Calculating MOA

[jimhoodag]

Several ballistics programs calculated my bullet drop at 2 MOA, but the point of impact is 6 inches low at 300 yards. What am I missing? I chronographed my 6.5 x 284 at 2,900 average with 140g Berger Hunting VLDs. According to the calculators by Berger, JBM, Ballistic, and Hornady, my drop at 300 yards with a 200 yard zero should have been 2 MOA, but it was actually 3.5 MOA. Why? My scope is a new Vortex Viper PST 6 - 24 x 50 front focal plane, so the MOA should not change based on the magnification setting.

I am going on my first elk hunt Friday and need help fast.

Jim

 

[Killshot]

That is correct output. A 200yd zero needs 2moa (1.89 actually) Up in Elevation for a 300yd target.
The total drop from point of firing is six inches (5.95"). Your firing point and zero range differ so the numbers aren't linear.

 

[fr3db3ar]

Yep, 2 MOA = 2" @ 100 - 300 yards. 2 X 3 = 6" Roughly. Until you reach longer ranges I don't find the fact that 1 MOA is not exactly 1" to make much of a difference.

MOA is not related to magnification at all.

 

[seanhagerty]

OK, There are a couple things I would look at:

Is your zero at 200 yds dead on?

Is the distance your scope is mounted above your barrel bore measured correctly and inputted into your calculators?

How sure are you of your velocity?

How sure are you on your target distances?

Sierra shows that at sea level, low humidity, and barometric pressure of 30; that you should have 6.93" of drop at 300 yds with a 200 yd zero. That makes it about 2.2MOA of drop from your 200 yard zero.

The day you shot, did you have a head wind?

Thats all I got

 

[300 RUM]

OK, There are a couple things I would look at:

Is your zero at 200 yds dead on?

Is the distance your scope is mounted above your barrel bore measured correctly and inputted into your calculators?

How sure are you of your velocity?

How sure are you on your target distances?

Sierra shows that at sea level, low humidity, and barometric pressure of 30; that you should have 6.93" of drop at 300 yds with a 200 yd zero. That makes it about 2.2MOA of drop from your 200 yard zero.

The day you shot, did you have a head wind?

Thats all I got

2nd the additional questions. Also can you adjust the parallax on your scope, were you close to the scope or farther away from the scope as when you shot at 200yards? Do you wear glasses? Was there a mirage?

 

[Nomad47]

A 2 MOA drop at 300 yards is 6.2832". Or going the other way, a 6" drop at 300 yards is 1.91 MOA.

Or looking at it a third way - if you have exactly a 6" drop at 300 yards and you turn your scope up 2 MOA, POI will be 1/4" high (actually .2832" high).

 

[lpreddick]

learned this the hard way...when entering data into the old hornady ballistic calculator i overlooked "sight height". the data already entered in the program was 1.5 in and my sight height was 2.5 and the poi was way off. once the 2.5 in was entered, the new table was spot on! a slight change in this variable makes a big difference at long range.

 

[XTR]

My experience with JBM says that good info in = good data out. You have something that's not correct in your inputs.

That said...

...6.5 x 284 at 2,900 average with 140g Berger Hunting VLDs. ...

I am going on my first elk hunt Friday and need help fast.

Jim

I'm sure someone has made dead elk with this combo or one like it, but personally I wouldn't try to use it. VLDs have a reputation for being fragile and a 6.5 is pretty light for an elk round.

 

[Jay Christopherson]

I dunno, 1 shot kill on a 6-point bull last year with a Berger hunting VLD. Ran about 40 yards and piled up. Works just fine as far as I am concerned, if you like frangible bullets. If it's not your thing, use a Barnes or something. Never heard about a reputation for being "fragile".

My experience with JBM says that good info in = good data out. You have something that's not correct in your inputs.

That said...

...6.5 x 284 at 2,900 average with 140g Berger Hunting VLDs. ...

I am going on my first elk hunt Friday and need help fast.

Jim

I'm sure someone has made dead elk with this combo or one like it, but personally I wouldn't try to use it. VLDs have a reputation for being fragile and a 6.5 is pretty light for an elk round.

 

[paulT]

Is the sight height the measurement from centre of bore to centre of scope.

 

[lpreddick]

yes and it's measurment is important. play around with a ballistic calculator by changing everything...temp, baro pressure, bc, mv and finally sight height ( do them individually) and look at trajectory beyond 400 yds. the sight height changes really changes you bullet's poi.