Ballistics for Excel Forum

BfX => Ballistics => Topic started by: admin on August 25, 2011, 10:45:03 PM

Title: g7 vs lapua
Post by: admin on August 25, 2011, 10:45:03 PM
Well I played a bit around with BfX_CD and the new abilities of the user supplied drag functions. It was a 1 minute activity to plot the measured drag function of the Lapua 6mm 105 gr scenar with g7 in a graph.

One might say that the shapes differ - rather much. The absolute magnitude is not very important as the ballistic coeficient is used to fix the agreement with data in the case of g7. It is a pitty that we do not know how Lapua did the measurements and how accurate they are. The discussions about the best drag function for long range shooting seam however rather pointless.

Title: Re: g7 vs lapua
Post by: admin on September 06, 2011, 12:13:51 AM
Kept thinking about the different shapes of the drag function. The max horizontal range of the 105gr 6mm scenar is 3990 m with the radar measurement, whilst using a g7 function with the Litz bc of .248 it would fly 4722m - both distances computed with quicktarget!

this is not what I did expect. I would expect that with the radar data the bullet would fly a longer distance, as the drag of bullets flying below the speed of sound is much lower as with the g7.
Title: Re: g7 vs lapua
Post by: mman on September 16, 2011, 09:29:48 AM
I don't see anything odd here. Subsonic drag coefficient is higher with 105gr. scenar radar data than with G7 data (see QTU lapua). This directly leads longer max horizontal range. This actually makes perfect sense. G7 data assumes that projectile is all stable and drag is low. Radar data shows that scenar will experience dynamic instability which increases drag. Subsonic velocity range and dynamic instability are not well behaved so you can't say that radar data is true for all cases.

I'm waiting for your 6 DOF calculator which might give us some answers regarding these and some other issues....

By the way I got some information of how lapua measured those atmospheric conditions. They used weather balloons. Temp, pressure, and humidity are pretty constant along the trajectory if they are know on every height. Wind is wild card but that does not do very much of error. For me this sound pretty good. Not exact but better than with litz's masurements. There is no problematic accoustic sensors or untrusty chronograps. Error boundaries would have been something that would have made lapua's measurements more beliavable. For my opinion Litz's +/- 1 % error is pretty optimistic estimate. It has been shown that in many case this haven't been true. BCs derived from lapuas radar measurements and litz's bcs also differ many cases more than 1 %.
Title: Re: g7 vs lapua
Post by: admin on September 17, 2011, 09:05:31 AM
This is indeed a very good setup of Lapua! Good news.