helpful. By your "lower start pressures," do you mean the 5500 to 7000 psi range you mentioned in Post #2? Or do you mean all the way down to 3625 psi?

Also, can you describe "Gordon's Reloading Tool." And where it can be sourced?
 
South Pender
Re: Do you think that it's possible that a faster twist can change the velocity (presumably reduce it) from what you'd get from a slower twist with all other factors held constant?
I didn't got as far as to chronograph them, but found that given all the same components with the only variable being barrel twist (yeah, yeah I know all barrels are different &c., &c., &c); I got pressure signs first with the barrel having a quicker twist. I hypothesize it's because the quicker twist creates more (back?) pressure.

Re: Gordon's reloading tool:
https://www.youtube.com/watch?v=BinciObVFJo

https://www.grtools.de/doku.php?id=download&do=register

cbh650:
Re: the primer issue, GRT allows a standard and magnum option for primer which is a difference of approximately 2200 psi higher for magnum rifle in regards to the start pressure.

Yes, but as we know not all standard primers, or all magnum primers or even the same primer by the same manufacturer over time for that matter, are created equally.
 
jamesharrison said:
South Pender
Re: Do you think that it's possible that a faster twist can change the velocity (presumably reduce it) from what you'd get from a slower twist with all other factors held constant?
I didn't got as far as to chronograph them, but found that given all the same components with the only variable being barrel twist (yeah, yeah I know all barrels are different &c., &c., &c); I got pressure signs first with the barrel having a quicker twist. I hypothesize it's because the quicker twist creates more (back?) pressure.

Re: Gordon's reloading tool:
https://www.youtube.com/watch?v=BinciObVFJo

https://www.grtools.de/doku.php?id=download&do=register

cbh650:
Re: the primer issue, GRT allows a standard and magnum option for primer which is a difference of approximately 2200 psi higher for magnum rifle in regards to the start pressure.

Yes, but as we know not all standard primers, or all magnum primers or even the same primer by the same manufacturer over time for that matter, are created equally.
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I agree with your point on the primers. But in my opinion it also proves that there is variability to be considered and that QL's 10825 minus 29 per thousandth formula is rigid enoguh that it doesn't even consider that variability and could easily over or under compensate for such a change in components.

todbartell said:
not adjusting the start pressure is foolish, and dangerous if you're loading higher charge weights
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Agreed, that's why I don't use the default pressure. But I also don't subscribe to QL's formula. It's my belief that the answering somewhere in the middle and is not determined by jump alone. As reloaders, we consider dozens of variables and the smallest of changes have real impacts. I think it's too simplistic to use QL's formula and that the "real value" should be determined for each combination of components for each rifle.
 
South Pender said:
That's helpful. By your "lower start pressures," do you mean the 5500 to 7000 psi range you mentioned in Post #2? Or do you mean all the way down to 3625 psi?

Also, can you describe "Gordon's Reloading Tool." And where it can be sourced?
Click to expand...

Yes, I mean the the mid range values that tend to be in the 5500-7000 psi range (depending on the load and rifle). The specific value is determined through field observations and chronographed velocities. This value is determined for each combination of components for each rifle.

I don't use the default 3625 pressure because it doesn't model correctly. But neither does the 10825 minus 29 per thousandth formula.
 
South Pender said:
Thanks for the links, James. I'll look into Gordon's Reloading Tool, although I don't really want to get too deep into the weeds on this....:)
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I was going to reply "S.P. Of course you do; what else is there to do over the long, dark winter months when the snow is blowing, the winds are howling, there's a blizzard in the making & winter's only started", but then I saw your flag avatar, so never mind. ;-)
 
The Quick Load objective for me is to get an Optimum Barrel Time (OBT) with a powder that consumes all deterrent chemicals at peak pressure, has a case fill of 95-105% with the components I'm using.
 
todbartell said:
not exactly, but an adjustment to the Ba (Burn rate) will line things up
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OK. That's interesting. I've seen references to adjusting Ba, but have been baffled by this. QuickLoad enters a Ba value that corresponds to the powder you've specified. Do you then tweak this Ba value, and, if so, on what basis do you do this? Do you need chronograph data to do this?
 
jamesharrison said:
I was going to reply "S.P. Of course you do; what else is there to do over the long, dark winter months when the snow is blowing, the winds are howling, there's a blizzard in the making & winter's only started", but then I saw your flag avatar, so never mind. ;-)
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LOL. Yeah, but things can be bleak here too!:) Given the difficulty I'm having trying to master QL, I'm not sure this old brain is up for Gordon's Reloading Took, with all its additional variables.
 
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