Help needed with Quick Load

[Langey]

Hello everyone - New guy here. I'm from New Hampshire and retired. I informally target shoot at my local gun club and have been reloading for about 35 years.

Just started using a chronograph (Labradar), and when used in conjunction with Quick Load, it has me confused. The cartridge I'm shooting is 6mm TCU. A load of H335 with a 70 gn. bullet chronos at 3075 fps.

Quick Load predicts the velocity at 2914 fps with a chamber pressure of 52,434 psi. QL was adjusted for the actual shooting range ambient temperature of 68 deg F. I should also note that the bullet is not jammed.

If I adjust Quick Load Ba (burning rate factor) until the muzzle velocity is 3075 fps, the estimated chamber pressure when using the original powder charge becomes 63,154 psi.

Here's my question: Do you believe the actual chamber pressure is this high, or am I misinterpreting the QL information?

 

[dcali]

Did you make sure the case capacity (in grains of water) was exactly correct? That's the first thing I'd check.

 

[Langey]

Did you make sure the case capacity (in grains of water) was exactly correct? That's the first thing I'd check.

Yes I did. Thanks for mentioning it. I should have noted that in my original post.

 

[savagedasher]

Barrel length ? Larry

 

[Sheldon N]

You could tweak the weighting factor a little bit, given that this is a case with a 40 degree shoulder and a touch overbore. Maybe something a little under the default 0.5 setting?

The other thing I've observed is that some bullets just run faster than others even with identical powder charges and identical bullet weights. The length of bullet bearing surface can affect this. QL doesn't really recognize this factor, so the only way to correct for it is to tweak the bullet weight by a few grains.

Have you shot other loads with this specific lot of powder that would suggest the burn rate is not as high as the "corrected" figure you had to use?

 

[Langey]

Barrel length ? Larry

16.5 inches

 

[Berger.Fan222]

Even if you chase down all the best details on the entry parameters for QL, I would not take the estimated peak pressures to be more accurate than 5-10%.

There are too many variables that cannot be accounted for.

 

[Ned Ludd]

Barrel length, case trim length, bullet length, and case volume are all user inputs that will affect the output, even if only by a little if the preset values are close. These input values should always be as close to actual as you can measure, in order to get the best predictions.

As far as the pressure prediction, the velocity increased by 161 fps from the prediction made using the preset Ba factor, if I understand your post correctly. That's a pretty big jump. In .223 and .308 cases, I find the predicted pressures goes up somewhere in the neighborhood of 400-600 psi per 0.1 gr charge weight increase, and velocity goes up around 5-10 fps. These are very crude estimates at best because it's not actually a linear response and will vary for different powders/cases/bullets/etc. However, using the median values of 7.5 fps and 500 psi per 0.1 gr charge weight increase, the increase you observed of 161 fps would roughly correlate to a 2.2 gr increase in charge weight. At 500 psi per 0.1 gr charge weight increase, that would indicate somewhere in the range of 11,000 psi total increase in pressure, very close to what the program predicted following adjustment of the Ba. Again, these are pretty crude estimates but there's nothing I see that would make me think the 63K+ psi prediction would be grossly wrong. The good thing about QL is that you can always check by running the numbers as many times as you think necessary if something doesn't look right.

 

[savagedasher]

A ppc has about the same capacity To get
3075 fps 63000 psi is needed .
What is your primers telling you ? Larry

 

[Langey]

A ppc has about the same capacity To get
3075 fps 63000 psi is needed .
What is your primers telling you ? Larry

The primers look fine, rounded edges, etc. This cartridge is being fired in a Thompson Center Contender and the primers being used are Remington 7 1/2's. The thicker cups on these primers make it a bit difficult to judge overpressure signs in these guns. There were no hard extraction problems.

I hope I was nowhere near that 63000 psi figure as these action were not designed for that amount of chamber pressure!

 

[Langey]

You could tweak the weighting factor a little bit, given that this is a case with a 40 degree shoulder and a touch overbore. Maybe something a little under the default 0.5 setting?

The other thing I've observed is that some bullets just run faster than others even with identical powder charges and identical bullet weights. The length of bullet bearing surface can affect this. QL doesn't really recognize this factor, so the only way to correct for it is to tweak the bullet weight by a few grains.

Have you shot other loads with this specific lot of powder that would suggest the burn rate is not as high as the "corrected" figure you had to use?

I load tested a 20 caliber wildcat (based on the 222 Rem. case) on the same day, under the same conditions, using the same lot of powder. The chronographed velocities were approximately 4% higher than the QL estimates. To equal the measured velocities, I had to increase the Quick Load Ba values by approximately 2.1%. And I should add, the actual barrel length, case length, cartridge length, bullet length and case volume were entered correctly.

I think I understand what is being suggested, that the discrepancy may be due to a combination of factors: weighting factor, bullet bearing surface and powder burn rate.

I'll tinker with the QL figures a bit and see what happens to the output.

I thank you all for your suggestions. I see I came to the right place for a likely solution.

 

[savagedasher]

I would check the means your using to get that much speed. Larry

 

[MrMajestic]

Sometimes you just go back to the old way of measuring case web increase, case extrusion from chamber after firing in the T/C Contender/Encore and primers condition. May want to stick a Federal 205M in place of the Remington 7 1/2 to see what it looks like. JMHO

 

[Langey]

May want to stick a Federal 205M in place of the Remington 7 1/2 to see what it looks like. JMHO

I'm thinking along the same lines ... establish a maximum load using a primer with a softer cup, and proceed from there.

 

[savagedasher]

I would have to check the Chronograph results . Larry

 

[Langey]

I would have to check the Chronograph results . Larry

Larry,
Thank you. I believe I understand where you're coming from ... the results from the first several loads from a new chronograph are considerably different from the expected results. Maybe I can find someone with a chronograph that can double-check my velocities.

 

[savagedasher]

Larry,
Thank you. I believe I understand where you're coming from ... the results from the first several loads from a new chronograph are considerably different from the expected results. Maybe I can find someone with a chronograph that can double-check my velocities.

Your welcome I never have found QL to be that far off . Larry

 

[Langey]

I never have found QL to be that far off . Larry

That's what worries me. After one outing and about 60 shots I love this thing (LabRadar)! I hope it doesn't have to be returned for repairs. For a quick and dirty velocity check I think I'll try a few 22 rimfires.

 

[BCBRAD]

What is the start pressure you entered into the program?