Dead Buff Posted July 1, 2004 Share Posted July 1, 2004 If this was answered already, please direct me in the right direction.... Has anybody done some practical research to determine by how much PF is affected by alititude? If've not been in a position to test it myself yet. Our club is at ±1800m above sea level - relativly high. Now we go to complete at 20m above sea level (or 900m for that fact) - by how much will my PF change all else remaining the same (temperature, gun, loads, humidity, etc)? The ballistic programs if used all give different answers - the one gives a vel change/decrease of 2 fps and the other with the same input perameters 75 fps. On a 180gr bullets that a diff of 0.36 to 13.5 PF Now what must I load - and where will my accuracy/POI go? Link to comment Share on other sites More sharing options...
.40AET Posted July 1, 2004 Share Posted July 1, 2004 (edited) I'll believe most anything Edited July 1, 2004 by .40AET Link to comment Share on other sites More sharing options...
Flexmoney Posted July 1, 2004 Share Posted July 1, 2004 I recall hear Jeff Maass talk aobut some of this stuff. I think he used data and infor found in the VV reloading manual. Link to comment Share on other sites More sharing options...
Detlef Posted July 1, 2004 Share Posted July 1, 2004 altitude alone has no statistically relevant measurable effect on PF/internal ballistics (i.e. there is no drift of the average PF to high or low). The IPSC PF is measured very close to the muzzle. But be careful with temperature changes that come with altitude. When you see PF drop because you shoot at 8000 ft altitude in CO, it is because it's COLDER up there than in Phoenix, AZ, not because it's higher! The ballistic programs you mention are relevant, and altitude alone becomes an issue, for terminal ballistics (usually rifle shooting) because of air density changes that measurably affect your flight velocity and path. --Detlef Link to comment Share on other sites More sharing options...
eric nielsen Posted July 1, 2004 Share Posted July 1, 2004 My power factors went up, moving from 1100' above sea level, down to sea level. A Navy gunner's mate could tell you exactly what happens & why. Link to comment Share on other sites More sharing options...
Dead Buff Posted July 2, 2004 Author Share Posted July 2, 2004 This is what happend to me: At our club I PF 180 with 180gr 40 short-and-weak std gun - this is at ±1800m a.s.l. At a shoot in South Africa with the same outside temp (±28 degC) I made 170.2 PF (yes, too close for comfort ) altitude was ±900m a.s.l. Rel hum was in both cases ±70% I don't want to run 190/200 PF just to be sure I'll make it where ever I go - thus guidline as to something like loose 10fps per 500m altitude loss or similar might be helpful - all other factors (temp, hum, etc) staying the same. Link to comment Share on other sites More sharing options...
Phil Dunlop Posted July 2, 2004 Share Posted July 2, 2004 I don't think this information is worth a lot, given the variances between chronographs, but my 180gn. Standard 40 load clocked 174p/f at sea level, but only managed 171 at 6000ft in South Africa. The load was N320, which has been very forgiving of temperature changes P.D. Link to comment Share on other sites More sharing options...
Dead Buff Posted July 2, 2004 Author Share Posted July 2, 2004 Your load just did the opposite from mine - now I'm really confused..... From all the PC software, my experiance and what I've seen here - there is no way of telling what will happen..... Guess I'll go to 200PF, put in some Buffs/springs/rubber/etc and be safe..... Link to comment Share on other sites More sharing options...
Skywalker Posted July 2, 2004 Share Posted July 2, 2004 Guess I'll go to 200PF, put in some Buffs/springs/rubber/etc and be safe..... Or...just shoot minor and never worry... Link to comment Share on other sites More sharing options...
Skywalker Posted July 2, 2004 Share Posted July 2, 2004 /rubber/ Rubber ?!?!?!?! Are you going to give a whole new meaning to "safe shooting" ???? Link to comment Share on other sites More sharing options...
Dead Buff Posted July 2, 2004 Author Share Posted July 2, 2004 Yep - thought of that shoot 175PF - if I make make major = hose more, make minor = aim more...... In the end it's about fun, right? Link to comment Share on other sites More sharing options...
Detlef Posted July 2, 2004 Share Posted July 2, 2004 I doubt a Navy gunner's mate knows more about pistol cartridges than us here... But of course, I am curious to hear the story! And surely, if you want to list your measurements here, do this only if you used *the same chronograph*, *the same load* (duh!) and don't forget to list temperature. --Detlef Link to comment Share on other sites More sharing options...
Bill Schwab Posted July 2, 2004 Share Posted July 2, 2004 I used to shoot a lot of NRA Highpower rifle when I lived in Colorado (sure do miss living there) and when we went to the Nationals in Camp Perry Ohio, so this is 5,000+ elevation to pretty much sea level, our ammo would hit low on the target at 600 yards. After a year or two of this I caught on and figured our exactly how much the difference was; I don't recall off the top of my head, but it was substantial. I never was given a good reason for this, but we surmised that the thicker atmosphere at sea level caused more bullet resistance and therefore a more rapid rate of deceleration of the bullet. Whether this would would be seen with a handgun when chrono'ing at a few feet in front of the muzzle I don't know.... Link to comment Share on other sites More sharing options...
eric nielsen Posted July 2, 2004 Share Posted July 2, 2004 In my case there was a temperature increase [but not much] and also humidity increase. It was WSL powder in 40cal ammo. I have found that sometimes the pf of ammo goes up just from travelling in general, could be related to the powder getting shook up & broken up inside the case. That definitely happens when you tumble ammo with certain powders. Link to comment Share on other sites More sharing options...
kevin c Posted July 2, 2004 Share Posted July 2, 2004 Dead Buff: The difference of 55 fps seems mighty big to me. Big enough that may be it was more a chrono difference than an altitude difference (the only way to know for sure is to use your own chrono in both locations under the same conditions). Did you read the thread on chrono sample size? How many rounds do you chrono at home, and how many were chrono'd at the match site? Perhaps you just got stung by a small sample size in each case. Just my guess. kevin c Link to comment Share on other sites More sharing options...
shred Posted July 2, 2004 Share Posted July 2, 2004 The only thing the atmospheric pressure could effect is the bullet drag. Hence the long-range differences, but nothing noticeable up close. Link to comment Share on other sites More sharing options...
Detlef Posted July 2, 2004 Share Posted July 2, 2004 same for humidity: It changes the air density. No statistically relevant internal ballistic effect, but important to the rifle shooter for long-range/terminal ballistics and flight path. --Detlef Link to comment Share on other sites More sharing options...
Gun Geek Posted July 3, 2004 Share Posted July 3, 2004 Guys - The issue here is going to be temperature and chronograph variances. Neither are generally predictable. Some powders go up with temp, some go down. Who knows about chronos? Link to comment Share on other sites More sharing options...
Detlef Posted July 3, 2004 Share Posted July 3, 2004 disagree: powder variation characteristics w/ temp are very well known, you just need to read the data sheet (and of course experiment w/ your own load). And for the chrono/operator/setup variations, you *can* model this w/ simple statistics (e.g. in a spreadsheet) after you kept track of your own measured PF at matches. --Detlef Link to comment Share on other sites More sharing options...
Dead Buff Posted July 5, 2004 Author Share Posted July 5, 2004 I had a chat with some guys around here over the weekend and all agree that vel wil go down with a decrease in alti due to the higher resistance of the "thicker" air on the bullet. We also came to the conclusion that the biggest unknow factor is the calibration of the Chrono - since temp and alti are known. At the distances that the chronies are set up alti should not make that big a diff. Skywalker: You should see how safe a "rubberised" pistol is - can't even get to the trigger. But since this is a family forum I'll not post a pic......... Link to comment Share on other sites More sharing options...
Gun Geek Posted July 6, 2004 Share Posted July 6, 2004 Well, I wasn't real clear, and that's what I get for not being clear: On the powder: Temp variation is well know but for a VERY specific situation: in a given lot, load (bullet weight, charge weight, type of bullet, etc,etc), specific firearm, number of rounds through the firearm, etc. I agree that if you very carefully made a bunch of rounds and tested them only changing temp, you could get a good idea with the spreadsheets, etc. However, by "generally predictable", I mean that I don't think we can say something like: Powder Name Change in velocity/ degree F A XX B YY C ZZ etc You have to know much more of the particulars. For the chrono stuff (and I think this will actually be more variation than temp). Since there is no way to test the calibration of chrono, who knows if they are correct. For a 45 going about 850 (200.0gn bullet) the actual correct statement of the pf is somewhere between: 855 x 200 = 171000 AND 845 x 200 = 169000 Both actual speeds of 845 and 855 could give the reading of 850, so other chronos would read anywhere between 845 and 855 (insert significant figures discussion here) for the same actual velocity. Point is, we don't really know that last digit. You might be able to model this if you knew that a particular match would use a particular chrono certian way (i.e. time of day, orientation to the sun, etc) and compare it to your readings, but again, you are counting on some VERY specific circumstances, so I can't say that it is "generaly predictable" Just build in a saftey margin of 10 pf and fuhgetaboudit. I just plugged in one of my loads into RSI Shooting Lab and can only get a 0.5 fps calculated difference in muzzle +5ft velocity from sea level, 95F, 90%RH to 10,000ASL 25F, 10%RH. Don't think this would be an issue. Link to comment Share on other sites More sharing options...
Dead Buff Posted July 9, 2004 Author Share Posted July 9, 2004 That about the conclusion I poped up with as well - load a bit higher and forget about it..... Thanx guys......... Link to comment Share on other sites More sharing options...
shred Posted July 9, 2004 Share Posted July 9, 2004 Heck, my Oehler with 2-chronos-in-one frequently has the two values differ by 10-20 fps, and not in a predictable way. Link to comment Share on other sites More sharing options...
D.Hayden Posted July 16, 2004 Share Posted July 16, 2004 We were just talking about this... We were wondering if the lower oxygen levels could afect the bullet at higher elevations? Link to comment Share on other sites More sharing options...
Detlef Posted July 17, 2004 Share Posted July 17, 2004 give us a break! The oxygen is built into the powder, in very high concentrations. No air oxygen is involved in gunpowder burning... --Detlef Link to comment Share on other sites More sharing options...
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