external ballistics programs only rely on bc, not weight?

[chop house]

what started as a simple question (to myself) has led already to multiple 'if, ands, buts, and what if's' about bullet weight. i was curious about some inexpensive 22 cal speer tnt's so i weighed a few hundred and found the 50's to range from about 49.85 to 50.25 with a few outside that range. i went to an online ball calc (happened to be nikon's) and immediately found i could not input tenths in the weight field, then went on to find it didn't matter anyway, because changing only bullet weight without changing bc input had zero effect on drop/path/drift. ok. if i made tiny changes to bc i could of course see miniscule effects on d/p/d.

revealation number one: so this is why folks sort by ogive, because length effects sectional density and bc... which is 'the' factor in the exterior ballistics world...?

Now, not having quickload and not being at the range today, i could neither predict nor measure the muzzle velocity difference across this weight spread. Factor in the other internal ballistics considerations of barrel time, pressure curve, burn rate, bearing surface, start pressure,.... crap.... it gets complex real quick. SO:

1. How much muzzle velocity variation from 1.0% bullet weight spread?

2. How much effect at short range (300 yds) is expected with a 1.0% extreme bullet weight spread? (Assumes non-vld bullets with bc's in the 0.2's and 0.3's)

3. Are some powders, or classes of powders (fast end vs slow end of loading spectrum, i.e) more tolerant of bullet weight spreads?

4. Are these effects observable from rifles that shoot only in the 0.5 moa range, or is this a part of why they only shoot 0.5 moa?

thanks... i left the load specifics out, hoping for a tutorial response - thinking the concepts apply across the board.

 

[alinwa]

yes
none
none
no
no

 

[Laurie]

1. How much muzzle velocity variation from 1.0% bullet weight spread?

The ballistic equivalent for different bullet weights is the MV that produces a constant ME. For example, if a 200gn bullet sees 1% overall weight variations, let's say they're evenly spread around 200 as an average and are therefore 199-201gn weight. At 2,600 fps MV, the nominal 200gn bullet produces 3,003 ft/lb ME. The MV that produces that ME for a 199gn bullet is 2,607 fps; for the 201gn bullet 2,594 fps. Therefore, a 2gn bullet weight spread here would in theory produce a 13 fps velocity ES.

I say in theory because there are many other factors that produce MV variations. Then too, the factors that cause the bullet weight variations might also change other factors that influence MVs, for instance bearing surface length variations which would change the degree of in-barrel friction which in turn influences both the powder charge burn rate and peak pressure.

2gn is an unholy size of spread in a modern 200gn match bullet - few precision shooters would be happy with such a range these days. In a light 224 dia. bullet, say 50gn at 3,000 fps MV, the MV range that covers a 1% weight variation is pretty similar - around 15 fps (but again if this is the only factor which affects MVs).

People don't do BTO (base to ogive) checks and if needed batching because they believe this metric affects SD and/or BC - they do it because it affects both bearing surface length and the position of the ogive in relation to the lands. It's about consistency in key metrics and settings. SD will be marginally affected by weight variations which will in turn affect BC as the bullet SD and form factor (bullet form ie shape efficiency) are the two metrics that determine BC.

As it's easier to work with big numbers, let's use the 200gn bullet as an example again and we'll say it's 0.308" diameter. At exactly 200gn, its SD is 0.301, and varying weights by 1% gives a range of roughly 0.300-0.303 using an off the shelf online calculator whereas we really need to do this to four or five decimal places. But let's say the lightest bullet has 0.300 SD and the heaviest 0.303. If we use Berger's 200gn 200-20X bullet as an example, it has an average G7 form factor of 0.919 and with 0.301 SD we get a G7 BC of 0.318, ie SD over Form Factor to get there.

Assuming all of the sample have identical form factors despite their weight variations, our lightest bullet in the sample has a BC of 0.300 divided by 0.919 = 0.326; the heaviest is 0.303 divided by 0.919 = 0.330 rounding to three decimal places.

So, one can input these two BCs into a ballistics program. However, the lighter, lower BC extreme in the sample has a higher MV than the heaviest, highest BC extreme - 2,607 fps v 2,594. Input those MVs and revised BCs into the (Berger online) program and you get for 300 yards:

Light example ............ 2,209 fps / 5.41" drift in a 10 mph 90-deg crosswind
Heavy example ........... 2,201 fps / 5.38"

ie 0.03" drift change in a (huge) 10 mph wind strength / speed shift between shots and 8 fps in retained velocity which won't affect the POI elevation more than fractionally.

at 1,000 yards where a bullet like this justifies its price (or not as the case may be) we get

Light example ............ 1,412 fps / 73.24" drift in a 10 mph 90-deg crosswind
Heavy example ........... 1,415 fps / 72.62"

Differences that are so small other factors would swamp them. But this likely all hogwash anyway as the form factor hence BC is likely to change as much as this, if not more anyway, through variations in the meplat size in non-pointed HPBT type bullets.

As to questions 2 to 4, I'd agree with alinwa - ie none, none, no, and no.

 

[dstoenner]

The ballistic equivalent for different bullet weights is the MV that produces a constant ME. For example, if a 200gn bullet sees 1% overall weight variations, let's say they're evenly spread around 200 as an average and are therefore 199-201gn weight. At 2,600 fps MV, the nominal 200gn bullet produces 3,003 ft/lb ME. The MV that produces that ME for a 199gn bullet is 2,607 fps; for the 201gn bullet 2,594 fps. Therefore, a 2gn bullet weight spread here would in theory produce a 13 fps velocity ES.

I say in theory because there are many other factors that produce MV variations. Then too, the factors that cause the bullet weight variations might also change other factors that influence MVs, for instance bearing surface length variations which would change the degree of in-barrel friction which in turn influences both the powder charge burn rate and peak pressure.

2gn is an unholy size of spread in a modern 200gn match bullet - few precision shooters would be happy with such a range these days. In a light 224 dia. bullet, say 50gn at 3,000 fps MV, the MV range that covers a 1% weight variation is pretty similar - around 15 fps (but again if this is the only factor which affects MVs).

People don't do BTO (base to ogive) checks and if needed batching because they believe this metric affects SD and/or BC - they do it because it affects both bearing surface length and the position of the ogive in relation to the lands. It's about consistency in key metrics and settings. SD will be marginally affected by weight variations which will in turn affect BC as the bullet SD and form factor (bullet form ie shape efficiency) are the two metrics that determine BC.

As it's easier to work with big numbers, let's use the 200gn bullet as an example again and we'll say it's 0.308" diameter. At exactly 200gn, its SD is 0.301, and varying weights by 1% gives a range of roughly 0.300-0.303 using an off the shelf online calculator whereas we really need to do this to four or five decimal places. But let's say the lightest bullet has 0.300 SD and the heaviest 0.303. If we use Berger's 200gn 200-20X bullet as an example, it has an average G7 form factor of 0.919 and with 0.301 SD we get a G7 BC of 0.318, ie SD over Form Factor to get there.

Assuming all of the sample have identical form factors despite their weight variations, our lightest bullet in the sample has a BC of 0.300 divided by 0.919 = 0.326; the heaviest is 0.303 divided by 0.919 = 0.330 rounding to three decimal places.

So, one can input these two BCs into a ballistics program. However, the lighter, lower BC extreme in the sample has a higher MV than the heaviest, highest BC extreme - 2,607 fps v 2,594. Input those MVs and revised BCs into the (Berger online) program and you get for 300 yards:

Light example ............ 2,209 fps / 5.41" drift in a 10 mph 90-deg crosswind
Heavy example ........... 2,201 fps / 5.38"

ie 0.03" drift change in a (huge) 10 mph wind strength / speed shift between shots and 8 fps in retained velocity which won't affect the POI elevation more than fractionally.

at 1,000 yards where a bullet like this justifies its price (or not as the case may be) we get

Light example ............ 1,412 fps / 73.24" drift in a 10 mph 90-deg crosswind
Heavy example ........... 1,415 fps / 72.62"

Differences that are so small other factors would swamp them. But this likely all hogwash anyway as the form factor hence BC is likely to change as much as this, if not more anyway, through variations in the meplat size in non-pointed HPBT type bullets.

As to questions 2 to 4, I'd agree with alinwa - ie none, none, no, and no.

Laurie

That was an excellent tutorial with well derived straight forward answers.

Thanks

David

 

[chop house]

The ballistic equivalent for different bullet weights is the MV that produces a constant ME

Laurie... thanks for reminding me of this equivalence, i should have remembered that. AND your excellent reply was clear and concise: written like a pro!

I shall put down the calculator and go enjoy shooting.

 

[Laurie]

I shall put down the calculator and go enjoy shooting.

Always the best thing to do!

 

[johnfred1965]

Laurie

That was an excellent tutorial with well derived straight forward answers.

Thanks

David

I always enjoy reading Laurie's posts. As they say "If you can't explain it simply, you don't understand it enough." His grasp on these subjects is voluminous, and his explanations are concise and easy for the layman to understand. I think Bryan Litz took a cue from Laurie, since his books are also clear and understandable. Thanks for making this forum the fountain of firearms knowledge it is.

 

[Shoebox]

The ballistic equivalent for different bullet weights is the MV that produces a constant ME. For example, if a 200gn bullet sees 1% overall weight variations, let's say they're evenly spread around 200 as an average and are therefore 199-201gn weight. At 2,600 fps MV, the nominal 200gn bullet produces 3,003 ft/lb ME. The MV that produces that ME for a 199gn bullet is 2,607 fps; for the 201gn bullet 2,594 fps. Therefore, a 2gn bullet weight spread here would in theory produce a 13 fps velocity ES.

I say in theory because there are many other factors that produce MV variations. Then too, the factors that cause the bullet weight variations might also change other factors that influence MVs, for instance bearing surface length variations which would change the degree of in-barrel friction which in turn influences both the powder charge burn rate and peak pressure.

2gn is an unholy size of spread in a modern 200gn match bullet - few precision shooters would be happy with such a range these days. In a light 224 dia. bullet, say 50gn at 3,000 fps MV, the MV range that covers a 1% weight variation is pretty similar - around 15 fps (but again if this is the only factor which affects MVs).

People don't do BTO (base to ogive) checks and if needed batching because they believe this metric affects SD and/or BC - they do it because it affects both bearing surface length and the position of the ogive in relation to the lands. It's about consistency in key metrics and settings. SD will be marginally affected by weight variations which will in turn affect BC as the bullet SD and form factor (bullet form ie shape efficiency) are the two metrics that determine BC.

As it's easier to work with big numbers, let's use the 200gn bullet as an example again and we'll say it's 0.308" diameter. At exactly 200gn, its SD is 0.301, and varying weights by 1% gives a range of roughly 0.300-0.303 using an off the shelf online calculator whereas we really need to do this to four or five decimal places. But let's say the lightest bullet has 0.300 SD and the heaviest 0.303. If we use Berger's 200gn 200-20X bullet as an example, it has an average G7 form factor of 0.919 and with 0.301 SD we get a G7 BC of 0.318, ie SD over Form Factor to get there.

Assuming all of the sample have identical form factors despite their weight variations, our lightest bullet in the sample has a BC of 0.300 divided by 0.919 = 0.326; the heaviest is 0.303 divided by 0.919 = 0.330 rounding to three decimal places.

So, one can input these two BCs into a ballistics program. However, the lighter, lower BC extreme in the sample has a higher MV than the heaviest, highest BC extreme - 2,607 fps v 2,594. Input those MVs and revised BCs into the (Berger online) program and you get for 300 yards:

Light example ............ 2,209 fps / 5.41" drift in a 10 mph 90-deg crosswind
Heavy example ........... 2,201 fps / 5.38"

ie 0.03" drift change in a (huge) 10 mph wind strength / speed shift between shots and 8 fps in retained velocity which won't affect the POI elevation more than fractionally.

at 1,000 yards where a bullet like this justifies its price (or not as the case may be) we get

Light example ............ 1,412 fps / 73.24" drift in a 10 mph 90-deg crosswind
Heavy example ........... 1,415 fps / 72.62"

Differences that are so small other factors would swamp them. But this likely all hogwash anyway as the form factor hence BC is likely to change as much as this, if not more anyway, through variations in the meplat size in non-pointed HPBT type bullets.

As to questions 2 to 4, I'd agree with alinwa - ie none, none, no, and no.

Always the best thing to do!

Great question !
Very well explained answer !!!
Thank you for your time and effort !!!
This shows the quality of this forum !

 

[Bc'z]

I've read in nosler manual that it doesn't matter if it's 30 caliber or 20. If it has same bc flying at same speed drop will be the same.
Know your load, know your rifle, know your ability.

 

[Laurie]

I always enjoy reading Laurie's posts. As they say "If you can't explain it simply, you don't understand it enough." His grasp on these subjects is voluminous, and his explanations are concise and easy for the layman to understand. I think Bryan Litz took a cue from Laurie, since his books are also clear and understandable. Thanks for making this forum the fountain of firearms knowledge it is.

Thank you for the praise, but please ........ I'd never claim to influence Bryan, rather look up to him as a model technical writer and supremely good at making the complex understandable. I'm simply not in his class - he is literally a(n) (ex) rocket scientist.

............ now, if only I could read the wind as well as Bryan.

 

[josh shrum]

People don't do BTO (base to ogive) checks and if needed batching because they believe this metric affects SD and/or BC - they do it because it affects both bearing surface length and the position of the ogive in relation to the lands. It's about consistency in key metrics and settings..

Now if could get this across to everyone who is telling me I am wasting my time sorting...

 

[Bart B.]

If you can see statistically significant differences in accuracy by sorting bullets by any parameter, do it.

Once you've got a batch of good bullets, will several people testing your rifle with them produce the same accuracy?

PS:
Bullet base or rimless case head to bullet ogive diameter contact point with rifling is not the critical dimension for bullet jump uniformity. It's case shoulder contact diameter with chamber shoulder to bullet ogive rifling contact diameter.

 

[johnfred1965]

Thank you for the praise, but please ........ I'd never claim to influence Bryan, rather look up to him as a model technical writer and supremely good at making the complex understandable. I'm simply not in his class - he is literally a(n) (ex) rocket scientist.

............ now, if only I could read the wind as well as Bryan.

Rocket scientist or not, you are both WAY out of my league. Luckily, I take pleasure in shooting, and not just trying to win. If it came down to winning, I would have to sell all my rifles and consider the proceeds my prize money!

I have commented on your writing before. Your insight is always spot on if you ask me.