nhglyn Posted May 19, 2006 Share Posted May 19, 2006 Ok, so I have been using 4756 for my open guns for quite a while now. Always used 9.0 grs with a 125 HAP for my Brazos Sx shorty. In February (here in NH, temp 35) I chronoed at average of 1390 fps through a CED chrono. Just chronoed a new batch toay (temp 60) and got an average of 1314 fps. A drop of 76 fps with the same # grs of powder? What the heck is going on here? I called IMR/Hodgdon and asked one of their "experts" if this particular powder is reverse temperature sensitive, temp goes up=fps goes down. He swore to me that it wasn't and they had never experienced this. So, how does the same # grs cause a decrease of 76 fps? Could it vary this much from lot to lot? What else could cause this. Nothing else has changed. Thanks for the help. Link to comment Share on other sites More sharing options...
9x21 Posted May 19, 2006 Share Posted May 19, 2006 What were your crony conditions? inside, outside? No direct or indirect sunlight? no florescent lights? Shot thru a box with controlled lighting to crony specs? Link to comment Share on other sites More sharing options...
nhglyn Posted May 19, 2006 Author Share Posted May 19, 2006 Sorry, should have mentioned. Both sets shot outdoors but with infrared lighting screens on the CED. Link to comment Share on other sites More sharing options...
9x21 Posted May 19, 2006 Share Posted May 19, 2006 I'm not familar with that model, check instuctions for outside use. Most cronys I've used don't like outside light, gives crazy readings, hence you always see light boxes at big matches. I used 4756 years ago, don't recall having had temp problems. Put a couple in the freezer & crony'em see if come back up. 9x21 Link to comment Share on other sites More sharing options...
al503 Posted May 19, 2006 Share Posted May 19, 2006 4756 is reverse temp sensitive. The colder it is, the hotter the load is. Link to comment Share on other sites More sharing options...
ipscbob Posted May 24, 2006 Share Posted May 24, 2006 Both of the popular IMR powders, 4756 and 7625, show huge lot to lot variability. I stopped using 7625 for 38super because of this. After experiencing this myself with different lots of 7625, I did a search here for loads vs. fps and found a very large variability here significantly greater than other powders. I suspect that the burn rate characteristic of these powders in the upper pressure ranges is very steep, which would explain this behavior. Link to comment Share on other sites More sharing options...
XRe Posted May 24, 2006 Share Posted May 24, 2006 The solution to lot-to-lot variations in any manufacturer's powder is to buy in bulk. The IMR powders are cheap enough to easily buy 16 pounds at a time (13K-14K rounds worth - about $200, total). Or, be dilligent about your chronoing.... My understanding (from reading things on this forum) is that these powders are engineered as shotshell powders, and as such are engineered to maintain consistent energy per volume, not consistent energy per weight... Shotshells are apparently loaded using a bushing, usually, and not strictly by charge weight?? So, they're not engineered to be consistent when you measure them by weight If this is true - and I don't know if it is, for sure, or not, though it makes sense... - it would seem to imply that you could just leave the charge bar set to where it is (ie, same volume of powder) and arrive at the same "charge energy" and same (or similar) PF load, regardless of the weight change between the two lots for the same charge volume. I don't know if this is the case or not, having never tested it - I don't have sigificantly different lots of either of those powders to experiment with to give it a whirl. It would make for an interesting experiment to find several lots of one of the two, and then develop loads that make the same velocity, and compare charge volumes (easiest way would be w/ a UniqueTek micromenter powder bar - very slick device). If this is the same lot (or even same can) of powder - has it been sitting in the powder measure for a while, or something?? Or not totally sealed in the can??? Link to comment Share on other sites More sharing options...
ipscbob Posted May 25, 2006 Share Posted May 25, 2006 The solution to lot-to-lot variations in any manufacturer's powder is to buy in bulk. The IMR powders are cheap enough to easily buy 16 pounds at a time (13K-14K rounds worth - about $200, total). Or, be dilligent about your chronoing.... Yup, I usually buy at least that much at a time. Unfortunately there is enough variability in 7625 that I have seen batches that will just barely make major with a full case and others that make it comfortably at 80% full. Not enough margin for my liking and definitely not worth the hassle to me. My understanding (from reading things on this forum) is that these powders are engineered as shotshell powders, and as such are engineered to maintain consistent energy per volume, not consistent energy per weight... Shotshells are apparently loaded using a bushing, usually, and not strictly by charge weight?? So, they're not engineered to be consistent when you measure them by weight Not my experience at all. Fortunately there are lots of other good powders out there for super. Link to comment Share on other sites More sharing options...
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