Subsonic computation

[rickp]

Hey guys,
I came across a post about distances when .308 rounds become subsonic.

168gr SMK becomes subsonic at 891 yards
168gr AMAX becomes subsonic at 1,007 yards
175gr SMK becomes subsonic at 1,038 yards
178gr AMAX becomes subsonic at 1078 yards

Can someone tell me how to figure that out? I'm shooting BH 175gr with an average MV of about 2585-90fps from my current lot.

I'm also thinking about loading so I'm trying to educate myself about using 175gr SMK or 168gr AMAX. I'm leaning more for the 175 SMK.
I'm using an AW with a 1:10 twist rate and 24" barrel with a can on it.

Thanks

 

[MitchAlsup]

Re: Subsonic computation

Go to JBM ballistics
select trajectory
select bullet kind (use "[LITZ]" if present)
select muzzle velocity
select 1500 yards (so the bullet goes Mach in the table)
select 20 yards per increment in the table
press the "compute" button
look at the table and find where the Mach number is closest to 1.000

 

[rickp]

Re: Subsonic computation

Mitch,
For the info I punched in the range is 980y with a mach # of 1.006. At 1000y it drops to .988

I use 175gr
MV 2590
Temp 80
Hum. 70
Press. 30.10

 

[MitchAlsup]

Re: Subsonic computation

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Rickp</div><div class="ubbcode-body">Mitch,
I use 175gr
MV 2590 <-- probably a little slow:: try 2650 fps
Temp 80
Hum. 70
Press. 30.10
</div></div>