Range Report .308 Ballistics with 20" barrel?

[Dumb_Like_Fox]

Lazy noob question here. I've been searching around the net and even tried the search function on this forum and found nothing (although this has undoubtably been discussed I'm sure).
I'm wondering the difference in bullet drop and wind-drift from 100-1000 yds between a 24" barreled .308 and a 20". I've assumed that the data on Federal's ammo boxes corresponds with a 24" barrel. I've got a compact remingtion 700 so I'm trying to figure out the difference.
If someone is generous enough to post ballistics tables for Federal 165 grain BTHP Gold Medal Match and/or Federal 150 grain Power-shok (with a 20" barrel), that would be awesome.
If not a point in the right direction would do. Thanks.

 

[NukeMMC]

Re: .308 Ballistics with 20" barrel?

Actually the only difference you will see on changing barrel length is a change in velocity. You should only see <span style="text-decoration: underline">about</span> 100fps difference in velocity between a factory 20 and a factory 24. Of course there will always be variances and exceptions to the rule, but it's a good ballpark estimation.

 

[Dumb_Like_Fox]

Re: .308 Ballistics with 20" barrel?

Thanks. So the decrease in velocity isn't enough to effect drop or windage? If not thats great, less work for me. I just figured that the slower velocity would make the round drop more or be more affected by a cross-wind. Not enough of a difference then?

 

[deadly0311]

Re: .308 Ballistics with 20" barrel?

you will have to use a tad bit more elevation but your wind shouldnt be changed. Run the numbers in JBM ballistics (google this) and see for yourself

 

[Dumb_Like_Fox]

Re: .308 Ballistics with 20" barrel?

federal rounds in JBM are all .223 strangely...wonder if I could substitute winchester or something...

 

[MitchAlsup]

Re: .308 Ballistics with 20" barrel?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Dumb_Like_Fox</div><div class="ubbcode-body">Thanks. So the decrease in velocity isn't enough to effect drop or windage? </div></div>

The shorter barrel will have greater drop and more windage. However, all in all, you lose access to the last 100 yards.

Properly loaded; a 308 can shoot out towards 1300 yards with a 24" tube with the bullet remaining supersonic. A 20" tube will go subsonic around 1200 yards. Properly loaded.

Many factory ammos will not lose very much velocity (if any). So the actual answer is dependent on what ammo you shoot. Ammo that takes advantage of the barrel length loses more than ammo that is completely burned at 20".

 

[kraigWY]

Re: .308 Ballistics with 20" barrel?

Maybe this will give you an ideal.

From my Grandaughters science project: All ammo and guns are differant, this was done using Factory Rem 180 Gn corlocks. Started with a 26 in barrel cutting it 2 inches at a time to 16 inches.

Barrel Lenght Vel
26 2826 fps
24 2722 fps
22 2713 fps
20 2677 fps
18 2654 fps
16 2552 fps

 

[Lindy]

Re: .308 Ballistics with 20" barrel?

That works out to 27.4 fps per inch of barrel reduction on average, not far off from what I might expect.

 

[ReaperDriver]

Re: .308 Ballistics with 20" barrel?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Dumb_Like_Fox</div><div class="ubbcode-body">Thanks. So the decrease in velocity isn't enough to effect drop or windage? If not thats great, less work for me. I just figured that the slower velocity would make the round drop more or be more affected by a cross-wind. Not enough of a difference then? </div></div>

No, a decrease (or increase) in velocity will most certainly affect both drop and windage. What is stamped on the factory box for those values will NOT be the same for a 20" gun. At shorter range (100yds) the delta is small. At 1000 yds, the delta will be huge. Here's what I get for elevevation if I run the 168 FGMM at factory specs (2650) and about 100 fps less with 10mph full wind value.

2650fps (Drop / windage in inches)
100 - 0 / .02
200 - 3.95 / 1.85
300 - 14.66 / 5.71
400 - 33.30 / 11.85
500 - 61.31 / 20.57
600 - 100.5 / 32.19
800 - 221.1 / 65.50
1000 - 417.5 / 114.0

2550fps (Drop / windage in inches)
100 - 0 / .02
200 - 4.42 / 1.96
300 - 16.19 / 6.05
400 - 36.59 / 12.57
500 - 67.22 / 21.63
600 - 110.0 / 34.15
800 - 242.0 / 69.36
1000 - 456.6 / 120.0

So you can see there is a LOT of difference, especially at longer ranges, as you decrease MV as a result of a shorter barrel. But as you can see, it even shows up by 200 yds if you need to be precise. It really makes no difference though, as long as you know what your dope is for each distance. As long as you know the correct values for the bullet/gun combo - you will hit your target.

 

[badbot]

Re: .308 Ballistics with 20" barrel?

use the new hornady ammo it's faster

 

[Dumb_Like_Fox]

Re: .308 Ballistics with 20" barrel?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ReaperDriver</div><div class="ubbcode-body">

No, a decrease (or increase) in velocity will most certainly affect both drop and windage. What is stamped on the factory box for those values will NOT be the same for a 20" gun. At shorter range (100yds) the delta is small. At 1000 yds, the delta will be huge. Here's what I get for elevevation if I run the 168 FGMM at factory specs (2650) and about 100 fps less with 10mph full wind value.

2650fps (Drop / windage in inches)
100 - 0 / .02
200 - 3.95 / 1.85
300 - 14.66 / 5.71
400 - 33.30 / 11.85
500 - 61.31 / 20.57
600 - 100.5 / 32.19
800 - 221.1 / 65.50
1000 - 417.5 / 114.0

2550fps (Drop / windage in inches)
100 - 0 / .02
200 - 4.42 / 1.96
300 - 16.19 / 6.05
400 - 36.59 / 12.57
500 - 67.22 / 21.63
600 - 110.0 / 34.15
800 - 242.0 / 69.36
1000 - 456.6 / 120.0

</div></div>
Yeah, it makes sense that the change in velocity would change the ballistics. What are you using to get your figures? I usually run a 200 yard zero so again the dope would change a bit. I need to get a good ballistics calculator program or something...

 

[Lindy]

Re: .308 Ballistics with 20" barrel?

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I need to get a good ballistics calculator program or something...</div></div>

http://www.jbmballistics.com/cgi-bin/jbmtraj-5.0.cgi

 

[Dumb_Like_Fox]

Re: .308 Ballistics with 20" barrel?

Ok am I just a dumbass cuz I can't seem to find Federal .308 in that calculator...I used Hornady 168 gr BTHP and got some figures. How do you determine muzzle velocity for your input? Might have to get a ballistics program for the ipod

 

[Rob01]

Re: .308 Ballistics with 20" barrel?

Look for the bullet, Sierra 168 Matchking

 

[Dumb_Like_Fox]

Re: .308 Ballistics with 20" barrel?

Ah, so it's under Sierra not "Federal". Tricky bastards. Thanks for the help, all.

 

[ReaperDriver]

Re: .308 Ballistics with 20" barrel?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Dumb_Like_Fox</div><div class="ubbcode-body">Ah, so it's under Sierra not "Federal". Tricky bastards. Thanks for the help, all. </div></div>

And see, that's the beauty of the Ballistic FTE program on the iPhone. You can search under bullet or cartridge load. I could search under both 168 SMK OR 168 Federal Gold Medal match. The nice thing is with using the "LOAD" database - it gives you the factory ballistics. Remember though, that those MVs are only good for their test gun, you will have to modify the MV to match your actual values if you want the program to give you accurate dope.

Garbage in/garbage out.