Nimitz Posted May 3, 2013 Share Posted May 3, 2013 Was working up a new 9mm minor load with N320 and the limited chrono data I shot showed something unusual ... I was using 124g Extreme plated bullets w/mixed range brass & Fed 100 primers. It was an overcast morning at about 75 degs The load was 4.2g of N320 (hand measured for each rd) with an OAL of 1.152. I only fired 6 rds but each rd was faster than the previous rd: In order, the velocities were: 940 1040 1053 1059 1077 1092 Anyone have an idea of what is going on? Link to comment Share on other sites More sharing options...
alecmc Posted May 3, 2013 Share Posted May 3, 2013 (edited) Weird... As the barrel warms rounds slide out faster? Loose crimp? As the new round seats the bullet head pushes inward a little bit? Rack a few rounds a few times and measure their OAL before and after. Edited May 3, 2013 by alecmc Link to comment Share on other sites More sharing options...
thermobollocks Posted May 3, 2013 Share Posted May 3, 2013 (edited) Firing a gun brings it up from room temperature to twice room temperature very quickly. Greater temperature makes powder burn faster. Some powders will show greater variation in temperature than others. Varget and IMR 8208, for example, have very little temperature sensitivity, while many handgun powders can show 20 to 50 fps of change between summer and winter. Couldn't tell you why the first shot is so low -- fouling? Edited May 3, 2013 by thermobollocks Link to comment Share on other sites More sharing options...
kevin c Posted May 3, 2013 Share Posted May 3, 2013 (edited) I see that pretty routinely, though the effect seems to be bigger from the first to the next round that between subsequent rounds. I've gotten two explanations for the effect. The first is that the heating up of the barrel is changing the amount of friction as subsequent rounds pass through. The other is that the rounds are heating up and the powder is temperature sensitive. I suppose it can also be both together. Alecmc's proposal may have merit too. It matters mainly chrono at major matches, where you may need to build in an extra PF cushion to compensate for the first round being slower. eta, after reading more closely: the difference in the first and subsequent velocities is a lot more that I usually see. Edited May 3, 2013 by kevin c Link to comment Share on other sites More sharing options...
jlamphere Posted May 3, 2013 Share Posted May 3, 2013 That is weird. I ran the stats on it and they are way outside what I have experienced with N320. My Std Dev is usually around 11 and this was at almost 54. You really need about 25 to have a accurate Std Dev, but this is still way outside the norm. Interested to see what you find out on the OAL. If that is shifting, that could explain it. Link to comment Share on other sites More sharing options...
Nimitz Posted May 3, 2013 Author Share Posted May 3, 2013 I'll have to check that to see on my next batch and see if the pattern repeats. I was a little surprized between the 1st & 2nd rds as well ... I guess it's possible I didn't have 4.2 g in the 1st rd? I'll make a larger batch as well and see if the velocity stabilizes at some point ... Link to comment Share on other sites More sharing options...
RH45 Posted May 3, 2013 Share Posted May 3, 2013 Maybe dump the mixed brass. Try once fired, all of the same batch and see what you get. You would be surprised at the difference brass makes. Link to comment Share on other sites More sharing options...
Nimitz Posted May 3, 2013 Author Share Posted May 3, 2013 That might be an interesting academic exercise but since I have over 25,000 pieces of 9mm brass and get free brass at every local match I shoot I don't plan on ever buying brass so I need to make this work with what I have .... If this is repeatable, as long as my first rd fired is about around 1000 fps I would never have to worry about the chrono stage ... Link to comment Share on other sites More sharing options...
Neomet Posted May 3, 2013 Share Posted May 3, 2013 (edited) Measure the rounds before you put them in a mag. Shoot a couple them measure them again. See if they are getting shorter. Sounds like setback. 320 is not particularly temp sensitive. Edited May 3, 2013 by Neomet Link to comment Share on other sites More sharing options...
Steve RA Posted May 3, 2013 Share Posted May 3, 2013 With that much brass, just sort out 25 or so of same brand and try those. There is a considerable difference in brass. In fact, I'd recommend that you sort it when you load, or when TV is boring, and try to load all one brand at a time. Link to comment Share on other sites More sharing options...
Justsomeguy Posted May 4, 2013 Share Posted May 4, 2013 Generally, when I test a load I run 20 shots over the chrono. If you throw out the first one you shot, your ES was not too bad. Load 'em up again and try a larger sample and see what you get. It could be you just got one underloaded round or a weak primer in that one case. Link to comment Share on other sites More sharing options...
Croomrider Posted May 4, 2013 Share Posted May 4, 2013 Was the barrel clean/oiled when you fired the first shot? Link to comment Share on other sites More sharing options...
Nimitz Posted May 5, 2013 Author Share Posted May 5, 2013 yes, gun was clean. I'll be doing a larger sample next test. Could have been a quirk since I only fired 6 rds but it looked funny that every round was faster than the one before it. We'll see what happens with 20 rds & I'll check for setback as well ... I might be temped to sort some brass for use at level II & III matches if that turns out to be the culprit since I only shoot a couple of those a year ... Link to comment Share on other sites More sharing options...
Croomrider Posted May 5, 2013 Share Posted May 5, 2013 If the barrel is clean and oiled when you start a chrono session, I would burn a few shots through it before you start recording velocities. That can definitely have an effect. Chances are you will not go to chrono at a big match with a completely clean and oiled barrel. Mike Link to comment Share on other sites More sharing options...
Nimitz Posted May 5, 2013 Author Share Posted May 5, 2013 true, but in order to simulate match conditions I need to see what happens when I fire only 6 rounds from a cold gun ... after I get the load stable I'll then randomly take a few rds at the start of a training session & fire them through the chrono to make sure I have enough margin to account for slight variations in charge weight. I use WST for training and it meters very consistently, is that also true of N320? Link to comment Share on other sites More sharing options...
jlamphere Posted May 6, 2013 Share Posted May 6, 2013 Yes, N320 meters very well. If I go kinda slow so as to not sling any powder out of the cases, I can get very consistent powder drops and my loads run in the single digit Std Dev and have a 15-20 spread. Link to comment Share on other sites More sharing options...
Nimitz Posted May 7, 2013 Author Share Posted May 7, 2013 It's a JS-100XV electronic scale. When I checked it against the 100g STD weight it showed 99.92g Link to comment Share on other sites More sharing options...
Nimitz Posted May 7, 2013 Author Share Posted May 7, 2013 sorry, wrong thread reply ... Link to comment Share on other sites More sharing options...
Shadyscott999 Posted May 7, 2013 Share Posted May 7, 2013 Or... maybe your chrono sucks. I had a chrony F1 that showed crazy variations. Got a better one and the STD DEV when down dramatically on the same batch of ammo. Link to comment Share on other sites More sharing options...
Nimitz Posted May 7, 2013 Author Share Posted May 7, 2013 It's a CE ProChrono so that's probably not the issue .... Link to comment Share on other sites More sharing options...
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