Verify a JBM calculation.

[pmclaine]

Loaded up some Win .308, IMR 4064 at 42.6 grains, in WCC 1X cases topped with a Nosler 168 CC fired by a Win LRP. 2.8 is the COAL. These are being shot from a LMT MWS, 16 inch chrome lined AR.

I zeroed at 100 than moved to the 300 yard line. My adjustment was +1.2 mil at 300. I have no actual chrony data.

Playing with the velocity on JBM I come up with 2800FPS at the muzzle in order to match my 1.2 mil adjustment.

This seems like a high velocity for the load and considering I find this AR to start to flatten/push out primers much above 2600.

Im prepping for the chance to shoot at a range out to 800 yards and Im looking for some come ups based on my known 300 data. Using the 2800 muzzle velocity JBM provides the following

Yards Mils

100 - 0
200 - .5
300 - 1.2
400 - 2.1
500 - 3.0
600 - 4.1
700 - 5.4
800 - 6.8
900 - 8.5
1000 - 10.3

Any help with confirming I will be on the right track for making hits on US Army spec pop up torsos?

Thank you for your time.

 

[FLIGHT762]

Did you input the proper sight height above the bore in your calculations???? This will make a difference. If you're shooting an AR-10 platform, your sight height can sometimes be between 2"-3"above the bore, depending on you mount rings.

 

[.50 Cal Cow]

Flat terrain or chance of having a little upwind?

Also how many shots did you fire. 300 yards is going to be a bit close to get a solid accurate velocity since the group dispersion in relation to bullet drop is not as great as it will be at longer range and lead to a greater error.

 

[.50 Cal Cow]

You will also want to ensure your turrets track true and if not that they are adjusted for if not.

 

[djskit]

300 is not a distance where you can make any solid conclusions on speed based on the drop. You really need a chronograph or have some solid feedback at longer distance to get the right dope.

For example, if at 800, you find the correct dope to be 8 mil, then drop the speed in JBM to match that - then you have a good idea of your speed absent a chrono. But you need one or the other.

2800 is an impossible speed for 168's out of a 16 inch barrel and 42 grains of powder. If I were to guess, I'd say 2500.

 

[pmclaine]

300 is not a distance where you can make any solid conclusions on speed based on the drop. You really need a chronograph or have some solid feedback at longer distance to get the right dope.

For example, if at 800, you find the correct dope to be 8 mil, then drop the speed in JBM to match that - then you have a good idea of your speed absent a chrono. But you need one or the other.

2800 is an impossible speed for 168's out of a 16 inch barrel and 42 grains of powder. If I were to guess, I'd say 2500.

Thank you. Agreed 2800 was not realistic to me but it is what made 1.2 mils work. Your educating me regards distance required to extrapolate data was what I needed. I think Ill be in the ball park with my come ups to a degree - maybe to 500-600 - than at some point I expect the table above will fall apart. I should have spotters with me and the targets have a pretty good splash pad in front of them. No matter what it will be fun.....