Impact energy

[memilanuk]

Kind of a random question that popped to mind... has there been any sort of testing done to measure actual impact energy at distance, vs. the calculated energy values displayed in typical ballistic tables? Kind of curious how well the calculations hold up relative to actual real-world results?

 

[gnochi]

Not that I’m aware of.

Energy is very very difficult (not saying impossible, but I can’t think of any ways off hand) to measure directly; you almost always need to measure several properties that collectively equate to energy.

So, for impact energy specifically, there are a few options:

1. 1/2 * [bullet mass] * [velocity at impact] ^2
2. Shoot into a large water jug and precisely measure the temperature change, then use [mass of water]*[specific heat capacity of water]*[change in temperature]
3. For a known mass swinging plate, measure how far the center of mass moves vertically when the plate is shot and moves back, and how loud the impact is at a known distance, then use [swinging plate mass]*[gravitational acceleration on earth]*[height change in center of mass on first swing]+[frustrating math that converts impact sound pressure level at known distance, that distance, and the speed of sound into the energy of the impact that has been converted into sound instead of pendulum energy]

It is by far the easiest to get an accurate velocity, compared to any of the other limiting measurements, so we don’t bother, especially because they all must have the same result within measurement and calculation accuracy bounds.

 

[Steel head]

Well I really have no reason to question the ballistic apps/tools.
As long as BC and air values are correct speed and mass are pretty easy to convert to an energy value.
At 1000 yards my 260 definitely hits the plate harder than my buddy’s various 6mm and 308 ‘s, my 7 saum definitely hits em harder yet and the 338NM even more so.

 

[memilanuk]

@gnochi what I was thinking of was something like #3... but in terms of an accelerometer or other device attached to the backside of a plate to measure the g-force imparted to the target. The details, though... might take some doin'

 

[Matthew488]

Much easier to simply measure velocity at target and then mathematically convert to energy.

 

[memilanuk]

How are you measuring velocity at the target? Something like a Silver Mountain or ShotMarker e-target?

For the most part, the 'chronograph' features of e-targets aren't especially accurate/reliable, in my experience. Most conversations with the manufacturers indicated that the feature is there mainly because users want it, not because it's good.

 

[Matthew488]

I haven personally have not tried to do it but you would have to have some form of chronograph down range. I dont know what would be the best option to do it though. I'm thinking the ballistic apps have to be pretty close on velocity through the trajectory to do all its calculations that it's good enough for my purpose. The bigger companies use a full doppler radar and can track the bullets through the entire flight. IE. Berger, Hornady etc.

 

[memilanuk]

you would have to have some form of chronograph down range.

Yeah... that's the problem with that idea - people hit their freakin' chronos regularly when they are 15-20 feet down range. Trying to put rounds over the (very small) skyscreen sensor window reliably at distance... it's been done, but generally the chrono becomes a consumable at that point. There's a reason Litz used acoustic systems (dedicated for chrono use, not the ones on e-targets) for his early works.

Doppler would be great for that... but most setups suitable for that kind of distance aren't exactly consumer-ready.

It'd be kind of cool if there was some way to trigger a downrange LabRadar using the external microphone pickup so that it would turn on and record the shot...

 

[gnochi]

How accurate are you trying to measure energy? 5% accuracy with 1% bullet mass accuracy would require ~2% velocity accuracy - 20fps per 1000fps. Pretty much any ballistic calculator will give you that with a couple passes to true muzzle velocity and ballistic coefficient.

 

[memilanuk]

Mostly just curious. Why are you so opposed to some sort of direct measurement?

 

[Matthew488]

I dont think it's that we are opposed to doing it or trying it. It's just that its unlikely to be feasible for us as individuals to do ourselves without the resources and budgets of what the companies can do.
The best way I can think of for us to try would be if a Labradar with an air gun trigger would fire from the sonic crack of the bullet passing by. You would then have to point the Labradar perfectly for the bullets trajectory at distance and shield it from a bullet strike as the bullet would have to be very close to have any chance.

 

[gnochi]

What Matthew488 said. I’m an engineer - my job is either to do what’s necessary with minimal resources, or do the best I can with a specific amount of resources. Note that every halving of error costs at least 10x.

 

[Steel head]

I could see measuring true energy of bullet at distance to fairly complicated and not cheap.

 

[gunsnjeeps]

The "easiest" way to get velocity at target would be to put a Labradar about 30 yards in front of the target and shoot. Much easier than trying to shoot over a chronograph 1000 yards out.

For any ballistic app that gets you on target from a known range and velocity to a further known range is close enough to give you a velocity and impact energy. For example, known velocity at muzzle and 100 yard zero. Calculate zero at 1000 yards and shoot. Personal experience put the first shot using that method in the lower part of the 8 ring and tells me a .308 175 grain SMK has the more energy (611 ft lbs) as a .45 ACP point blank (369 ft lbs). I'm well behind the times I use Sierra Infinity V7 on a laptop.

Come up was 37 inches using 8 clicks a minute. 296 clicks calculated for 8 clicks an inch put me in the 8 ring, 310 clicks put me in the X. One of these days I might figure the difference at 8 clicks a minute and 8 clicks an inch and see which one is closer. And before we get the MOA/MIL argument going don't. I have 3 iron sight NMAR's in MOA, 1 scoped NMAR in MOA, the Savage Palma for Long Range in MOA, and a couple cheap scopes in MOA.

ETA: on my phone, computer has data. Updated.