Weight ES for LR bullets ?

[Old Corps 8541]

While listening to my TV there was a show about snipers and their stories, then it switch's to some USMC armor's , and one o them was shown weighing and separating the bullets by + or - either 2 or 4 gn. differences. So, what do you consider an practical ES for LR shooting, F-TR, F class or PRS ?

 

[straightshooter1]

While listening to my TV there was a show about snipers and their stories, then it switch's to some USMC armor's , and one o them was shown weighing and separating the bullets by + or - either 2 or 4 gn. differences. So, what do you consider an practical ES for LR shooting, F-TR, F class or PRS ?

Just to be clear. . . are you asking about the sorting of bullets or cartridges?

 

[Old Corps 8541]

Bullets ,

 

[memilanuk]

some USMC armor's , and one of them was shown weighing and separating the bullets by + or - either 2 or 4 gn. differences.

There's your problem, right there.

Some of the old (really old, now) military bullets had... pretty iffy quality control, compared to what we have now, commercially. They were making do with what they had. Probably had to pull the bullets out of loaded rounds, sort and re-assemble.

I wouldn't go basing your reloading processes off of *that*.

 

[straightshooter1]

Just to be clear. . . are you asking about the sorting of bullets or cartridges?

These days, the higher quality bullets, like Berger's, are less that 1 gr difference . . . even less than .5 grs; a lot better than decades ago. But there still people who'll sort their bullets by weight.

 

[Ledzep]

I've done some testing with this and the example that I did the biggest test with was 6mm 109 ELD-Ms. I weight sorted through 1200 of them to get 30x each of the "high" and "low" brackets. Low was 108.7gr, high was 109.2gr.

The high and low MV separation on 30x ea. was less than 1fps on the average velocity. Either 0.3fps or 0.7fps I can't remember which. There must be some self-correcting physics going on with the powder burn because if you do it proportionally scaled to the weight difference it should have been more like 19-20fps.

MPOI and dispersion over that 0.5gr spread was undetectable.

The only place I could see it mattering, and it was very small net effect at 1000yd was with drag. Mass is a direct 1:1 scalar on total drag. If you have bullets that are 5% different in mass, but otherwise externally/physically the same, you will have very near 5% difference in drag. This will hardly manifest to anything inside of 600yd, will start to show beyond, and the effect just grows the further you shoot. For example, in my situation it was a 0.4% extreme spread in mass and drag on the averages between those two 30x samples. Remember that those two samples were picked over 1200 rounds of sorting and the vast majority of bullets would be much smaller spread.

The weight distribution in most cases is a normal distribution so you can figure what the SD math will play out as for what you can expect to see in X number of rounds. Add in inherent drag variability, MV spreads, mirage conditions, etc... I'd say it's not worth your time in PRS. On paper for F class or BR scores and how close those can get, it's probably worth doing. YMMV.

 

[Old Corps 8541]

There's your problem, right there.

Some of the old (really old, now) military bullets had... pretty iffy quality control, compared to what we have now, commercially. They were making do with what they had. Probably had to pull the bullets out of loaded rounds, sort and re-assemble.

I wouldn't go basing your reloading processes off of *that*.

I didn't go into detail with my question, the bullets I saw being weighted was 142 Gn. The bullets were in a large box and they were using a plastic ice scoop to fill their smaller containers , which they took a bullet out and weighted it on a $$$$ scale.

 

[Doom]

I've done some testing with this and the example that I did the biggest test with was 6mm 109 ELD-Ms. I weight sorted through 1200 of them to get 30x each of the "high" and "low" brackets. Low was 108.7gr, high was 109.2gr.

The high and low MV separation on 30x ea. was less than 1fps on the average velocity. Either 0.3fps or 0.7fps I can't remember which. There must be some self-correcting physics going on with the powder burn because if you do it proportionally scaled to the weight difference it should have been more like 19-20fps.

MPOI and dispersion over that 0.5gr spread was undetectable.

The only place I could see it mattering, and it was very small net effect at 1000yd was with drag. Mass is a direct 1:1 scalar on total drag. If you have bullets that are 5% different in mass, but otherwise externally/physically the same, you will have very near 5% difference in drag. This will hardly manifest to anything inside of 600yd, will start to show beyond, and the effect just grows the further you shoot. For example, in my situation it was a 0.4% extreme spread in mass and drag on the averages between those two 30x samples. Remember that those two samples were picked over 1200 rounds of sorting and the vast majority of bullets would be much smaller spread.

The weight distribution in most cases is a normal distribution so you can figure what the SD math will play out as for what you can expect to see in X number of rounds. Add in inherent drag variability, MV spreads, mirage conditions, etc... I'd say it's not worth your time in PRS. On paper for F class or BR scores and how close those can get, it's probably worth doing. YMMV.

To the point here. There are several things that come into play with a change in weight. It you change only the weight of a 168 gr 308 bullet in 308W in Gordens Reloading Tool by 1/2 gn you get about 2 fps. That estimate assumes ONLY the weight changes and nothing else. But for the weight to change other dimensions must change, which could be bearing surface diameter or length which can and do vary from bullet to bullet and are part of the normal variance in bullet performance. So in this test those variables are also a part of the result. Simply sorting by weight may or may not show a significant effect.

As for shooting at distances of 1000yds there is an external ballistic benefit of a heavier bullet in that with all else being equal the BC of the heavier bullet will increase.

I would suspect that weight may show an effect but it may only be a surrogate for other dimensional changes and whether a small difference matters will vary from lot to lot and by manufacturer to manufacturer.

 

[RegionRat]

For a simplified version based on the concept that weight is the only variant while energy and friction are held constant, the math can be simplified for the projectile weight.

KE=1/2MxV^2
for Mass as weight in grains and Velocity in feet per second we can simplify as follows using an assumed GC of 32.174 ft/sec^2

KE= (MxV^2)/450436

So let's say you are wondering about the 143 grain bullet and what happens if it happened to be 144 by accident. If we plug in a nice round 2500 fps to make the math pretty we would get a KE of 1984.2 ft*lbs. We then hold that amount of muzzle energy as the baseline constant and plug in a 144 grain bullet. The velocity of that bullet would then be as follows

1984.2 = (144 x V^2)/ 450436
((1984.2 x 450436)/144))^0.5=V

and solving for V we get 2491.3 fps

So, for the energy balance version, with that 143 to 144 grain example the rough figure of change due to weight as an independent variable (which is an oversimplification) the worst case is we see a change of 8.7 fps. If we assume a goal of staying under ES of 30 fps due to the assumption of mass being the only variable, we would need to hold the weight spread to 3.4 grains.

The implications of that math exercise is than unless your bullets are very poor quality in weight spread, the spread in weight is not going to account for a difference on the target.

The real reason it is done, is to screen for escapes where there is a very gross bullet that gets into the mix, and also to use weight difference as an indicator that the aerodynamics or friction are also affected. The folks who do this then must decide if they are also going to sort on lengths and/or subsequently "point" their bullets.

Once in a while, you find a bullet that comes from the start of a run where the weight is way off before the process is stable. These are generally culled and recycled, but every once in a while one "escapes".
Semper Fi