Archive | June 2015

For @VillianusPoonII

This is a reply to Twitter user @VillianusPoonII regarding an interesting exchange we started. VP, you deserved more than 140 characters could provide and so here you go.

Our Twitter conversation suffered from all of the problems of any Twitter conversation. Lack of depth which cannot lead to any useful conclusion or information exchance. It’s entirely likely that we share a largely common view on matters of policy and that we differ substantially in what things we’re taking into account in our opinions. That provides a rich and fertile ground for discussion. Our exchange also contained something not common to Twitter at all: Honesty. In an effort to do justice to someone who was honest I’m going here to expound on the conversation. You are free to reply and I will approve replies from you that are at least respectful regardless of how I feel about them intellectually.

So here’s what we started with:

A.S.A.P. Sperg ‏@VillianusPoonII Jun 19

Ballistic_XLR ‏@Ballistic_XLR Jun 20
@VillianusPoonII Actually you can. In Mexico it’s documented to have happened twice. Your point is valid only in non-lawless countries.

A.S.A.P. Sperg ‏@VillianusPoonII Jun 20
@Ballistic_XLR Mexico is a shining beacon of gun control.

Ballistic_XLR ‏@Ballistic_XLR Jun 22
@VillianusPoonII That reply doesn’t make your initial point less spurious. It does make another more accurate statement which is orthogonal.

A.S.A.P. Sperg ‏@VillianusPoonII Jun 22
@Ballistic_XLR My reply was an exasperated generalization of Mexico’s seemingly lackidaisical enforcement of legislation.

A.S.A.P. Sperg ‏@VillianusPoonII Jun 22
@Ballistic_XLR Its a reply to your reply, not mine.

Ok, on Mexico and gun laws: Mexico is not just vigorous about enforcement of gun control laws, they’re draconian about it. That said, Mexico is a land of endemic corruption as well as crippling poverty and harrowing social issues. They have an illegal immigration problem of their own and a history of military-economic collusion that makes it unfair to say that they don’t enforce their laws. They do but the consistency is frustrated by the realities of law enforcement in Mexico. To their credit Mexico has made massive strides in the last 30 years.

Guns are not illegal in Mexico. They’re actually quite common though restrictions are pretty heavy. That said, Mexicans have a constitutional right to own firearms (with substantial restrictions on military chamberings) but not to carry them. Subtle distinction.

You made two statements about Mexico and gun control which appear to and do directly contradict each other: First that Mexico is a shining beacon of gun control and that they completely fail at usefully enforcing it. Which is it? If you’re saying both then I might agree with you if we stipulate that the only reason that they’re so bad at enforcement is that the general state of affairs of Mexico (rampant corruption, poverty and lacking: opportunity, education, infrastructure, good governance, etc…) directly preclude the enforcement of any legislation being done to a usefully consistent level.

Your first statement that nobody had ever been criminally killed with a .50BMG was your error. That was a factually wrong statement. If you’d have qualified it with the words “in the United States or Canada” you would have saved your point, been right and been immune from gain-saying or argument. The rest of the conversation appears to be me picking at you like a mosquito and you replying (sorry to say) somewhat disjointedly in a way that was difficult for me to grok.

You’re right, poor quality enforcement leads to poor quality results. Mexico is a bit of a shit hole still but they’re improving and it’s not that Mexicans are bad but that they’re at this point in their history and have to live through it for better or worse. You’re exceptionally on target with your first implication that the most devastating weapons available in the USA appear to be those least used; like actually carried into and/or fired, during criminal acts. There’s an unsaid corollary in there too: The most common, least expensive, least complicated and most obtainable firearm that you can get will be used disproportionately in crimes because: There are a shit ton of them, they’re everywhere and they’re not expensive.

So I’m going to predict that we agree on all or most of the above and that I was picking at you for a gross bit of hyperbole that was factually inaccurate while being only a few words from being unassailably accurate while still being laced with bits of insight.

How To Use Ballistic_XLR: Extended Data

Extended data is meant to be used to interpolate and for direct results. The table consists of windage and elevation settings for increases in barometric pressure and wind speed. Wind drift data is keyed for varied wind speed speeds of 5mph, 10mph, 15mph, 20mph, 30mph at specific barometric pressures. The pressure variances are in increments of 1 inch of mercury (about 34Hpa) and go up and down from your station pressure for 3 intervals each. So if you list 27inHg as your station pressure on the Pocket PC Input page then you will have both wind and drop data for 24, 25, 26 and 28, 29, 30inHg on the Extended Data tab.

Now let’s cover a use case. You’re out in the field doing some extreme long range precision training. You’ve set your chart for nominal barometric pressure of 27.5inHg and your area of operations tends to languish around that value and it’s been hanging there just fine for 2 days. Your target is set up and you’ve spent the day clanging steel with good success. You wake the next day to find that the weather has taken a serious turn and you pull out your Kestrel and find that the barometric pressure has dropped to 26.7inHg. You know that this is going to affect your DOPE and you’re running low on ammo and you want to get some shots on video for a little bragging rights. How to solve the conundrum. Easy.

Your target is at 1050m which means even a couple clicks off and you’ll have a wasted shot. You pull out your Extended Data table and look up the drops at the nearest 100yrd/m intervals to where your target is. In this case you find the drop for the temperature and pressure you’re at at 1000m and at 1100m. We’ll call this 8.62MRAD and 10.23MRAD. Almost 2MRAD betwen those two points is a LOT of difference. Now go back to your Table 10 data (it’s important to use Table 10 if at all possible) and get the indicated drop at station pressure and at the temp you’re at as well as the indicated drop for 1000m and 1100m. We’ll call this 9.62, 8.81 and 10.48 respectively. Now we see that at 1000m the difference of 1inHg meant .2MRAD at 1000m and .25MRAD at 1100m. So we see the difference is .2MRAD on average and we pull .2MRAD out of our elevation calculation.

From this we see that localized interpolation can be very flexible and provide useful and accurate data. The key of this is to find your indicated data from your station pressure tables and then to find the differences between comparable points on the trajectory that are nearby and at 100m/yrds brackets. Note that at 1050m .2MRAD at 1050m is just about 8 inches. That’s the difference between a hit and a miss and it’s the reason that Ballistic_XLR is stuffed full of points of interpolation meant to get you within about .5MOA or .2MRAD (2 clicks) of your target even when conditions change.

The way we did the barometric pressure also applies to the windage. Windage changes at long range due to barometric pressure changes can easily be not insignificant. As well differences in wind speed dramatically affect trajectory and need to be carefully doped. A 10mph wind can easily put in 3x as much drift as a 5mph wind and things get really dramatic after 15mph. Interpolation is done the same way. Find the difference between your bracketed 100m intervals and then find the data for your actual range and apply that difference.

How To Use Ballistic_XLR: Table 10

I’ll cut the story a tad short and note that the Table 10 tab is used exactly the same way Table 100 is used. The reasons for using Table 10 instead of table 100 is when you’re shooting at a target at such long range that the dope changes so quickly that range estimation or interpolation errors that are very small can cause a miss or to strike the target in a non-critical area. In general beyond 400m I would recommend using the Table 10 tab. The other reason is that Table 100 is limited to 1800m due to available space while Table 10 goes out to 2490m which is close to being a practical limit on what even the most advanced shoulder fired precision rifles are capable of. Yes you can reach farther than that but almost nobody is able to shoot anything usefully small at such ranges. Even with a .1MOA rifle you’re looking at a target with a radius of 30 inches as a useful minimum. Well if your target is 5 feet across you should probably be using something more than shoulder fired small arms to destroy it. The primary reason for even creating a 10 meter/yard increment table was to eliminate the need for significant interpolation when shooting at extreme long range, not simply to increase the range you can shoot at. As long as your weather and weapon conditions are stable you’ll be able to stay “on-tab” (as I call it) and not have to do much in the way of interpolating and that’s the benefit in the 10m tab. If you’re shooting at beyond 400m I would say that Table 10 is the best option.

How To Use Ballistic_XLR: Table 100

Table 100 is designed to provide a single page with all the data you need to get your rifle on target out to 1700m. Beyond 800m most rifle rounds are going to be coming down pretty sharply and it’d be better to use Table 10 for your data. This gets into danger space and swept space and if you’re not familiar with those please watch this before deciding to use the Table 100 data instead of Table 10 data:

Now, provided that Table 100 is suitable to your needs the usage is pretty straightforward. The table is keyed for your normal station pressure and includes wind and drop deltas for -1inHg. The reason for that is, it’s not entirely unlikely that you’ll have a hill to climb or descend or that one day could be cloudy and cold while the previous day might have been warm and sunny. These are barometric pressure changes that need to be accounted for. Those data are there to be used and they should be used by subtracting the values listed in those blocks from the data you calculate AFTER your final firing solution has been calculated. The reason to do it afterward is just to keep all these numbers from confusing the hell out of you (and me).
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I mostly use Table 100 for hunting purposes or informal target shooting where first round hits aren’t a priority. I also use it at High Power Metallic Silhouette matches at ranges I haven’t been to before to get my initial DOPE settings (these get refined during the sighter period before the match). I do not use it in competition unless the way the match is shot doesn’t lend itself to using Table 10.

Moving on: Let’s assume we’re proned out behind the rifle and we’ve got the 12×20″ iron maiden target on-sight and ranged using the mil-scale reticle. We range the target at 815m. First thing first, it’s not at an increment of 100m. Assuming a 6.5×284 performing at 2900fps and coin with a 140gn slug, we look up our 800m elevation and see 5.65MRAD in the 800m slot at our temperature regime. Above that we see 4.55 at 700m and 6.85 at 900m. This means that over this 300m range we’re adding approximately .1mrad per 10m. This is called interpolation and that’s how it works. It’s nothing but simple quick averages. 800m indicated 5.65 + 15 yards worth which would be about .15 we’re at 5.8MRAD elevation. Write that down somewhere or just enter it in your calculator (or head).

Sample Metric 6.5×284 Table 100

Now we need to finish elevation. That happens at vertical coriolis. So we look at V-Cor for 800m we get .05MRAD being full value. We need to get our compass heading now and find that we’re pointing 10deg north of due East. We need to find our degrees away from North from that and we find that it’s 80deg. Look now at the compass rosette and find the 80deg mark near the East point. The cosine is just outside the rosette for each bracket of angle.
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In this case the cosine is .985 which is close enough to full value and full value is so small that we’ll just take the full value number and (because we’re facing East) subtract that from the elevation setting for range (for westward shots you add the V-Cor value to elevation). This gives us 5.75MRAD and if we round up we end up back at 5.80MRAD.

Now to the windage correction. It is vital to keep track of which direction you’re adjusting windage since it will get adjustments in both left and right in many circumstances. If you lose track of which direction things are moving then you need to assign little arrows or something on your calculation sheet or find a way to keep that data sane. Windage is the source of most misses for the new long range shooters and seasoned ones that I play with in the real world. Assuming that you’re going to dial for wind and not hold for wind (holding wind is arguably better for a number of reasons but beginners should learn to dial things and hold steady until they can read the wind a little, then start holding off for wind). Also realize that windage and wind are different words. Windage refers to the adjustment we put into the sighting system and wind refers to the air moving.

Get a wind reading in whatever way you can. If you’re at ground level and using a meter like a Kestrel then you should verify that reading against flags, trees, brush, mirage, etc… Why? Well because of the way air moves over the surface what we end up with is lower wind at ground level than at 10m. Similarly when you fire from 10 inches off the ground at a 800m target you’re looking at the bullet being 4ft or more off the ground at the maximum ordinal. Wind will be stronger at 4ft than at 10 inches.

We take a wind reading and look at the range flags and we’re seeing the Kestrel read 7mph and the flag is showing 10-12mph. We’ll call it at 10mph for simplicity. Look up the value on the windage sheet for our temp regime and we get 1.54. The wind isn’t directly against us though, it’s coming from 5 o’clock so we look at the compass rosette and get the cosine for winds coming from 5 o’clock (so wind is to the left) which is .5. So we multiply that cosine by the indicated wind adjustment and get .77MRAD right. Now find the Horizontal Coriolis which is always full value and requires an adjustment to the left in the northern hemisphere and we get .2MRAD. Since one is to the right and the other to the left we subtract and get .57MRAD right.

Look over at the Spin Drift which is always full value (and we’ll assume left adjustment from a right twist barrel) and we get
.13MRAD left. So subtract .13 from .57 because they’re opposing directions again and we get .44. Round that and we get .4 right total windage.
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Now for the fun part, dial 5.8MRAD of up and .4MRAD right. Hold center of the target, get a natural respiratory pause and let fly. Wait the 1.1 seconds for the bullet to get there and another 2.5 seconds for the sound of the impact to get back to you (via distance-in-ft / 1050 = time of sound propagation) and revel in your awesome. You can even use the time of flight chart to figure out the time it’ll take for the bullet to get there, how long before the target hears your rifle boom, how long between the bullet strike and the target hearing the boom and lots of other fun and tactically interesting uses.
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