GRT numbers

[marchx]

I am losing confidence in GRT. Wanted to model velocity using n165 in .284 win.
VV reloading database shows 57.7 as max load and shows 2651 fps for a 24" barrel shooting 180gr Scenars.

I put exactly the same data into GRT and it shows overpressure and 2955 fps out of 24" barrel. nonsense.

 

[jepp2]

Can't really comment much unless you provide more information.

I don't know if you have entered your actual water capacity of your brass. Don't know if you have fired any other loads with the N165 powder lot you have and entered the actual velocities into GRT to find the adjustment to the powder burn rate necessary and used that in this model calculation. You surely wouldn't use the 57.7 gr max load listed in your database for your starting load. So why not use the VV reloading database starting load and record the velocities into GRT and let is create a more accurate model for future loads?

I was impressed enough with GRT that after using QuickLoad for every load I worked on over a 10 year period, I switched to GRT and am very happy with the results.

 

[Allen Corneau]

The VV page says,

".284 WINCHESTER
Test barrel: 610 mm (24”), 1 in 10” twist
Primers: Large Rifle, Remington 9 1/2
Cases: Peterson Cartridge Co, trim-to length 55,12 mm (2.170”)"

Did you adjust all the GRT settings to match?

I've been using GRT for a little over a year now and have found that it gets you in the ballpark but won't match exactly. Still, 2650 vs. 2955 is a considerable discrepancy.

 

[jxb]

in GRT need to get h2o, case length, coal, and bullet length all correct. some of those vary the data considerably. I measure my h2o with a primer in. I shoot some cartridges in Alpha and Hornady and those two in 6GT are considerably different h2o.

There's also temp, make sure that's set to something like 70 unless VV shows the temp, then match it. If I'm shooting in the summer I set my temp in GRT to 85 to start so I'm not missing it and putting the high end over pressure for those hot days. Obviously if a person shoots in hotter or colder then adjust as needed.

 

[Murray Brook]

As @jepp2 posted you must enter all your specs in the program , ( OAL--case cap- Sebert factor- MV from a chronographed load from the same lot powder - primer type etc) if you want a reasonably accurate output. JMO

 

[marchx]

case length is correct, as well as barrel length. we are not talking about precise modeling here.
using standard values we get 2955 fps out of 24" barrel. it's just wrong.
try repeating that in your GRT.

 

[Evan]

case length is correct, as well as barrel length. we are not talking about precise modeling here.
using standard values we get 2955 fps out of 24" barrel. it's just wrong.
try repeating that in your GRT.View attachment 1591505

It’s hard to tell on my phone, but it looks like your OALis 71mm, and you have 22mm of bullet in the neck. That seems wrong. Shouldn’t your OAL be in the 80+mm range?

 

[dhagerty40]

Ok, already found one error.. VV load data shows a COAL of 74.0mm. You are using a COAL of 71.12mm.

That is going to make a big difference in pressure!

 

[chop house]

Personally I like grt and when my results don't pass the reasonableness test I dive deep IF it might be a safety issue.
Sometimes I don't expend the energy to get good correlation. That is when I when I say GIGO and move on.

I agree with the case capacity comments! Just today was loading some 7-08 and when I looked into the block of charged cases i saw powder up the neck of one, where all others were half down the shoulder. It was a ppu that got mixed in with the Hornadys.

 

[tthomp302]

GRT has simplified my reloading. Garbage in garbage out. It’s a model and only as good as the info you’re feeding it. Reloading all day has an advanced load development course that shows you how to utilize GRT to its fullest. I took it and it was the best money for knowledge that I spent.

There are many things you need to do to calibrate the model to what you are seeing in real life.

 

[jelenko]

I used all the defaults from GRT but COAL to 2.91" to match VV's load data. 57.7 gr of 165 still shows Fill Ratio of 114.6% and a peak pressure of 84.4K psi.
Either VV's load data or GRT is incorrect.

 

[Fast14riot]

What is the scale of confidence in the powder burn model when selecting N165 from the list? Thats the red to green scale next to powder in the selection window. It might have low confidence from only a few examples submitted.

 

[jelenko]

What is the scale of confidence in the powder burn model when selecting N165 from the list? Thats the red to green scale next to powder in the selection window. It might have low confidence from only a few examples submitted.

The scale for n165 is well into the green zone

 

[dhagerty40]

I also found another problem is in the bullet. Lapua shows the bullet is a boat tail bullet, While both GRT and quickload shows the bullet as a flatbase bullet. Without having the actual bullet measurements though, no way to fix as Lapua does not provide that data. I found this once with a Nosler 44M bullet where the starting loads provided by Nosler would show as an overload in Quickload. after measuring some bullets, I found out that the bullet length was wrong that Nosler listed. After entering the bullet length I measured, the Data almost matched.

 

[William Vogel]

case length is correct, as well as barrel length. we are not talking about precise modeling here.
using standard values we get 2955 fps out of 24" barrel. it's just wrong.
try repeating that in your GRT.View attachment 1591505

Why don’t you load it and shoot it. Whichever prediction you choose. I use GRT and it predicts the same phenomenon for me and when I shoot it my velocity is higher than its prediction. If I loaded off a reloading manual, I’d probably blow up my gun. Hornady says I can use 31.7 grains to get 3350fps. I use 30.8 and it goes 3390. If I put 31.7 in it, I’d be in trouble. I don’t trust reloading manuals.

 

[AccelR8]

Yeah...for 300 BLK (not everyone's favorite for accuracy ) I plugged in in all my measured values to get an initial out put from GRT before loading. At Hornady's maximum charge GRT says 87,051 psi and 2,803 fps from a 16" barrel (110 grain V-Max). Obviously not realistic. I figured the powder model is not accurate. Also the scale of confidence is low for the powder in question.

Still scary though. So I started with Hornady's minimum and worked up watching velocity and pressure signs. GRT is way out in left field on this one.

 

[marchx]

I agree with the above remarks. So, what’s the point of using reloading program such as GRY if the data it produces are highly inaccurate. Also OBT times are questionable then.

 

[AccelR8]

I agree with the above remarks. So, what’s the point of using reloading program such as GRY if the data it produces are highly inaccurate. Also OBT times are questionable then.

I have had GRT initially predict close to actual results many times. And after going back in an entering actual velocity measurements it seems to then predict velocities pretty good and therefore I feel the pressures it outputs are close to reality.

edit to add: for example my previous post here I said GRT returned over 82K psi. After entering measured velocities and generating a new file with the calibrated values the pressure says 42.3k psi. (but I'm not at Hornady's max load as I originally said I'm about 6% below max).

Still, I don't over trust it and don't push my luck!

Maybe put it this way; if given a choice between GRT and published load data, I'll go with published data every time.

 

[jelenko]

I agree with the above remarks. So, what’s the point of using reloading program such as GRY if the data it produces are highly inaccurate. Also OBT times are questionable then.

I haven't run into a case like this => generally/almost always, GRT is in the ballpark.

When I want to develop a load, I load up 9 rounds [the velocity for the first shot I always throw away and count only the next 8] with what a mid range load, fire them across a chrono and enter the observed velocities in GRT. Then use the OBT function to automatically calibrate GRT to the observed velocities.

 

[dhagerty40]

lets say this: GRT and Quickload are only as accurate as the data provided by the MFR's. Wrong data will give bad results. The programs themselves work fairly well!

Sierra's reloading manual has close to same problem. Since they list all bullets of same weight in the load data, it may not be the max loads for each bullet.