Shooting /Chrony question

[Silentstalker]

Im a newby at distance shooting. So, please bear with me. I have worked up a great load for my .270 Win. It shoots under 3/4" with 57 gr. H 4831 and the 150 gr. BT. I took it out and shot it through a chronograph a few months ago. It averaged 2875 fps. I was using a borrowed Master Chrony. It was set up approximately 10 feet from the muzzle. I plugged my info into the Nosler ballistic app and went shooting this past weekend. We were shooting rocks at 560, 800, and 1060 yards. At 560 yards I hit the rock easily. It was a large rock. The two further rocks were much smaller. At 800 I was about 2 feet high. I had to tweak my fps all the way up to 3025 fps to accurately reflect my true drop. The new fps/drop had me right on at 800, and 1060.

My question is this, is it normal for the chrony to be off this much or is it more likely the app just gives you a starting point and you always need to tweak each load for true drops? Or could it be something else?

 

[Oldtrader3]

I think that your listed ballistic coefficient is off? It could be your chrono but I doubt it is that much off? Also in your drop, you have to correct for the scope height, angle change in your barrel.

Having loaded for and hunted with a .270 Win for 50+ years, your 150 grain velocity sounds right on with IMR 4831?

 

[Silentstalker]

I used the BC listed by Nosler, .496. So I should most likely be adjusting the BC rather than velocity? As for the scope I measured from center of my scope to center of the barrel (is that the right way?) and I think everything else is input correctly. Thanks for the help!

 

[Ridgerunner665]

I used the BC listed by Nosler, .496. So I should most likely be adjusting the BC rather than velocity? As for the scope I measured from center of my scope to center of the barrel (is that the right way?) and I think everything else is input correctly. Thanks for the help!

That BC (.496) is way too high...I don't know the exact BC of that particular bullet...but its safe to say that the advertised is a bit optimistic.

 

[.300winmag]

PM with details of your experience and equipment and I will be more than happy to help any way I can. I've got several suspects and questions, your velocity should not be that far off. Also, if you are interested in long range shooting, I recommend highly in investing in this book.

http://store.appliedballisticsllc.com/m ... tCode=0001

 

[Silentstalker]

Thx! Pm sent

 

[muleman]

Silentstalker,
Regardless of what any program tells you; you should verify drops for every load. That data will give you the information you need to "program" your program. Chronographs will generally give you good data if set up correctly. Altitude, temp. and barometric pressure can and will effect the bullet. Your measured drops will help you calculate the actual B.C.s of the bullet. B.C.s will vary with velocity as well. Sierra posts several different B.Cs for various velocities. Finally, once you get your "new" B.C. you'll be better able to calculate wind drift. Wind drift estimating is the real art of long distance shooting. :grin: Enjoy the long range shooting.... lot to learn and a lot of fun.
Scott

 

[bobnob]

Speed sounds about right for that load.

The BC is more likely something like 0.45. Atmosphere is a factor too.

Are you dialing for drop on your scope? If so, some scopes don't track linearly when winding on drop. I am new to the LR sport too and learned this point quickly!

 

[Oldtrader3]

Plus, just to make it more confusing, BC changes as velocity goes lower.

 

[Silentstalker]

I have got a lot to learn! Thanks guys!