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MV vs. Barrel length


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#1 OFFLINE   meh77gmc

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Posted Nov. 28 2009 - 08:01 PM

Evening ya'll,
I was looking at the Savage 12 F/TR in .308 and noted the 30inch barrel on it. I'm curious what the 30 inch BBL does for the muzzle velocity compared to the 24 or 26 inch barrel on my dad's 308. I imagine it allows for hotter loads to be used (being the rifle is a long range competition piece) but just how much velocity change are we looking at?

Thanks for your time,
Mike


#2 OFFLINE   bruce

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Posted Nov. 29 2009 - 12:16 AM

The 30 inch barrel would allow you to get more out of hand loads with slow burning powders. Although I think case capacity would quickly become a limiting factor. I researched changes in velocity between 16, 18, 20 and 24 inch barrels. My findings are here http://www.ar15armor...or-308-barrels/ . For factory ammo there was very little change in velocity between 16 and 24 inch barrels. The biggest documented velocity change I could find between 16 inches and 24 inches was less than 150 fps. The increase in velocity per inch decreased as the barrels became longer.


#3 OFFLINE   glock10mmman

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Posted Nov. 30 2009 - 12:51 AM

One of the reason's the 20" is more accurate than the 24" is due to the hyrmonics (sp) of the barrel. The longer barrel had to much vibration and caused the bullet to pull. I cant for the life of me see why you would want a 30"????

I guess you might get a few FPS more, but if the difference is only 150 or so with a 20-24, then 30 would likely be around 200-300 faster.

Not to mention the weight and trying to manuveur that sucker around :unsure:


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#4 ONLINE   MontanaLon

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Posted Nov. 30 2009 - 02:18 AM

Not really a use on an F-class rifle but my match rifle has a 28" tube for the longer sight radius as well as the higher muzzle velocity. Shooting long range which is what F-class is about IIRC is a game of keeping the bullet from going subsonic before it gets to the target. It may not seem like much but a few extra FPS can be the difference between X's or 10's and just somewhere on the paper. I had a load that worked fine until I had to shoot it into a headwind. With just a 10 mph headwind the bullets went subsonic before they got to the target and left some nice keyholes for about 1/2 of the shots. They were not far off the mark and it was actually a pretty good group for me, but the target puller confirmed he was hearing a buzz from the bullet instead of a crack most of the time. YMMV

ETA that load was the most accurate load in the rifle and was a little off max velocity. I switched to a hotter load that was minutely less accurate.

Edited by MontanaLon, Nov. 30 2009 - 02:21 AM.



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#5 OFFLINE   meh77gmc

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Posted Dec. 01 2009 - 08:05 PM

Not really a use on an F-class rifle but my match rifle has a 28" tube for the longer sight radius as well as the higher muzzle velocity. Shooting long range which is what F-class is about IIRC is a game of keeping the bullet from going subsonic before it gets to the target. It may not seem like much but a few extra FPS can be the difference between X's or 10's and just somewhere on the paper. I had a load that worked fine until I had to shoot it into a headwind. With just a 10 mph headwind the bullets went subsonic before they got to the target and left some nice keyholes for about 1/2 of the shots. They were not far off the mark and it was actually a pretty good group for me, but the target puller confirmed he was hearing a buzz from the bullet instead of a crack most of the time. YMMV

ETA that load was the most accurate load in the rifle and was a little off max velocity. I switched to a hotter load that was minutely less accurate.


Thanks for the info MontanaLon (everyone else as well),
So I guess when the target is at 1000, you have the limited case capacity of the .308, and are trying to push the heavier (high BC) slugs out to fight the wind you need every last bit of velocity you can get. I guess that is why they offer The 12 F/TR with the 30 inch BBL. So out of curiosity, how high would the MV have to be for the average 175 grain .30 cal bullet not to fall below 1150-1200 fps at the end of the 1000 yard range?

y'all are bad-a** for taking time to answer these silly questions for folks like me,

Mike


#6 OFFLINE   bruce

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Posted Dec. 01 2009 - 08:39 PM

2550 fps muzzle velocity for a 175 grain Sierra Match King HPBT falls to 1157 fps by 1000 yards.

Federals 175 grain Gold Medal Match using the SMK HPBT bullet is reported to have a 1200 fps muzzle velocity from a 24 inch barrel.

You can calculate the ballistics for other bullets at http://www.jbmballis...aj_simp-5.0.cgi


#7 OFFLINE   meh77gmc

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Posted Dec. 01 2009 - 08:45 PM

cool beans!
I'm gonna play with the jbm ballistics tool for a while.

Mike :thumbsup:


#8 OFFLINE   akxx

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Posted Dec. 03 2009 - 12:23 PM

There was an article a while back regarding barrel length and velocity. For tactical use, it was determined that 20" was about ideal for the .308, in terms of velocity/length tradeoff. There was little velocity lost cutting from 24" to 20".

20" to 18" showed a greater linear loss, and 18" to 16" was even more drastic.

From what I understand, a 20" .308 should be more than plenty to reach out to 600 yards effectively. It likely won't get you to that magic 1000 yards, though.





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