ASSRA Forum - PA and DA

joeb33050 Frequent Elocutionist Offline Posts: 2613 Location: Marathon, FL Joined: Apr 20^th, 2004	PA and DA Nov 23^rd, 2004 at 6:29am	Print Post
	I've been reading about PA and DA on the net, and am starting to understand what's going on. The whole business is an elaborate construct to enable pilots to turn a set of Altitude, Barometric Pressure and Humidity conditions into an equivalent altitude. And I understand why pilots might need and use that construct and the information. For shooting, wouldn't it be easier to measure absolute barometric pressure and humidity, and use that info to make inferences about sight settings? After all, the bullet doesn't know the altitude, it only responds to the density of the air. Isn't there a simple absolute barometer available for short $$? We stand a chance of making it so hard to understand tha folks won't use the info. joe b.


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FAsmus Ex Member	Re: PA and DA Reply #1 - Nov 23^rd, 2004 at 7:34am	Print Post
	Joe, A so-called "Absolute Barometer" is a real thing. Any mercury barometer will give you the atmospheric pressure where ever you go. But! the item is large, clumsy and delicate! Enter the regular barometer that takes its readings from a parcial-vaccume cannister. This machine compares the internal pressure of the cannister to the local atmospheric pressure and reads out in inches of mercury. The trick is to calibrate them. This is typically done by referancing a "real" mercury barometer in a lab somewhere, entering the correction for local elevation above sea level and setting ALL barometers, everywhere to "standard" datum such that their readings may be more readily compared across the country and around the world. Having said all that, I mention again that in the DA data I collect and use I seldom find it necessary to worry much about the PA anyway. Temperature is far, far more important to the calculation. Good morning, Forrest


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PETE Ex Member	Re: PA and DA Reply #2 - Nov 23^rd, 2004 at 10:32am	Print Post
	Forrest, And as you've pointed out to me you don't consider humidity as being inportant, altho it might possibly be at shorter ranges. I'm looking at DA as being a convenient number to use. If a portable rugged barometer is used.... and such are available. The Kestrel 4000 being one. There seems to me to be one problem with just using a barometer. Barometric pressure will vary with altitude, and as such would have to be set for each range you shot at or the readings you've set on the home range might not apply. To me Barometers are a "fixed in place" instrument and moving them around would skew any readings. With DA you are correcting for this difference in pressure and altitude, and adding in temp. Altho it might not be a perfect way of doing things, it's sure better than nothing. The old Creedmoor shooters had a thermometer, and barometer in their loading tents for doing just what we'r talking about. I wish there was an article on how they used these two figures to determine their sight settings. I'm pretty sure they didn't have an E6-B, or Kestrel 4000! Perhaps if David Minshall reads this he can come up with something since the British spent a lot more time at LR shooting than we did back in the Creedmoor days. Since the first shot counted they must have known something we don't. PETE


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ResearchPress

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Re: PA and DA
Reply #3 - Nov 23^rd, 2004 at 2:35pm

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Pete, can you keep the challenges a little simpler in the future please! Wink

Must admit to having to look up what PA and DA were! I found more information than I needed at: (You need to Login or Register to view media files and links)

William Ellis Metford produced tables of elevation settings for his rifles. Published tables I have seen have one for 'usual conditions' and two alternatives dependent on conditions. Although Metford acknowleged that on different days, even supposing the wind is nil, there are variations in elevation I have not read any specific comment about pressure.

Incidently, Metford did write about the "Trajectory of the Modern Match Rifle" for the Spirit of the Times magazine in the US. His work was reprinted in Judge Gildersleeve's 1878 "Rifles and Marksmanship".

Alex Lord Russell notes in his 1869 "Handbook of Rifle Shooting" that at Wimbledon height of the barometer is daily recorded and referred to, as well as the hydrometer, which records the relative moisture in the atmosphere. A consultation of both conjointly is necessary in correcting for elevation".

Forest and Stream published a little "Hand-Book For Riflemen" in 1876 (just timed nicely I presume for the Centennial Match at Creedmoor). It contains reference to atmospheric conditions but like so much I have found does not give specific observations as to effects of their changes. Observatiosn are general in nature: "If the day is clear and bright the elevation should be less than when the sky is overcast." etc.

Volume 2 of J.H.Walsh's 1884 "The Modern Sportsmans Gun and Rifle" has more specific information. Reference is made to a Government text for officers at the School of Musketry with regards to the Martini-Henry. There's an interesting table with barometer reading, height above sea level, velocity of bullet at 1000 yards, drop of bullet in 1000 yards, height of bullet above mark. eg

Barometer	Height	Velocity at 1000yds
30in	0 feet	672ft/sec
28in	1832	696ft/sec
26in	3926	720ft/sec
24in	6186	746ft/sec

Sir Henry Halford wrote a little in his 1888 "The Art of Shooting With The Rifle". He referred to Metford's elevation tables and noted they were draw for temperature at 60degF and a barometer standing at 30in. Halford states that every decrease in temperature of 4.5degF will at 1000 yards require an increase of angle of 1'. He also noted that "the effect of the state of the air, as shown by the barometer, is not so great as that shown by the thremometer, but nevertheless, it must not be neglected. I find that a variation of, an inch in height of barometer at 1,000 yards, makes a difference of about 2' of elevation."

It seems to me that the top riflemen probably shot far more than we (or at least I) do at long range. Copious note taking to build up elevation tables and experience would seem to be the order of the day, rather than any 'easily applied' rule.

David

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PETE Ex Member	Re: PA and DA Reply #4 - Nov 23^rd, 2004 at 4:10pm	Print Post
	Quote: Pete, can you keep the challenges a little simpler in the future please! David, Will try, but I knew if anyone could come up with something you'd be able to. The best that I can recall is having read once that the Creedmoor shooters kept a thermometer and barometer in their loading tent. Unfortunately I can't recall if this was a normal thing for both or if just one of the sides used them. Will have to look at that web site you mention. Sounds interesting. Quote: Alex Lord Russell notes in his 1869 "Handbook of Rifle Shooting" that at Wimbledon height of the barometer is daily recorded and referred to, as well as the hydrometer, which records the relative moisture in the atmosphere. A consultation of both conjointly is necessary in correcting for elevation". This the part I wish there was more info on. Did they just record the readings and the sight settings, and then when needed, match up as closely as possible, similar conditions so as to set their sights so the first shot would be on paper. I don't know this for a fact about Creedmoor, but having shot in pastures, the impacts are about impossible to see and you better have a good idea of where your bullet will hit under varying conditions. Since, as I got it, you didn't get any sighter shots, you would want your first shot to be a center if possible. Quote: Observatiosn are general in nature: "If the day is clear and bright the elevation should be less than when the sky is overcast." etc. Yes! This is a pretty well known fact among high power shooters using iron sights. Also the POI will follow the sun as the sight being brighter on one side or the other will cause the eye to see that side clearer and tend to shift POI slightly into the bright side. Do you have a page number for that table of Walsh's in his Vol. II? I've got the Wolfe reprints of both so would like to take a look at them. Quote: He also noted that "the effect of the state of the air, as shown by the barometer, is not so great as that shown by the thremometer, but nevertheless, it must not be neglected. This is what Forrest is saying. He's found that humidity can be ignored...... at least in the dry country of the northern great plains...... but that temp. produces the greatest differences in DA. But we also need the barometric pressure to calculate PA. Looks like Halford was doing what he could with the equipment he had, but still begs the question how, or did, he combine the two to make some kind of sense out of those readings so he could get proper sight settings. Apparently this seems to be one of those things that you were supposed to know so therefore wasn't needed to be explained. Sometimes I wish I could grab those guys by the neck and tell them a 100 yrs. from now people will need a little more detail! Quote: It seems to me that the top riflemen probably shot far more than we (or at least I) do at long range. Copious note taking to build up elevation tables and experience would seem to be the order of the day, rather than any 'easily applied' rule. I agree! But, it seems to me, from my own experience, that you just can't go in blind. You have to have some idea of where to set your sights. If conditions are bad enuf you could miss the whole target at 1000 yds. on your first shot, and if you didn't see your strike, you'd be up the creek because you wouldn't have the foggiest idea of which way to turn your sights. You have to have a base to work from, and enuf knowledge of the current conditions so you would know which way to move the sights under similar ones. Thanks David! Interesting bits of info, and lots of food for thought! PETE


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ResearchPress Full Member Offline Posts: 107 Location: UK Joined: Aug 18^th, 2004	Re: PA and DA Reply #5 - Nov 24^th, 2004 at 2:14am	Print Post
	Quote: Did they just record the readings and the sight settings, and then when needed, match up as closely as possible, similar conditions so as to set their sights so the first shot would be on paper. That's the impression I am getting. Halford writes of his observation of the effect of pressure, "It is not easy to prove by calculation that the quantites I have given, both for temperature and weight of atmosphere, are correct, but long practice and close observation have shown me that they are practically so." He refers to calculating tables of trajectory and also stresses the importance of record keeping; "keep full records of diagrams of every shot fired, faithfuly taken, with full notes of wind and weather, distance, charge, and position." Ultimately I suppose it is down to the individual riflemen, we all do something different and applying an absolute set of standards is not going to work. Two people shooting the same rifle and ammunition will often find that the site settings are markedly different. Each needs to know their own rifle. Quote: as I got it, you didn't get any sighter shots, you would want your first shot to be a center if possible. Certainly in the International series of matches held in the 1870's sighters were not permitted. Maximum scores (or very close to) were not unknown, so these riflemen obviously knew what they were doing! Milner of Ireland scored 15 consecutive bull's-eye's at 1000 yards in 1876... Chapter IV of Volume II of Walsh's book covers 'Atmospheric Resistnce' and the table I referred to was printed on page 71. We have so much to learn... David

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PETE Ex Member	Re: PA and DA Reply #6 - Nov 24^th, 2004 at 10:19am	Print Post
	David, Quote: Halford writes of his observation of the effect of pressure, "It is not easy to prove by calculation that the quantites I have given, both for temperature and weight of atmosphere, are correct, but long practice and close observation have shown me that they are practically so." I'll agree. Looks like the best most of the shooters could do was to keep a record of every shot along with as much atmospheric conditions as they could and then hope they could find a close match when needed. Even tho we have access to instruments that will aid us we're still in the same boat. Might be able to just use DA, but we still have to have enuf data so we can select a match on any given day. Like Halford said....... Keep plenty of records! I've been corresponding with "ol7groove" on various things and yesterday he sent me some computer data that shows how temp. will effect drops over the different ranges. I hope he'll chime in with some of the figures he came up with as I don't think it's my place to get the credit for someone elses work. To sum up a little..... As Forrest says, you can forget humidity as even very large differences make less than 1/4" difference at 200 yds. But from the data it appears as tho even at 200 yds. temp. differences will make some difference, altho not as much as I was thinking might be the case. But, we also have to remember that this was a "static" test, and altho the initial figures were taken from an actual test, the "changes" were just pulled out of a hat in order to see what would show up. What I found out using the Kestrel is that if you read PA & DA inside there is a wide difference from what you will get outside under "natural" conditions. This is due to the fact that the barometric pressure will pretty well reflect the outside conditions, but the temp. was from the inside, so there was, at the time, about 30 degrees difference. Also, if the Kestrel uses humidity in it's calculations, then there was about 30% difference. Quote: We have so much to learn... Ain't that the truth! What is really frustrating is that as you have pointed out many times on the different Lists/Forums with your posts, that everything we've been wanting to know over the past 25 yrs. or so, has all been done before, and written down somewhere. If we would know where to look we could have saved ourselves the trouble of re-inventing the wheel. Thanks for the info on Walsh's charts. Will look at the the first chance I get. Been meaning to ask you...... Have you ordered that reprint of "The Irish Riflemen in America" by the Capt. of the Irish team in 1874.... Major Arthur B. Leech, that Precision Shooting is doing? PETE


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ResearchPress Full Member Offline Posts: 107 Location: UK Joined: Aug 18^th, 2004	Re: PA and DA Reply #7 - Nov 24^th, 2004 at 12:27pm	Print Post
	Quote: Been meaning to ask you...... Have you ordered that reprint of "The Irish Riflemen in America" by the Capt. of the Irish team in 1874.... Major Arthur B. Leech, that Precision Shooting is doing? Yes! It is currently sat on my bookshelf next to the orginal copy I have signed by the author! (smug) David

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PETE Ex Member	Re: PA and DA Reply #8 - Nov 24^th, 2004 at 2:49pm	Print Post
	David, I haven't gotten my copy yet, but it should be here any day. You've got an original copy! WOW! From what I understand there were only about 1000 of them made up, and from the way the editor of PS talks they sell for about $400..... if..... you can find one for sale. Guess you have right to be smug! PETE


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