Which do you believe – SMT or Berger online Ballistic Calculator?

[Jdne5b]

How far apart are they on the SMT?

~3.75 inches on Shotmarker

 

[Webster]

Alex, what BC input did you use for the for the 180 Hybrids? Factors such as Lot-to-Lot bullet BC variance, or whether you pointed the bullets and then estimated the increased BC could be contributing factors. Also, I have accidentally failed to notice with some ballistic calculators that the default BC used is G1, rather than G7. Mixing the two will definitely cause a large difference in output.

If you are confident you input all the correct values, the next question I would ask is whether your actual elevation adjustment was more consistent with the drop predicted by the Berger ballistic calculator velocity or the SMT velocity? A 200+ fps difference in predicted muzzle velocity is huge. You should easily be able to determine from the actual vs predicted elevation settings which of the two is the better 1000 yd velocity estimate.

Not an expert in the BC area. BC is not a constant it varies with velocity. Don't remember which bullet Co. but I once saw BC data published for the same bullet at different velocities. Once in a while a well know bullet co. publishes a BC correction. I think you need the G7 it accounts more for velocitiy drop. G1 is more for hunting bullets at shorter ranges?

 

[FatBoy]

I think it's important to add that on all three open face targets we use, "ghost shots" can and do appear from time to time, especially when we are not leaving a firing point between the targets. The reason I bring this up is it is usually very easy to figure out if you had a crossfire or a "ghost shot" based on the velocity reading at the target.

100-200 fps different, crossfire usually and these come up in normal places. We can usually track down where these coke from. (Leo? Gary? Tim?)

200-400 fps different, system issue and these almost always come up a 6 in the corners.

3500-5000 fps, ghost shot.

So, beyond any load statistics that the target velocity may be useful for, the chrono at the target helps us understand what is happening at the target and what to do about it.

When we skip a firing point between targets the 2nd and 3rd situations rarely happen.

 

[JimSC]

Using the JBM calculator a 180gn 7mm bullet with a BC1 of .680 will need a muzzle velocity of 2680 FPS to be traveling at 1528.5 FPS at 1000 yards. That would give a drop with a drop of 318.2 inches or 30.4 MOA,

edit - at 2610 the FPS at 1K would be 1480 with a 338 inch drop or 32 MOA

what is your scopes elevation setting telling you ?

 

[Ned Ludd]

Not an expert in the BC area. BC is not a constant it varies with velocity. Don't remember which bullet Co. but I once saw BC data published for the same bullet at different velocities. Once in a while a well know bullet co. publishes a BC correction. I think you need the G7 it accounts more for velocitiy drop. G1 is more for hunting bullets at shorter ranges?

G7 BC remains relatively constant with respect to velocity...more than enough to consider it as a single value without resorting to "velocity-banded" values as is done with G1 BCs. The main reason for the difference is the shape of the G7 and G1 standards, and their resultant drag functions. The G7 projectile is very similar in shape to the modern boattail bullets we typically use in precision rifle shooting. The G1 standard is much less similar in shape to the projectiles we normally use, therefore the predicted drag functions are not as good a match and must be adjusted to account for velocity.

 

[Dos XX]

It might be calculating it front to back but assuming a straight line, not taking in consideration angle of approach? If it traveled the distance from front to back at an angle, it would have traveled further than if in a straight line, thus taken longer?

I am not a rocket scientist, nor do I play one on tv.

 

[natdscott]

Get a different chronograph, and get actual data at 1,000 yards out.

 

[memilanuk]

I recently had a discussion about this with someone involved with e-target design... If I recall, the gist of what he had to say was that the 5 mic system used by a lot of SMT G2 units is highly dependent on the bullet path being aligned as closely as possible to the target. The math involved in the calculations makes a few assumptions - primary among those being that the bullet is coming in 90 degrees to the target face. Any variance from that basic assumption and it throws off the calculation of the velocity at the target.

Interestingly, some 8 mic systems have the ability (don't know if they all take advantage of it) to compensate for a certain amount of misalignment based on the difference between the timing of the shock waves arriving at the sensors.

That said: I've seen 8 mic SMT systems (at Lodi) report velocities that weren't physically possible. Maybe that's been addressed in later firmware updates...?