I just got came back from the range yesterday and was suprised to see my BC I got using muzzle/100 yard velocities. My rifle is a Model 70 .270WSM with about 200 rounds through it. The bullet used is a Nosler Accubond 140 grain with a published BC of .496. While at the range I recorded muzzle velocity and 100 yard velocity with three round as follows.
1# 3250 and 2962
2# 3253 and 2962
3# 3275 and 2982
I used these velocities and came up with a .313 BC on JBM calculations. I was suprised at the large difference in the BC which will change my bullet drop about 22 inches at 700 yards. I called Nosler and they seemed suprised and stated that the bullet might be having a difficult time stabilizing out of the muzzle, causing the BC to drop. I am interested on your thoughts. Thanks, Huntingbigsky
There many things that will effect a bullets bc,your barrels lands and groves, the condition of your barrels throat, etc. Shoot at longer ranges and confirm your actual bc. Ken Olher siad that the distance between chronographs is when calculateing bc is only good for about double the range between the chronographs........ [img]/ubbthreads/images/graemlins/smile.gif[/img]
range it,check the wind, dial in correction, aim and only one shot
Welcome to the wonderful world of ballistics, ha ha. Yah I've had pretty much the same thing happen from my 7 RM and 140g NBT's. I was 3-4 MOA low at 800 yards. I had it crono'ed at 3250fps, entered .485 BC, and came up with however many MOA"s at 800. Well I was really low. I figured 1 of 3 things from help on this board. Either my MV was not 3250fps as the crony stated, (probly closer to 3100) the BC was not .485, actually closer to .350, or my scope was not giving true 1/4" MOA clicks. Well I worked everything out, backwards, yadda yadda, I just figure that you write down how many actual clicks it takes and shoot and be happy. Otherwise you'll find yourself frustrated and trying to figure things out that really cant be explained. Computer ballistics are only a guideline, I now know that for sure.
Oh yah, forgot to mention Nosler is out of Bend which is definately not sea level. Its considered high desert. Not to sure exactly what the elevation is, but I'm betting its close to somewheres around 3,000.
Are you sure both chronos match? Are they really accurate?
Have you swapped them?
A .496bc would have dropped from 3250 to 3047fps. Let's say your chronos are good within typical 5% accuracy. Here, that's ~160fps worse case right?
Or looking at it another way 1-3047/2962 = 2.9% error
I could see this coming into play. My Oehler w/20' screen spacing still has ~1-2% possible error.
Altitude is 4000 ft. The two chronographs I used were the Shooting Chrony Beta and the Competition Electronics ProChrono. I did not swap them but I will the next time I go to the range. I will also extend the 2nd chrony to 200 yards. My friends chrony is the one at 200 yards.... or maybe I should shoot 400 yards.[img]/ubbthreads/images/graemlins/grin.gif[/img] Have you guys also experienced a considerably lower BC than what was listed? Thanks for the help.