GRT for 6 ARC Load Development

[chromatica]

I have been using Gordon’s Reloading Tool to help me do load development for a 6 ARC bolt gun. Berger 105 HTs have become my bullet of choice. My measurements of velocity diverge from GRT predictions by quite a lot. I have a 30 inch barrel so I was expecting some very good speeds. Instead, with Reloder 16, my velocities are almost 200 fps lower than the model predicts. With Varget my velocities are about 120 fps low. I have been careful to set the cartridge dimensions. I suspect there are some thermodynamic properties of these powders that need to be tweaked. Does anyone have a knowledge of the parameters for powder that affect burn rate? Is there a source for these thermodynamic property values or was Gordon measuring these in his lab?

Oops, just saw this: https://forum.accurateshooter.com/t...lets-with-gordons-reloading-tool-grt.4113273/

 

[SeabeeKen]

I recommend calibrating the powder model to match measure velocities, assuming all your measurements are accurate. The powder models are getting out of date. Hopefully GRTnextgen comes out next year

 

[Krogen]

I have been careful to set the cartridge dimensions.

This caught my eye. Do you mean outside dimensions or internal water capacity measurements? I'm more familiar with Quickload, but I've also used GRT. I found the default water capacity for .223 Rem in Quickload was wrong for my cases. Quickload predicted enormous pressures and velocities. Tweaking water capacity aligned predicted velocities with chronograph data. Predicted pressures dropped from an action-popping level to normal. Perhaps water capacity is gumming up GRT.

 

[chromatica]

This caught my eye. Do you mean outside dimensions or internal water capacity measurements? I'm more familiar with Quickload, but I've also used GRT. I found the default water capacity for .223 Rem in Quickload was wrong for my cases. Quickload predicted enormous pressures and velocities. Tweaking water capacity aligned predicted velocities with chronograph data. Predicted pressures dropped from an action-popping level to normal. Perhaps water capacity is gumming up GRT.

I did weigh an empty case and then weighed it after filling it with water. The capacity was 33.66 grains.

 

[jxb]

Yeah want to make sure you updated your case settings to be correct for your 6 ARC cases which will also adjust bullet seating. Then if you input your range data it should true up. Pretty sure they are already well into 6 ARC work for the existing release over on the forum, did you get the model from there? I thought I saw it mentioned but I don't have 6 ARC so don't follow it. Charlie over there is probably your best source to getting this trued up.

# reloading someone asked just that a few minutes ago
<@Charlie> Good morning. When your real world results don't match GRT predictions, what values are you adjusting to bring them more in-line with each other?

Charlie posted a detailed answer over there, should get you fixed up.
'https://discord.com/channels/621039234404581406/694108252824273046'

 

[JLT]

A couple of things. First, @SeabeeKen is right on the money. GRT's OBT function allows you to enter measured velocity for a given powder charge and this, in turn, is used to predict OBT charge weight. Second, RL-16 may be too slow for the ARC case, even with heavier bullets. RL-15 may be better-suited. I use RL-16 in a Dasher but I have to use a fairly long drop tube to get enough in the case, and the Dasher has quite a bit more capacity than the ARC.