GRT accuracy and results from projections

[William Vogel]

I just started plugging in my loads with GRT and its predictions for MV are coming up lower than my chronograph results, not a big deal. My question is that the pressure prediction on some cartridges is lower than expected and higher on others. Also I load CFE223 25.5 grains 75ELD bullet for my 223 rifle and get big ES and SD 70 ES but land 3 inch groups at 700 yards. It’s prediction is that burnout is past muzzle. I’m using a 26 inch barrel. This might explain my ES numbers but the ballistic calculator predicts a 1 MOA variation in impact at 700 yards and I’m hitting less than 1/2 MOA. Could it be pure coincidence?

 

[jepp2]

I just started plugging in my loads with GRT and its predictions for MV are coming up lower than my chronograph results, not a big deal.

Some thoughts.
1. Have you run OBT and input your actual velocities? When you do, GRT will adjust the powder burn rate to match your actual.
2. On pressure predictions not matching actual, it is just a mathematical model simulating burning of powder, so I always consider close is good enough for the intended purpose.
3. On the difference in MOA variation vs your actual - there are many variables used to model the actual flight of the bullets. For example BC changes as velocity changes, and you are probably only using 1 value. I'd just smile with 1/2 MOA actual variation.

 

[jelenko]

Just to confirm. You've measured the capacity of your brass and measured the length of the bullets you're using?

As Jepp mentions, even with these measurements, the predicted velocity/pressure usually is different from your actual results => plug in the muzzle velocities from 7-10 shot of your load through the specific rifle and use the OBT calculator to have GRT calibrate the powder to your rifle/load.

 

[William Vogel]

Some thoughts.
1. Have you run OBT and input your actual velocities? When you do, GRT will adjust the powder burn rate to match your actual.
2. On pressure predictions not matching actual, it is just a mathematical model simulating burning of powder, so I always consider close is good enough for the intended purpose.
3. On the difference in MOA variation vs your actual - there are many variables used to model the actual flight of the bullets. For example BC changes as velocity changes, and you are probably only using 1 value. I'd just smile with 1/2 MOA actual variation.

Do you think the lack of burnout before the bullet exits the muzzle could be causing my ES being fairly high? My brass work, seating etc is proving to be consistent with my tight group sizes at short range (100 yards) and long range.

 

[jepp2]

Do you think the lack of burnout before the bullet exits the muzzle could be causing my ES being fairly high?

Typically my ES and SD for 223, regardless of powders are MUCH higher than all my larger case cartridge loads. I use an A&D fx120i, so I know my powder loads are very consistent. I attribute it to the primer being a larger portion of the energy of the load as well as the lower case volume. Occasionally, I get lower values, but not on average.

I don't limit my loads to those that achieve 100% burn before bullet exit.

 

[vonb]

GRT will provide you an idea. It is a estimated idea that can be close provided you provide accurate case length, cartridge length, cartridge volume and bullet length. Some of the powder profiles are not accurate while some are. I find it will give me an idea before I even start loading.

 

[William Vogel]

I haven’t m

Just to confirm. You've measured the capacity of your brass and measured the length of the bullets you're using?

As Jepp mentions, even with these measurements, the predicted velocity/pressure usually is different from your actual results => plug in the muzzle velocities from 7-10 shot of your load through the specific rifle and use the OBT calculator to have GRT calibrate the powder to your rifle/load.

I haven’t measured my brass volume in grains of H2O. I have adjusted the COL of my cartridge and adjusted the chamber measurements. I did use the OBT calculator with my 6.5 Creedmoor round and for some reason it adjusted my powder weight input along with the parameters of the powder. I shoot 43.5 grains of win Staball 6.5 with a 147 ELD bullet and 9 1/2 mag Remington primer. I get 2760 in a 26 inch barrel. It’s sub .5 moa at 100 yards and 5 inches at 700 yards. This rifle is a Christensen Arms Mesa Long Range. I swapped in a Triggertech Special but otherwise factory built.