Quickload and Gas Guns?

[GeoffreyKolbe]

For those who want to estimate gas port pressure (the pressure at the barrel's gas port) take note that you can create a pressure curve that indicates pressure at the base of the bullet rather than chamber pressure (the default pressure).

Bullet base pressure is lower than the mean gas pressure because the bullet is moving at speeds which are a significant fraction of the speed of sound in the powder gasses. That is not true of the gas port. I think you would be better to use "chamber" pressure as a function of projectile distance travelled rather than bullet base pressure. This is not actually the chamber pressure, but the mean gas pressure in the gas column behind the projectile at the time the projectile has travelled a certain distance up the barrel.

 

[243Mendoza]

Reading the instructions that comes with the software will probably help. You can change several inputs to help adjust the predicted velocity to match the measured velocity.

Case capacity
Weighting factor
Powder burning rate factor
Shot start pressure
Ambient temperature

All of these factors can be adjusted and if you read the manual you will find recommendations of what to adjust first.

The temperature adjustment is enabled when you click on the small icon to the right of the Ratio of specific heats.

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After you click the icon a temperature adjustment icon will appear above the Apply&Calc button
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After you click on the Temperature icon this window will appear

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It's entitled Powder Temperature Variation so the software expects you use the powder temperature but in most situations your powder will be the same temperature as the ambient temperature and it's pretty hard to actually measure the temperature of the powder.

There is no information that I've found that defines a limit to the Ba adjustment but I don't like to change my numbers by more than .1, most often I can get good results with far less of a change, more like plus or minus .02.

Before I start changing the Ba value (which can vary from lot to lot of the same powder) I make sure that I use the proper Shot Start Pressure value based on the recommendations that the software displays in a popup if you hold the your cursor over the value for a second or two.

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The Weighting Factor is another value that I change before I start adjusting the Ba value. This represents the amount of powder that follows the base of the bullet. Most ballisticians use .5 (50% of the powder charge) as the standard value. You can change this up or down a couple tenths (.4 to .6). This value is just a guess and it is controlled by the case shape and powder characteristics, I don't know of any standard that specifies what this value should be in relation to the case or powder so it's just a guess.

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When you decide to change the Ba value some people recommend changing the case capacity instead. I wont change the case capacity by more than half a grain if I've measured it.

A lot of these adjustments are going to be based on what input data you feel has a better chance of being correct. For example, if you have to choose whether to change the Ba value or the case capacity I'd base it on whether I know one or the other for a fact. If I've actually measured the case capacity (in accordance with the software manual's instructions) the I'd probably change the Ba value since I couldn't measure it.

Great info. I guess if all else fails, read the instructions.

 

[243Mendoza]

I use ambient temp, but am careful not to let the barrel heat up too much while measuring velocity. Measuring ambient temperature may not be perfect, but it is much simpler than trying to accurately measure barrel temperature, something many shooters are not equipped to do properly. I do use temperature strips on the side of the barrel near the action (https://www.mcmaster.com/temperature-strips/, Cat. #59535K13). These are not at all what I would call accurate, but they are an acceptable indication of when the barrel is sufficiently cooled to continue shooting. You can also simply place your hand on the barrel 2 to 4 inches in front of the action before you start shooting, then again while you fire a string, just to ensure the barrel isn't becoming noticeably warm to the touch. None of these methods is perfect, but they suffice for most people's needs. As with many things, I generally view simpler as better, even though the software is capable of far more than I actually use it for. Certainly for someone just learning to use the software, it may not be necessary to attempt to incorporate the most sophisticated capabilities at the very beginning.

As @rammac noted, the manual is a good place to start learning about the software and how to use it more proficiently. It can sometimes be a bit difficult to find a very specific piece of information, but it's a good resource nonetheless. It has a procedure for "calibrating" the software not too dissimilar from the rather simplistic method I described. It also provides estimates for the maximum adjustments to Ba that are advisable. The more you use the software, the better feel you will develop for making the appropriate adjustments, and the manual can be a big help in that process.

Thanks for the info. May look into the temp strips. I try to space my shots to avoid a hot barrel during a test but then, during a match, one tries to shoot as fast as possible to avoid wind change.

 

[243Mendoza]

I have checked velocity difference of gas/no gas by shooting with an adjustable block wide open and completely closed on multiple barrels of different lengths and gas tube lengths, The difference was less than shooting the same load in a bolt action. The variation was less than the same load used in two different rifles with the same barrel length, 20-30 fps.

Thanks, this is helpful info.