Trajectory mysteries.

[Bear44]

Here's a weird one.

I use AB ballistics software for calculations and it has worked very well on several firearms. I have also used The Nikon ballistics software to calculate the best match up for the round I shoot with one of their Monarch scopes with a BDC reticle.

To make a long story short, in my M1a the AP software has me dead on at 200 and 600 yards, but two MOA low at 300. This exact same thing is happening when I use the BDC reticle on my 300 WSM, on at 200 and 600, but two MOA low at 300.

So, umm...

 

[Horsehat]

Never seen a bullet move in quite that path.

 

[Bear44]

Never seen a bullet move in quite that path.

LOL! Let me be clear. I'm talking about after the calculated adjustment has been made for each distance.

 

[Redleg]

what is the distance you are zeroed and the bullets?

 

[Kyzrsoze]

Obviously there's a drop in your torque curve midrange. Probably need a different cam.

 

[Bear44]

what is the distance you are zeroed and the bullets?

Original zero on the M1a is 200 yards, but am making adjustments for 300 and 600. 600 works, 300 is two minutes low.

On the 300 WSM center cross-hair is zeroed at 200. 300 yard reticle is two MOA low, but 600 yard reticle is on.

The bullet for the M1a is the 175 grain Lapua Scenar L. For the 300 WSM it's the Barnes 168 grain TTSX.

 

[Bear44]

Obviously there's a drop in your torque curve midrange. Probably need a different cam.

That's what I'm thinking, too.

 

[Bear44]

Using the Nikon, their ballistics calculator says that my load should be very close to being on with the scope set at 11 power, but I have to turn it down to 8 to be on at 300, which makes it very high at 600.

 

[Jeddak]

Stupid question, but aren't BDC recticles designed for specific calibers?
My Russian POSP has a BDC for 7.62x39, but since my Vepr has a 20" barrell, not 16", bullet moves faster than BDC is calibrated for, so I have compensate for it.

 

[Bear44]

Stupid question, but aren't BDC recticles designed for specific calibers?
My Russian POSP has a BDC for 7.62x39, but since my Vepr has a 20" barrell, not 16", bullet moves faster than BDC is calibrated for, so I have compensate for it.

The BDC reticles have preset magnifications for general use. However, the Nikon Ballistics calculator that I mentioned is designed so you can plug in the data for your load and find out what the actual distance each reticle would be on at. The program also gives you the ability to change magnification on your particular scope to get the best match for your load at specific distances. This program tells me I am within a few yards of the 300, 400, 500 and 600 yard reticles when set a 11 power. I would say the program was just screwed up, if it wasn't for the fact it nailed the 600 yard.

The same is true for the M1a. The calculation is off at 300, but on at 600. This is with a Nightforce scope and a ballistic calculator that has proven to be very accurate with several other rounds in other rifles.