BC for 69 SMK

[mdmp5]

Hi guys, I am gonna trek out to Young's Longshot tomorrow morning. What drag model should I use in my ballistic calculator for the Sierra 69 gr HPBT SMK? Reason I ask is because if I plug in G7 or even G5 for that manner, it shows the 69er supersonic out past 1000 yds, and I don't find that plausible.

 

[Lindy]

Re: BC for 69 SMK

Go to JBM Ballistics:

http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

Pick from the Bullet Library Sierra, 0.224 cal, 69 gr, Matchking (Litz)

That will automatically select the G7 drag model - don't worry what it shows on the setup page. Plug in your other information, i.e., muzzle velocity, atmospheric data, etc.

Believe what it says.

 

[mdmp5]

Re: BC for 69 SMK

Thanks Lindy. I am using Ballistic! right now, and I think the discrepancies arise because it asks for both native drag model and use drag model. Sometimes it kicks out some funky numbers with my .308 also.

 

[treebasher]

Re: BC for 69 SMK

Be sure you are using the correct BC for the drag model that you choose.

Below is data from the book "Applied Ballistics for Long Range Shooting" by Bryan Litz

G7 Drag Model
Average Form Factor (i7) = 1.161
Average BC G7 = 0.169

G1 Drag Model
Average Form Factor (i7) = 0.596
Average BC G1 = 0.331

At sea level and approximately 3000 fps a SMK 69gr 223 bullet will stay super sonic out to about 850 yards.

 

[mdmp5]

Re: BC for 69 SMK

My calculator shows G7 BC of 0.153, so I guess it is close enough for usage. 3000 fps is a bit ambitious for the 69er; I was using about 2750 or so. Could be a bit understated, but I am not sure I can get to 3000 with that bullet in my AR.