GRT and Working pressure Graph

[Abram]

Hello all,
My apologies if this has been asked, if it has please show me the thread, I searched but did not find the info I wanted.

I downloaded Gordons Reloading tool out of curiosity. I have a question regarding the pressure curve diagram in results. I am working with the following:

Winchester 1x Fired cases, L=2.00" - I measured the capacity at 56.2 grains of water in a fired case
IMR-4064
Sierra 2130 SPT Pro Hunter
Ruger American Predator Gen 2 - 22" Barrel

The maximum charge that does not give a warning is 45 grains and it falls within the Pmax-15% this is at 70 Degrees. Anything more than this it flags it "Working pressure close to the maximum permissible gas pressure!". Of course adjusting the temperature up pushes it closer to the Pmax absolute. My questions are these

1. Do you include the maximum temperature you intend to shoot the rounds or in this case set the temp according to the use of the cartridge, in this case it is a hunting round that will never see 90 Degree weather unless of course I go Hog hunting and I don't think that will happen as it is too dang hot.

2. If you set the temp to be say 70 degrees how close do you get to the Pmax absolute?

Before GRT I looked up the max charge in several manuals and then dropped back 10%, then loaded in .5 grain increments until I reached the max charge I had set. Perhaps I am over thinking this thing.

I knows GRT is just meant to get you in the ball park and five you some idea of the ballistics of the rounds created.

Thank you

 

[chromatica]

Cartridge please?

 

[Abram]

My apologies, 308 Winchester

 

[dstoenner]

Using the given powder parameters will not lead you to real numbers. You need to plug in actual velocities even for 1 powder level and then run OBT. OBT is a curve fitting solution of the powder parameters that gives you exactly what your system is giving and then you can see the actual pressure. I usualy run OBT and then back annotate (copy and paste) the OBT derived BA and K back into my base file. I have a whole paper on here about developing a load with GRT in less than 50n bullets. I am about ready to help my wife with dinner so when that is done I will go and find it and give you the reference.

Here

How to get a load in 50 bullets with Gordon's Reloading Tool (GRT)

There have been a few threads on here about GRT. I have been using it for over 2 years now. As an electrical engineer, I am used to simulating systems to see the response. GRT is just exactly that but the "trick" is how to coax it into giving you a good answer. That took me some time to...

forum.accurateshooter.com

 

[Wild Bill IV]

Hello all,
My apologies if this has been asked, if it has please show me the thread, I searched but did not find the info I wanted.

I downloaded Gordons Reloading tool out of curiosity. I have a question regarding the pressure curve diagram in results. I am working with the following:

Winchester 1x Fired cases, L=2.00" - I measured the capacity at 56.2 grains of water in a fired case
IMR-4064
Sierra 2130 SPT Pro Hunter
Ruger American Predator Gen 2 - 22" Barrel

The maximum charge that does not give a warning is 45 grains and it falls within the Pmax-15% this is at 70 Degrees. Anything more than this it flags it "Working pressure close to the maximum permissible gas pressure!". Of course adjusting the temperature up pushes it closer to the Pmax absolute. My questions are these

1. Do you include the maximum temperature you intend to shoot the rounds or in this case set the temp according to the use of the cartridge, in this case it is a hunting round that will never see 90 Degree weather unless of course I go Hog hunting and I don't think that will happen as it is too dang hot.

2. If you set the temp to be say 70 degrees how close do you get to the Pmax absolute?

Before GRT I looked up the max charge in several manuals and then dropped back 10%, then loaded in .5 grain increments until I reached the max charge I had set. Perhaps I am over thinking this thing.

I knows GRT is just meant to get you in the ball park and five you some idea of the ballistics of the rounds created.

Thank you

Watch that powder!!! 4064 is very temperature sensitive!! Don't leave a loaded cartridge in the chamber, and let the sun heat it up!!! I had a 7-08 using 4064!! Did this once!! Blew the primer out!!! Had to fist hammer the bolt a couple of times to get it open!!! This was on a 700 BDL Varmint Special Rifle!!! 24", 9-1/4:1 Twist Target/Varmint barrel!! I'm like you, I loaded and tested below 75° for fall/winter hunting!!! But, it is a very, very good powder in 308 cases and its wildcats!!!!
Bill

 

[jelenko]

Do you include the maximum temperature you intend to shoot the rounds or in this case set the temp according to the use of the cartridge,

You can change the powder temp to see what the change in pressure it makes.
As mentioned above, you really need to enter the case volume for the brass you're using and enter the shot values or the average velocity for a sample a load using the brass, primer, bullet and powder that you'll be using. This will enable GRT to provide a more accurate estimate of the actual pressure for your load in your gun. Depending on the brass and the barrel, the difference can be non trivial.

 

[SeabeeKen]

With the info you give I'm showing 56,500 with a COL of 2.75". Like @dstoenner says, Run OBT with your load and ACTUAL temperature and put in you actual measured speed of the load. Then, click on create load file with the calibrated values. This will be your actual calibrated load. You can save the updated calibrated powder model in the user tab for future use. Be sure to measure everything possible so that you get good information out of it
The app is going to tell you when your pressure is close, thats normal

 

[Abram]

So GRT is as dstoenner indicated in his article GRT is a simulation software but for it to be accurate as possible it needs all of the real world info you can plug into it. makes sense, I am going to study the write up you did and see if I can actually use this software, then again I may just go back to the way I was doing it to keep things simple.

Thank you all

​

 

[Chestnut]

I've followed Dstoenner's write up and it helped me a lot to get multiple loads dialed in.

My only addition is to measure your bullets, brass, and COAL. This allows the seating depth to be accurate which is another important variable.

I did this yesterday with a rifle in 6.5CM and was at 1moa in 10 shots. 5 to tune the powder and 5 to verify. I'll be loading further to hopefully fine tune further but for hunting 1moa is good enough for the 150y max i ever shoot.

 

[Fast14riot]

As a prediction software, it can help you figure out a safe starting point, but because powder data is crowd sourced, it may be off from your lot.

As noted, run OBT with your measured velocity for a given charge to get better results. I helped a fellow shooter on here by plugging in all his measured numbers for case capacity, velocity at charge, bullet weights and dims, coatings, and output an OBT node that on target dialed in within 0.2gr of predicted charge weight and within 15fps for velocity.

As with any ballistic software, garbage in = garbage out. Plug in numbers "about" what it was and get info "about" what it should be.

Put in what it actually is and get results closer to real world results.