G-ManBart Posted October 2, 2009 Share Posted October 2, 2009 We (myself and some of my friends) have been testing ammo and found a source of erratic readings comes from illumination problems with the sky screens. So much so in fact, that in some cases putting the entire chrono in a sonnetube (big pasteboard tube for casting concrete) and supplying an artificial light source was the only way to get the chrono readings to settle down. Muzzle blast was indeed an issue but the light turned out to be very important. Lighting is absolutely a huge issue if you want any kind of consistency. I've been planning on building a chrono box, but man, the sonnetube is an excellent idea! I might just go that route and save myself some trouble....thanks for sharing. R, Link to comment Share on other sites More sharing options...
Braxton1 Posted October 3, 2009 Share Posted October 3, 2009 I wouldn't think that 1818 fps would be the speed of the muzzle blast. Wouldn't the muzzle blast, upon exiting the barrel, immediately slow down to the speed of sound (roughly 1125 fps at sea level)? (Enough science content). I would think that this is a skyscreen spacing issue, since there seems to be some consistency to the error. Most chronos have an adjustable spacing setting so that you can use screens spaced 2 feet, 4 feet, 6 feet apart. If this is inadverently set wrong, it will cause a gross error. I had a friend set his spacing wrong. When I shot some of his "Major" ammo that wouldn't cycle his gun, I told him "This stuff is way too light". He swore on his chrono results, so I ran it over mine. 122 Power Factor for "Major" 40. He had set the spacing wrong and had tuned the load to a very erroneous velocity. Link to comment Share on other sites More sharing options...
G-ManBart Posted October 3, 2009 Share Posted October 3, 2009 I wouldn't think that 1818 fps would be the speed of the muzzle blast. Wouldn't the muzzle blast, upon exiting the barrel, immediately slow down to the speed of sound (roughly 1125 fps at sea level)? (Enough science content).I would think that this is a skyscreen spacing issue, since there seems to be some consistency to the error. Most chronos have an adjustable spacing setting so that you can use screens spaced 2 feet, 4 feet, 6 feet apart. If this is inadverently set wrong, it will cause a gross error. I had a friend set his spacing wrong. When I shot some of his "Major" ammo that wouldn't cycle his gun, I told him "This stuff is way too light". He swore on his chrono results, so I ran it over mine. 122 Power Factor for "Major" 40. He had set the spacing wrong and had tuned the load to a very erroneous velocity. It certainly could be a spacing issue, but if it was set for 4ft and the screens were at 2ft the velocity would read twice what it really is, not just (ballpark) 400fps more than it is. I'm guessing it's not the muzzle blast itself, but the unburned powder and junk that exits the bore ahead of the bullet....it's light and getting squeezed through a small space, so I could see it going 1800fps for enough distance to trick the sensors. R, Link to comment Share on other sites More sharing options...
1911jerry Posted October 3, 2009 Share Posted October 3, 2009 Just a quick note to add that the screens must be also be LEVEL. If one end is higher or lower, the results will not be accurate. Just something else to check for. Jerry Snyder Link to comment Share on other sites More sharing options...
capdek Posted October 5, 2009 Author Share Posted October 5, 2009 (edited) Well, as everyone expected, the problem was with my distance from the chrono. I went out to the range today and set the chrono up about 20 feet away. I loaded up a couple of 7.5 grain loads (just in case) and got readings of 1160 and 1164 fps. Then I tried the 8.6 grain rounds and got 1282 and 1276 fps. Went up from there in .2 grain increments until I hit 9.2 grains with readings of 1337, 1336, 1343, 1355 and 1340 fps which averages to 1342.2 fps - perfect! So using a 125 grain bullet, I should be good to go at 9.2 to maybe 9.3 grains of the 3N38. And let me tell you, this is a great powder! The gun barely moves and it even cleans out my comp. Thanks to all of you who posted for helping me work through this. I owe you all a big debt of gratitude! DEK Edited October 5, 2009 by capdek Link to comment Share on other sites More sharing options...
Patrick Sweeney Posted October 5, 2009 Share Posted October 5, 2009 Problem solved. Science content; the escaping gases behind a bullet, as it exits the muzzle, are assumed for the purposes of calculating recoil, to be going 4,000 fps. So 1818 fps would be the average speed "read" by the chrono, as the shock front decelerates through air, until it dissipates. You can also trick the chrono into reading 'way low. If the screens are close enough that the front one is triggered by the shock wave, but the back one is too far away, you get the chrono started by the muzzle blast, but stopped by the bullet. That'll give you readings of 400 fps or so. Link to comment Share on other sites More sharing options...
Jaxshooter Posted October 5, 2009 Share Posted October 5, 2009 (edited) It must be the chrono. I load 10.1 gr of 3n38 with a 125 gr JHP at 1.250 OAL in my SV IMM 38 SC . The average for 10 shots is 1368 FPS. With a PF of 171.5 with no signs of pressure. Edited October 5, 2009 by Jaxshooter Link to comment Share on other sites More sharing options...
Whitefish Posted October 6, 2009 Share Posted October 6, 2009 9.3 gr 3n38 with a 125 gr Zero FMJ, Starline 38 Supercomp brass, Winchester small rifle primer, at 1.245" OAL yielded average 1365 fps out of my 4 port comp, 5 inch Nowlin barrel open gun on the chrono at recent section and Area 1 matches. That is about par for other shooters I know that use 3N38. Link to comment Share on other sites More sharing options...
G-ManBart Posted October 6, 2009 Share Posted October 6, 2009 (edited) I hit 9.2 grains with readings of 1337, 1336, 1343, 1355 and 1340 fps which averages to 1342.2 fps - perfect! Not really if you're going to go to any matches where they chrono. I've repeated this many, many times in the last year, but it's absolutely true. I loaded up a batch of ammo for the 2008 Nationals that was always right in the 173-175PF range. I shot this same batch of ammo at two major matches where it went 173 and 174PF as I recall (have it written down somewhere). It was loaded with the same lot of cases, same lot of powder, same lot of bullets and same lot of primers. At the Nationals it went 166.1PF with a slightly heavy bullet I loaded up some more of this for this year's Nationals (no changes to press or gun) with everything the same (the last of the same lot of powder and bullets even) except a switch from SR to SPM primers and it went 173PF at this year's Nationals. When I chrono'd it here before leaving it was at it's most typical 173-175PF range shooting multiple 20-shot strings. This is actually my normal load but for big matches I use new brass (slightly higher velocity with new). Unless you're extremely lucky 167PF is definitely going to jump up and bite you. 170+ is a pretty good rule to follow and it needs to be under varying conditions (temps primarily) for it to really mean much. If I was relying on a 170PF load last year I would have gone Minor and that would have really hurt. You will have no problem at all getting into the 170+PF with 3N38...it's slow enough you're not going to hurt anything, especially with a 125gr bullet. R< Edited October 6, 2009 by G-ManBart Link to comment Share on other sites More sharing options...
larry cazes Posted October 6, 2009 Share Posted October 6, 2009 I agree with Bart. I load to 173pf minimum and I still chrono 167 at times at majors. As I have stated many times before, I believe a significant buffer is necessary. Link to comment Share on other sites More sharing options...
matt2ace Posted October 6, 2009 Share Posted October 6, 2009 (edited) +1 Bart Three of us had the same experience at the 2008 Nationals shooting open. We all had ammo that chono'd 171-173+ through our chrono's and at recent major matches. I was the high PF 166.2, the others were 165.6, 165.4 at the Nationals. This was with new or once-fire, same headstamp brass for all of us. That experience reminded me just how important it is to allow for enough variability between chronos and range conditions when setting up a major load. Edited October 6, 2009 by matt2ace Link to comment Share on other sites More sharing options...
lcs Posted October 6, 2009 Share Posted October 6, 2009 2009 Nationals --38SC New brass, SRP, Silhouette. 165.2 I even added .10 to be on the safe side. Several Area matches and Sectionals---172 PF Link to comment Share on other sites More sharing options...
G-ManBart Posted October 7, 2009 Share Posted October 7, 2009 (edited) 2009 Nationals --38SC New brass, SRP, Silhouette. 165.2 I even added .10 to be on the safe side.Several Area matches and Sectionals---172 PF Yikes! Funny thing was this year my load was within 1PF (of what I was seeing at home) at Nationals....nothing changed but the primer. Same lot of bullets, same lot of powder, no change to the press etc. One year it's like 7-8PF low and the next it's pretty much on the money. Weird! Edited October 7, 2009 by G-ManBart Link to comment Share on other sites More sharing options...
calishootr Posted October 7, 2009 Share Posted October 7, 2009 please bow tothe wisdom of our man Bart +1 to the 170+ PF, if you flirt w/ 165ish your goingto get burned sooner or later, atthe 01 nats I was fine, golden gate champs I was fine, went low one yr atthe oregon sectionals....it all depends on how they are chrono'ing/what set-up they are using ....plug in the extra power factor if for no other reason, peace of mind.... Link to comment Share on other sites More sharing options...
capdek Posted October 7, 2009 Author Share Posted October 7, 2009 Good points on the power factor variations! I'll try 9.3 grains which should put me up around 1360 fps or a 170 PF. Thanks! DEK Link to comment Share on other sites More sharing options...
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