DogmaDog Posted January 31, 2003 Share Posted January 31, 2003 Hey all, Section 5.6 of the rules deals with chronographing, but nowhere in there can I find anything about how far from the firing line the chronograph is to be placed. As bullet velocity decreases most rapidly right at the muzzle (drag forces are proportional to the square of velocity), I would think that placing the chronograph a couple of feet further away than expected could significantly lower the measured power factors at a match. So, how far away from the muzzle are chronographs placed during matches? Is there any standard? Should there be? Thanks for your insight, DogmaDog Link to comment Share on other sites More sharing options...
SmittyFL Posted January 31, 2003 Share Posted January 31, 2003 I don't know what the standard is but I have always put mine 5-8 ft. away. And if I remember correctly that seems about right the last major match I was chronoed at. Link to comment Share on other sites More sharing options...
Flexmoney Posted January 31, 2003 Share Posted January 31, 2003 I know that you get screwy readings if the screens are too close. Muzzle blast gets picked up along with the bullet. Pact suggests eight feet in my Mk-IV manual. Link to comment Share on other sites More sharing options...
Vince Pinto Posted January 31, 2003 Share Posted January 31, 2003 There is no standard and I think it's impossible for us to state one, because each chrono is different. The key is for the chrono to be in the same position for all tests. This is not stated in the rulebook now either, however it's on my list for our rules committee meetings in Orlando soon, where calibration of poppers will be a major topic. Link to comment Share on other sites More sharing options...
DogmaDog Posted January 31, 2003 Author Share Posted January 31, 2003 Flex, My MKIV manual also suggests 8' . What I'm most worried about is that I'd chrono my loads at 8', and get a major power factor, and then go to a match and have to chrono at 12', and come up minor. I don't know how much velocity would be lost over the extra 4' interval, but I imagine it could be an issue, especially for those really fast, really light bullets some people shoot. I would suspect that most chronographs are similar enough that you could make a standard...say, the longest distance recommended by a common manufacturer as the shortest distance to the chrono (Pact says 8, CED says 9, and Oehler says 10; we say 10, for e.g.). If "power factor" isn't measured the same way, at all matches, all the time, then it doesn't make sense to set a single power factor floor. One other solution would be to have some "calibration ammo" of "known" power factor. Shoot it from whatever distance you have set up at the match, and the resulting measurement sets the power factor floor for that match. I think it would be easier just to say "center the skyscreens around a point 10' away". DD Link to comment Share on other sites More sharing options...
benos Posted January 31, 2003 Share Posted January 31, 2003 For my own consistency's sake, I've always placed the screens so the first one is ten feet from the muzzle. Can we put that in the rules Vince? Because, you know, as far as rules are concerned, it's best to eliminate variables. be Link to comment Share on other sites More sharing options...
L9X25 Posted February 1, 2003 Share Posted February 1, 2003 Just for kicks I checked the reloading tables to see how much velocity is lost on different bullets. A 180gr .40cal that leaves the muzzle at 900fps loses 21fps in the first 25yds. While the deceleration is not linear it is close enough for our purposes. It could lose as much as 1 fps per yard for the first few yards. A 115gr FMJ 38super leaving the muzzle at 1450fps loses 100fps in the first 25 yards or ~ 4-5 fps/yd for the first few yards. I do not think that anybody is intentionally loading that close to the limits. Leo Leo Link to comment Share on other sites More sharing options...
vluc Posted February 1, 2003 Share Posted February 1, 2003 Interesting in that when I chrono'd a load at 15 feet with my Shooting Chrony, it came out at about 130 PF, yet when that same load was done at two different competitions, distance about 6-8 feet or so with a differenct style chronograph in a box, it came out at 138PF. Link to comment Share on other sites More sharing options...
Vince Pinto Posted February 1, 2003 Share Posted February 1, 2003 BE, If the majority of chrono manufacturers recommend between 8 to 10 feet, then it's a no brainer to state a maximum of 10 feet in the rules, and I'd be a happy little camper having another definitive measurement. This way, I get to deal with questions like "Why not 9 feet?" Link to comment Share on other sites More sharing options...
L9X25 Posted February 1, 2003 Share Posted February 1, 2003 VULC, I would guess that the disparity in your chrono readings may have more to do with temperature, elevation, chrono to chrono variation and bullet to bullet variation than the distance to the chrono itself. I doubt that a few extra feel would affect the reading nearly that much. Leo Link to comment Share on other sites More sharing options...
shred Posted February 2, 2003 Share Posted February 2, 2003 Ahh.. Vince, this is what the evil metric system was designed for. Call it "Three Meters". When people say "Why three?", you can say "because two was too close and four was too far away" Link to comment Share on other sites More sharing options...
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