SD at the target

[Dave Way]

Problem is you are assuming they are all starting with the same ES and SD at the muzzle and I’m fairly certain that isn’t the case. Without having the data at the muzzle, comparing SD data at the SM is virtually meaningless. Just my two cents.
Dave

You are correct but in the absence of that data conclusions can still be drawn based on assumptions. The SD of a string would need to be compared to the SD at distance to see clear evidence. I maintain however that once the bullet leaves the barrel the only thing that will affect the velocity degradation will be wind or ballistic coefficient. Humidity would not change the SD nor would temperature because they would be constant during the bullets flight. We can’t accurately account for wind unless we are shooting in a tunnel.

I bet many would be surprised to see the actual BC’s generated by the Doppler Radar that the bullet companies use these days. The listed BC’s are an average.

Interesting topic for sure.

Dave.

 

[Ned Ludd]

Ned:
As mentioned in my post I was just posting info generated by the shotmarker targets at the match.
61 FPS is a big increase from match 1-4. As I mentioned we had to change targets after match 3 from target 18 to target 21 due to 18 not recording shots at the start of match 4. This change of target may have had an impact on the variance? I did not have to click down on the scope going to match 4.
Also on Sunday shooting on a different target.
I'm not making any judgements on the data reported as I have no idea how accurate it is.
What is important is I don't have elevation issues and can shoot tight tens and high Xs in good conditions.
An SD of 6 with the other third of shots being flyers causing elevation issues is not helpful to winning.

I agree with all of this. I was merely pointing out that it supports the notion that velocity data/stats at the target face from current e-target systems is of questionable value.

 

[Ned Ludd]

Ned,
My thoughts on the SD/velocity variations you have experienced could be attributed to the change in temps from the early morning start of the match and the last shots fired in the heat of the afternoon. You could begin with air temps in the high 60s and end the day in the low 90s. That might not account for 100 fps but certainly contributes to an increase in velocity simply due to density altitude. Also, variations in the actual bullets themselves would affect the the "At the target" numbers.
I agree that the velocity and SD numbers generated by the SM system are probably not accurate, but at least if you are shooting on the same target, you can compare the numbers from different calibers. The variation should be relatively consistent across a 20 shot string of fire.

Bob

Atmospheric changes during the day could certainly contribute to a velocity change, but 61 fps? Or changes of well over 100 fps such as I have observed from Shotmarler data on more than a few occasions? I am inclined to doubt that atmospherics would typically account for velocity changes of that magnitude in a well-developed load. So I'll re-iterate the following statement: If individual velocity values at the target face are inherently inaccurate, then so too are the statistics data derived from those values. That means any comparison made is potentially meaningless. Sure, you can make the comparison, but that's the easy part. The real problem is that it can be very difficult to know with any certainty whether the values derived from the e-targets are solid, or totally inaccurate, without some kind of external velocity data and/or ballistic program predictive data that is derived from solid chronograph data for comparison.

 

[mikecr]

What was the BCs of the different cal bullets?

 

[SG4247]

All I know is the SM says a SD of 10 with my 6BR, and a SD of 16-17 with the .284 at 600 yds.

My 6mm AR at 300, shows a SD of 15 on SM.

That’s when the rifles are shooting well with great loads. When the SD numbers go up from here, my score suffers - a lot.

I also see the SM speed increase significantly with increase in temp from morning to afternoon -roughy 40-50 fps.

 

[RegionRat]

In a club match setting, we allow folks to run a chrono that stays behind the line so long as everyone agrees and it doesn't turn into a distraction for any other shooters.

When your target scores go awry, if it is possible to get the muzzle velocity then I would.

Some of those observations at the SM may not be very accurate, but there may still be some information that is useful in there if you can tease it out on the basis of having the true muzzle value.

ETA: forgive me if you already mentioned it, but what powder were you running?

 

[Ned Ludd]

All I know is the SM says a SD of 10 with my 6BR, and a SD of 16-17 with the .284 at 600 yds.

My 6mm AR at 300, shows a SD of 15 on SM.

That’s when the rifles are shooting well with great loads. When the SD numbers go up from here, my score suffers - a lot.

I also see the SM speed increase significantly with increase in temp from morning to afternoon -roughy 40-50 fps.

There are potential issues with these lines of reasoning. First, have you ever confirmed this putative increase in velocity from morning to afternoon with a quality chronograph? If not, it's likely an artifact of measuring velocity at the target face via the SM. If such a velocity increase is real, a quality chronograph will have no difficulty recording it at the muzzle. Further, unless there are some pretty major temperature swings during the course of a typical day where you shoot, your load might need some adjustment if you're actually seeing 40-50 fps velocity variance during the course of a single day.

Second, I've shot cleans with high X-counts at 300 yds on many occasions using a practice load in one of my F-TR rifles that had an ES of 60 to 90 fps and SDs well over 30 fps due to a throat defect in the barrel. It still shot well enough to win matches. Velocity variance of 50 fps, or even slightly greater simply doesn't have that much effect on external ballistics at only 300 yd. I don't think anyone replying in this thread is trying to denigrate e-targets or prevent those of a mind to use e-target velocity data from doing so, we're merely pointing out the obvious caveats involved. It's something that is worth consideration.

 

[SG4247]

When my 6AR 300 yard loads are not in shape, and SM reports SD of 30-40, it shoots really badly.

Shotgun like pattern, I mean really badly. No chance of winning.

I am not assuming anything good or bad about SM, just reporting my observations.

Your SM may vary.

Like any measurement system, without any standard or calibration method applied - it is just that.

 

[Ned Ludd]

When my 6AR 300 yard loads are not in shape, and SM reports SD of 30-40, it shoots really badly.

Shotgun like pattern, I mean really badly. No chance of winning.

I am not assuming anything good or bad about SM, just reporting my observations.

Your SM may vary.

Like any measurement system, without any standard or calibration method applied - it is just that.

One thing that occurs to me as a possible way to explain our observations is that differences in barrel stiffness/contour are associated with greater changes in barrel launch angle for a given velocity variance. I'm running fairly heavy contour barrels on F-TR rifles, and they just don't seem to generate that much much vertical dispersion at [relatively] shorter distances (i.e. out to 300 yd), even with fairly large changes in velocity. Obviously, the velocity data from the e-target system is working for you in terms of analyzing your shot patterns.

 

[Chilly807]

To the original question, I do think the 223 is a more difficult cartridge to get "good" ES and SD numbers from, compared to larger cartridges such as the 308. That's been my experience, anyway.

I also think that holds true regardless of what distance you measure the velocity at.
Is it possible the lighter bullet is more affected by headwinds or tailwinds? Maybe..

I do know that that Adam Mac has said the SM's chronograph function is a secondary use, and isn't intended to be absolutely accurate.

I've shot the 223 and SM combination at longer ranges, out to 800m and I can say that vertical shot placement can usually be correlated to what the SM is saying. In other words, noticeably lower reported velocity shows up as a low shot on the target.

At 600m, the same holds true but to a lesser degree. Shots that would have been a 4 at 800 are still in the lower part of the 5 ring at 600. (We use V-5-4-3-2-1 scoring rather than X-10-9 etc)

At close range, you get away with poor velocity control, but it bites you at long range.