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Posted: 10/3/2008 7:19:01 PM EDT
| i use AA2230 for all my bullets for 55 grain FMJBT (hornady) 25.5 grains of powder COL 2.30 62 grainFMJBT (SS109) 24.5 grains of powder COL 2.55 on monday i chronographed 10 of each 55 gr avg 3051 fps 62 gr avg 2810 well within specs sunday i tested the same batch and the averages jumped up 110 fps faster . now it was a whole lot cooler on sunday . but im at a loss im open 2 suggestions. and yes the chrono was set at the standard 15 feet away . |
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As long as EVERYTHING was the same, the culprit could be Density Altitude. That is the Pressure altitude corrected for temperature. It is very important for pilots to keep track of the DA(density altitude) because it is how engine performance and take off distance is calculated. For bullits, it works the same. Since you have the exact same amount of energy each time the gun fires, the bullet travels and pushes itself through the air. The pressure in the air, can mean it is easy or harder for the bullet to push. The temperature also makes a diff. Cause if it is warmer or cooler, the air molecules are closer or further apart. The Density Altitude takes the temp into consideration so as to give a specific to location measurement that is read as a standard. Since almost everywhere has a different real altitude (compared with sea level) and the pressure and temp can vary from one place to another, this puts several variables into one reading. Keep in mind that the changes work for or against you in a percent type form. If you have a bullet traveling at 500fps, then the change happens the increase or decrease will be less. To kindof round it, at 3000fps you have a 100fps change. So a bullet that traveled 1000fps would have a change of about 33fps. A 500fps would change about 15 or 16fps. But, at 6000fps the diff would be 200fps. If the ammo is consistant and groups well, then there is a good chance that is the reason. However, if you are getting larger fps avg, then there would more than likely an ammo problem. If that is the case, then old powder, or powder measurements that are off a tad could be it, it could even be how the primers are seated. (if some are at diff levels that is) Hope this helps at least a little. lol Just keep it simple, hot and cold, and pressure and density all can change FPS. |
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Density altitude is a consideration down range. Across a chronograph 15 feet away, the ambient temperature effect on chamber pressure is dominant. We really don't have enough information to sort through the problem. As near as I make out from the post, the speed was higher on the cooler day. We need to know the temperature of each day, too. If you used a Chrony brand instrument, pay close attention that the body is fully open. If it's even slightly bent, meaurements will be high. Finally, the change could be caused by your components and loading practices. 100 fps extreme spreads might be normal for your ammunition. |
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