308 ballistics question.


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Max
June 4, 2009, 05:33 AM
I read somewhere that the 308's ballistics when fired out of barrels shorter than 18" or so are dropped severely. As in no better than 7.62x39 but, with twice the felt recoil, muzzle flash, and audible report. Thus avoiding 308 battle rifles with 16" barrels would be advisable.

Is their really that much drop in performance for the sake of a couple inches or, dose it really matter?

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jackdanson
June 4, 2009, 11:06 AM
Is their really that much drop in performance for the sake of a couple inches or, dose it really matter?

Short answer, no there isn't a drop to the point where it is in x39 ranges.

Long answer.

http://www.ballistics-software.com/shoot.htm

Personally, if I'm going with .308 weapon I prefer 20+ inches. I had a saiga that was 16in... the recoil and blast were acceptable, but it was a little brisk. I don't get the whole socom/16inch m1a thing.

LemmyCaution
June 4, 2009, 11:07 AM
In short, no, there is not that great a loss.

Taking the hypothetical M14 as an example, MV=850mps and ME=3350j with a 22" barrel and M80 ammunition. It's reported that a 16" barrel of a SOCOM 16 is about 790mps and 2850j with M80 ammunition. A hypothetical AK47 has a MV of 710mps and ME of 2010j with M43 ammunition.

The M14 has a Maximum Point Blank Range of 233m
The SOCOM 16 has MPBR of 217m
The AK47 has MPBR of 189m

Zeroed at 300M:

The M14 is
+12.6cm at 100m
+12.7cm at 200m
-52.7cm at 400m
-129.4cm at 500m
-247.6cm at 600m
-416.5cm at 700m
-650.9cm at 800m.

The SOCOM 16
+18.1cm at 100m
+20.9cm at 200m
-51.4cm at 400m
-140.3cm at 500m
-279.5cm at 600m
-479.6cm at 700m
-756.8cm at 800m

The AK47
+26.6cm at 100m
+30.7cm at 200m
-80.2cm at 400m
-227.2cm at 500m
-465.5cm at 600m
-806.3cm at 700m
-1262.9cm at 800m.

saturno_v
June 4, 2009, 11:10 AM
You will lose a good chunk of ballistic performance compared to a regular barrel length (22-24") but not that much to get really into 7,62 X 39 territory....

Carne Frio
June 4, 2009, 11:10 AM
Just a guess, but going from 18" to 17" you would loose about
50 to 75 fps. Close to the same each inch you reduce by.
www.ballisticsbytheinch.com actually has results for pistol
calibers. Hope that helps. :D

saturno_v
June 4, 2009, 12:21 PM
Just to be precise..

The gain/loss of velocity by every inch of increased/decreased barrel length is not constant, obviously....if you cut below certain length the loss will become increasingly more severe and if you add beyond a given value, the gain will become, eventually, negligible (in extreme cases you may start actually losing velocity)

Faster powders and pistol based rounds tend to lose/gain less for every inch of difference.

dagger dog
June 4, 2009, 01:12 PM
Read in reloading manual ,as rule of thumb a 20 fps velocity drop for every 1" of barrel loss. You can use a .308 Winchester ballistic chart in your favorite bullet weight to get your velocity and energy numbers

Art Eatman
June 4, 2009, 01:15 PM
Purely a generality, but the loss of velocity per inch of cutback seems to be pretty much consistent for any given cartridge whether rifle or pistol. There doesn't seem to be some "magic" length where there is any notable change in the loss.

saturno_v
June 4, 2009, 01:22 PM
Dagger

That rule of thumb need to be twisted considering the class of the cartridge....one inch of difference doesn't result in the same amount of change between, lets' say, a 300 Wby and a 44 Magnum or a 30-30.....you cannot generalize...

Art


At some point if you cut too much the loss will be fairly dramatic, especially for slow burning powders...

A nice article by Chuck Hawks sum it up well and he mentions some interesting reading materials on the topic

http://www.chuckhawks.com/rifle_barrel.htm

dagger dog
June 4, 2009, 03:50 PM
sat,

point taken!

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