Help me Calibrate my Bal.Calc. to actual Drop

[MMH]

I shot my AR out to 100o yards yesterday and my ballistic prediction (JBM) was off. Going thru the I was going to correct the velocity to 700 yards and then the BC to 1000. I am shooting a Hornady 75 gr. BTHP that has a published G1 of .395 @ 2800 fps (per labradar). To get the JBM drops to correlate reasonably close to actual I had to change the velocity to 3100 & drop the BC to .28. Does this seem reasonable or what else should I look in?

For comparison, I did the same for my .308 & the drops correlated 100% (all within .1 mil) to the JBM calculations using actual BC & MV - I was pretty excited about that!

 

[Near miss]

Is your scope height set correctly?

Do you get the calc to match by just changing MV or BC?
Litz library shows BC of 0.348 G1 / 0.179 G7 for 75 BTHP

 

[MMH]

Is your scope height set correctly?

Do you get the calc to match by just changing MV or BC?
Litz library shows BC of 0.348 G1 / 0.179 G7 for 75 BTHP

Yes, the scope height is correct. Also, the altitude, temp, baro.pres. & RH was input for actual conditions that day.

That is why I am unsure about what is going on. Both MV & BC are significantly off to get the drops to line up.

 

[Near miss]

Try the .179 G7 with different speeds.
Are you sure there was no mess up while dialing?
Could it be the scope?

 

[Bmghunter]

Your bullet is hitting the transonic zone around 700 yards. Maybe thats causing the calculations to go off when the bullet maybe destabilizes. Have you tried truing the bc or velocity using numbers only inside of 700 yards?

 

[Bmghunter]

Also, is your scope tracking perfect? If the tracking is off, you may be adjusting more/less moa/mils than you think you are.

 

[MMH]

Do not think that I made mistakes in dialing the scope as I would write down the values at 300, 400, 500, 575, 700 & 1000 yards.

Did not think that the scope may not be tracking properly, I guess that would make sense tp check next. Easiest way would be to switch scopes between the .308 & 5.56, as the 308 predicted/tracked perfectly & see if now the .308 prediction is off & the 5.56 is on. Do that or just a traditional tall target I guess.

If it is the scope, the error would be huge - approx 10% (for example, dialed dope 2.6 for a calculated was 3; dialed dope of 5.1 for a calculated 5.7). The scope is a 4-16 FFP Viper PST GenI 1. So although not super expensive, not a cheep scope either, but anything is possible.

 

[Near miss]

Try using some other calc also.

And post your dope here

 

[MMH]

Try using some other calc also.

And post your dope here

I am shooting Hornady 75 gr. BTHP w/ a published G1 of .395 & cronographed speed of 2800 fps. The first column is actual dope. The second two are ballistic calcs from JBM & Strelok @ MV=2800 & BC=.395. The fourth colum is JBM w/ a MV of 3150 & BC of .28. The last column is strelok w/ a MV of 3175 & BC of .31. I changed the MV & BC to get the prediction to get as close to actual as I could. Needed to shoot more at 800 & 900 yds, but the day was getting long.

Range	Actual
(yd)	(mil)	JBM	Strelok	JBM	Strelok
0
100	vel	2800	2800	3150	3175
	bc	0.395	0.395	0.280	0.310
200
300	-1.1	-1.0	-1.2	-0.8	-0.9
400	-1.8	-1.9	-2.1	-1.6	-1.7
500	2.6	-2.8	-3.1	-2.5	-2.6
575	3.3	-3.7	-4.0	-3.4	-3.4
600		-4.0	-4.4	-3.7	-3.7
700	5.1	-5.3	-5.8	-5.1	-5.1
800		-6.8	-7.4	-6.9	-6.7
900		-8.5	-9.3	-9.0	-8.7
1000	11.7	-10.4	-11.6	-11.6	-11.2

 

[Bmghunter]

I know you wrote MV to be 2800 recorded, but your table confused me a bit, is that 2800fps at 100 yards or actually at the muzzle? What's your chronograph?

 

[Bmghunter]

Also strelok pro has a multiple bc input. If your chrono and scope is good, then try multi bc. This will allow you to true it more accurately.

 

[MMH]

I know you wrote MV to be 2800 recorded, but your table confused me a bit, is that 2800fps at 100 yards or actually at the muzzle? What's your chronograph?

The chrono is a Labradar & the MV should be at the muzzle

 

[MMH]

Also strelok pro has a multiple bc input. If your chrono and scope is good, then try multi bc. This will allow you to true it more accurately.

I'm probably more concerned about the data at yardages below 700 not lining up very well where I thought that I should be adjusting the velocity.

 

[Bmghunter]

So your initial velocity is probably good. Once it is out of the barrel and starts slowing down, then the bc will affect how fast it slows down. So you would be adjusting bc to account for this slowdown. But, the bcs are different at different speeds, so multi bc will let you true up the bc at each velocity so that it matches your dope. By truing up bc , the velocity will adjust accordingly since the lower bcs will slow bullet more, and this the drop will then match up.

 

[6.5SH]

Happen to have a picture showing POA and POI for your 100 yard zero? Last time I saw something similar to this the zero was ~0.3 mil off

Also what is the scope height?

 

[MMH]

Happen to have a picture showing POA and POI for your 100 yard zero? Last time I saw something similar to this the zero was ~0.3 mil off

Also what is the scope height?

Yea, I'm wondering about that. Zeroed the rifle @ 100 yds. was off initially by 2" (high & to left). Re-zeroed & could not see bullet impact (was looking through scope) and assumed that group was in solid black square. Should not have been lazy & went down to check. At the same time, I would expect this to be an error that is consistently off and it seems to me to get greater w/ distance. It will be easy enough for me to double check zero at local range as scope has not been off of rifle. Hopefully I can do that in next day or two.

Scope height is 2.75 (shooting an AR).

 

[Bmghunter]

I been playing with the numbers on strelok with your settings and the only way to match the bcs up with your drops is that as the bullet hits transonic zone, it tumbles and the bc drops like a rock.
I put a bc of .24 at 2800fps, .22 at 2000, .19 at 1500, and .04 at 1250fps, gives a close approximation of your numbers. All G7 bcs.
Notice how the first 3 velocities are in the ballpark of published bc. Then as it gets to the transonic zone, the bc drops like a rock to match your drops. Do you have paper at those long distances to see if the bullet is tumbling? I think that will likely answer the question.

 

[Near miss]

Did you shoot steel or paper?
What is your twist? Have you confirmed it with a cleaning rod?
How many shots per distance did you shoot?

 

[MMH]

I been playing with the numbers on strelok with your settings and the only way to match the bcs up with your drops is that as the bullet hits transonic zone, it tumbles and the bc drops like a rock.
I put a bc of .24 at 2800fps, .22 at 2000, .19 at 1500, and .04 at 1250fps, gives a close approximation of your numbers. All G7 bcs.
Notice how the first 3 velocities are in the ballpark of published bc. Then as it gets to the transonic zone, the bc drops like a rock to match your drops. Do you have paper at those long distances to see if the bullet is tumbling? I think that will likely answer the question.

Good point. I did not even consider the bullet going transonic which it probably does between 900 to 1000 yards. That may explain the BC at long range, still confused why shorter ranges being so off. As 6.5SH said - I need to recheck zero.

 

[MMH]

Did you shoot steel or paper?
What is your twist? Have you confirmed it with a cleaning rod?
How many shots per distance did you shoot?

I was shooting steel, 3 to 5 shots for each range. The twist is 1/7, 4 groove Bartlein, but can't say that I verified it.

 

[socchia16]

I shot my AR out to 100o yards yesterday and my ballistic prediction (JBM) was off. Going thru the I was going to correct the velocity to 700 yards and then the BC to 1000. I am shooting a Hornady 75 gr. BTHP that has a published G1 of .395 @ 2800 fps (per labradar). To get the JBM drops to correlate reasonably close to actual I had to change the velocity to 3100 & drop the BC to .28. Does this seem reasonable or what else should I look in?

For comparison, I did the same for my .308 & the drops correlated 100% (all within .1 mil) to the JBM calculations using actual BC & MV - I was pretty excited about that!

I would use a closer range, like 400m to verify velocity. Then tru bc at 700m. your trying to make calculations off a .556 that is burning out.

 

[Near miss]

I was shooting steel, 3 to 5 shots for each range. The twist is 1/7, 4 groove Bartlein, but can't say that I verified it.

In my opinion, on steel with those shot counts, I'd say it is nowhere near enough to make 0.1mil calls.

You need to bracket the hit margin and work your calc dope with that.

When I make dope, I shoot paper and keep on shooting until I have 3 to 5 shots within moa. Even that is not accurate down to 0.1mil but very close

 

[MMH]

I would use a closer range, like 400m to verify velocity. Then tru bc at 700m. your trying to make calculations off a .556 that is burning out.

In my opinion, on steel with those shot counts, I'd say it is nowhere near enough to make 0.1mil calls.

You need to bracket the hit margin and work your calc dope with that.

When I make dope, I shoot paper and keep on shooting until I have 3 to 5 shots within moa. Even that is not accurate down to 0.1mil but very close

Agree with both of you. Will shoot paper out to 500 yards or maybe even take it out to 700. Use that to determine where I am really hitting. Then start worrying about truing. Unfortunately, it will be a pain to hang paper out there...