Whats up with my BC?

[bigsky]

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

 

[sharpshooter1]

What's your altitude? I'm not sure if nosler's are from sea level, but if theyre not, and you are lower than them, yours will be lower.

 

[jwp475]

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........ /ubbthreads/images/graemlins/smile.gif

 

[remingtonman_25_06]

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.

 

[remingtonman_25_06]

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.

 

[Mikecr]

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.

 

[bigsky]

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./ubbthreads/images/graemlins/grin.gif Have you guys also experienced a considerably lower BC than what was listed? Thanks for the help.
Huntingbigsky

 

[jwp475]

Yes, and sometimes it works out higher than listed

 

[Jeff In TX]

Huntingbigsky,

Lots of variables here to screw your readings up, so you need to eliminate some variables. The issue is more than likely with the two different chronographs and possibly the distant measurement between the two chronographs. I utilize two chronographs for calculating the exact BC's of my bullets. I use two CED Millenniums chronographs which when put back to back at 7 feet from the muzzle have an average 3 fps difference in velocities, which is very accurate. I also use a distance measuring wheel for measuring distances.

Let's say for the sake of arguments your shooting chrony is working fine and your Competition Electronics ProChrono is working fine. Being two different brands of chronographs each company more than likely calculates the bullet velocities differently. More than likely each chronograph is giving different velocities readings which are screwing up your results. BC's do change for a bunch of different reasons, but the change is usually slight and no more than a few % points. Your BC is not going to change by 100+ points. It can't it all comes down to math and physics.

Try using only one chronograph (I'd recommend the Competition Electronics ProChrono) and set it up 7 to 10 feet from the muzzle and shoot a string of 10 rounds to get a consistent average muzzle velocity. Measure accurately to a 100 yards (not with a laser range finder) and move the same chronograph out to the 100 yard line and shoot another 10 round string to get a consistent average 100 yard velocity. Now recalculate your BC, which should be more in line with the published BC.

Hope it helps

 

[bigsky]

Thanks for the advice.... I will keep you posted on what happens after going to the range.

 

[bitterroot bulls]

Your friend must be some kind of gentleman ... allowing you to shoot at his chrony like that. /ubbthreads/images/graemlins/laugh.gif

 

[Jeff In TX]

IMHO, If you can't shoot through a chronograph at 100 or 200 yards you have no purpose being on the range.

Screwy how people can shoot moa or sub-moa at 100 or 200 or 300 yards and feel they can't shoot through the opening of sky screens of a chronograph at those distances, especially at 100 yards.

Go figure!

 

[jwp475]

Most can hit a golf ball sized target (or smaller) at 300 yards put a chrono at 300 and some will hit that too............ /ubbthreads/images/graemlins/confused.gif

 

[Jeff In TX]

JWP475,

Here in Vegas for the week at the convention center for the NAB show. Other than the 50 million sites to see on the strip are there any others you can recommend that are off the beat'n path?

Thanks