Help with trajectory please "thing"

[groper]

Hi folks,

ive been shooting out a new load from my 300WM using 208gr amax`s.

The load is 76.2gr h1000, i have yet to chronograph the velocity tho. From other reports, it should be somewhere around 2950fps i believe from a 26in bbl.

So far i have shot it at 200, 600 and 900yds.

what i have is a 200yd zero, 3.1 mils @ 600, and 5.8mils at 900. This was shot at 90deg temp, 1000ft elevation, quite low humidity, and no wind.

from this data, i cannot find a trajectory that fits, no matter how i change the BC or the velocity, and ive even tried changing the mil value etc aswell using the "thing"

i have a feeling the 200yd zero, may be closer to 218yds (200m), not sure about the distance at our range upto 300...

Can somone figure this out and if so, teach me please???

Cheers...

 

[britz]

Hi, The first thing I would do is get your load chronographed over at least 5 shots and the more the better. I did a little checking and Hodgdon's data shows that you would be getting about 2850 FPS or maybe a little more. THe thing is you can't just go by what the book says. You can mess around with a calculator and the two known distances, drops and play with the numbers to find out what your velocity is... but it's a pain. Go to JBM - Calculations - Trajectory Enter in your velocity for 2850 and the 0.648 BC and you will be very close to what your field varifications were 2950 really sounds a little high for that bullet. I'm shooting a wsm and Pushing the thing pretty hard from a 24" barrel getting 2950 from the muzzle w/ 180 grain bullets. The win mag has a little more torque but not that much.

Good luck.

Also, I would start off by shooting at 200, 600, and 900 without moving the scope and shoot 5 rd groups at each. Measure in inches what the actual drop was. This will give you a very good measurement of the true BC of your bullet, or tell you if your measured velocity is accurate.

 

[Willys46]

just a stab at it for you. if i had to guess the 900 yard target had to be at 800 yards to be even close.

When looking at your numbers the drop from 600 to 900 is 2.7 mils. and that is in line with a 600 to 800 yrd drop. 600 to 900 should be close to 4mils.

Just a guess, but it makes alot more since.

Willys46

 

[Michael Eichele]

1: If youre using a second focal plane variable power scope, I would verify that you were on the appropriate setting.

2: See if you can duplicate your reults.

3: Verify your velocity. You are likely to be less than you expect.

4: You mentioned low humidity. Humidity has very little effect on a bullet. The difference between 0% and 100% is very minute even at 1K yards. You mentioned nothing of pressure. Barometric pressure even in small changes have a very drastic effect on bullet trajectory. Find an accurate pressure. This will help you find your true numbers.

5: Have you tried different drag functions?? This is most likely your issue. Try G5 and G7 over G1. One of these will likely get you closer. I use my own personaly developed software that allows me to make the drag function any number I wish. Between adjusting the drag function and BC I can match ANY real world trajectory to within an inch at any point in the range.

 

[groper]

scope is a FFP scope and so far its proven to track very well. distances are assumed from a palma range, we verified the 900yds via GPS also. I shot over 50 rounds to get these numbers, not just a few.


but not to worry, i seem to have gotten closer since my last post.

from this data, closest i can make fit is this...

2950fps, .62 BC
zero at 218yds. (1 inch groups centered at 218yds)
3.0 mils at 600, (i measured 3.1mils actual)
5.8mils at 900 (same as i measured)

only thing i can figure is that my 600yd drop is maybe 0.1 mil error???

does this sound ok?

i worked this out via trial and error using the "thing", is there a more methodical way other than subbing in random values to get a traj close to what you measure as actual???

Also as a side note, i needed 0.3mil left at 900 also, i didnt think spin drift would be that much, is it possible or somthing else???

 

[britz]

I'm going to guess your velocity is still off because the AMax has a higher BC than .62.

 

[BryanLitz]

FWIW,
I measured the G1 BC of the 208 Amax to be 0.633 (average value from 3000 to 1500 fps) and the G7 BC is 0.324 (valid for all speeds).
Using this BC, you're velocity would have to come down from 2950 to match your trajectory.

You asked about a more 'methodical' way. The first step is to measure your muzzle velocity.

You can expect about 5" of spin drift with the 208 Amax at 900 yards from a 1:10" twist barrel.

-Bryan

 

[britz]

"You can expect about 5" of spin drift with the 208 Amax at 900 yards from a 1:10" twist barrel"

I'm suprised that it's that much! learn something new every day!

 

[Michael Eichele]

only thing i can figure is that my 600yd drop is maybe 0.1 mil error???

does this sound ok?

A 0.1 mil error is normal. It is impossible to get any calculator to match EXACTLY your real world drops. This is where knowing your weapon comes in to play.

i worked this out via trial and error using the "thing", is there a more methodical way other than subbing in random values to get a traj close to what you measure as actual???

1: I agree with Brian, verify your velocity before sweating much more. Also it helps to shoot more ranges for a more acurate picture of what is happening.

2: Verify you barometric pressure. I cannot stess this enough. it is a MUST know.

Also as a side note, i needed 0.3mil left at 900 also, i didnt think spin drift would be that much, is it possible or somthing else???

0.3 seems a wee bit much for spin drift at that range but is close. Double check your scope level AND make sure youre not canting the rifle. Also depending on your global position you may see some coriolus effect which WILL add to your spin drift woes. The two combined can easily account for your 0.3 mil correction.

 

[RockZ]

I think you would be closer to 2850fps, going by the results I get with my 300win and 200smk's.
Of course all setups are different.

 

[groper]

thanks for the spindrift hint bryan my bbl is 1:10... i also trust your BC findings for this bullet, and so i used 0.633.

using this, i get my velocity @ 2940fps, and everything fits except the 600yd is out by 0.1mil, although this was shot on a different day compared to the 218 and 900, so i believe it to be accurate enough.

Ill shoot at 800 and 1000 next outing and i should have a pretty clear picture by then... by the way, for a stock remmy M700 SPS (only mod is a DE brake), the groups at 900 were sub MOA. I put almost every bullet (once dialed) into the V bull on a palma target from prone off a bipod and using my hand as a rear support. 1inch groups at 218yds... not bad for a stock bbl eh?

multiple people have published velocity very close to this for this recipe, i dont see it as abnormal. 77grns gets you circa 3000fps, but it wasnt as accurate in my rifle, neither was 78grns i also tried...