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Posted: 1/29/2011 7:23:11 PM EDT
[Last Edit: 1/29/2011 7:43:22 PM EDT by Accomplice1]
I have a rifle coming in that is 7.62X39 with a 16" barrel. I'm going to have ADCO cut/ crown/ thread/ perm attach a PWS FSC30 muzzle brake (bringing overall length back to 16"). The rifled portion of the barrel will drop to about 13.5". What kind of impact will this have on accuracy, velocity, stability of projectile, noise and effective range?
Link Posted: 1/29/2011 7:30:36 PM EDT
The claimed velocity of mil surp ammunition from an AMD-65 (12.6" barrel) was 80 fps slower, IIRC.
Link Posted: 1/30/2011 5:47:49 AM EDT
Originally Posted By POLYTHENEPAM:
The claimed velocity of mil surp ammunition from an AMD-65 (12.6" barrel) was 80 fps slower, IIRC.


Wow, that is a pretty small drop from 16". I guess 13.5" would only lose 60 or so. With a 9.5X1 twist rate, will the bullet have enough barrel to get good stability?
Link Posted: 1/30/2011 6:02:03 AM EDT
What's your reason for doing so?
Link Posted: 1/30/2011 6:22:54 AM EDT
Originally Posted By AEnemaBay:
What's your reason for doing so?


Easier/faster handling of rifle without having to go through the NFA hoop. Same reason I'm considering doing so to my next AK.
Link Posted: 1/30/2011 6:34:38 AM EDT
Originally Posted By unseenSNIPER:
Originally Posted By AEnemaBay:
What's your reason for doing so?


Easier/faster handling of rifle without having to go through the NFA hoop. Same reason I'm considering doing so to my next AK.


16" is 16". But now the rifle will have less velocity. If it's for a look then that's fine, but I don't see the point if the rifle is for any type of work.
Link Posted: 1/30/2011 7:01:31 AM EDT
[Last Edit: 1/30/2011 7:04:20 AM EDT by unseenSNIPER]
Originally Posted By AEnemaBay:
Originally Posted By unseenSNIPER:
Originally Posted By AEnemaBay:
What's your reason for doing so?


Easier/faster handling of rifle without having to go through the NFA hoop. Same reason I'm considering doing so to my next AK.


16" is 16". But now the rifle will have less velocity. If it's for a look then that's fine, but I don't see the point if the rifle is for any type of work.


True. Now if he throws a 2.6" compensator at the end of a 16" barrel it now becomes over 18". So I can understand why he would want to chop the barrel down to 13.5" and have the comp perm attached to bring the length of the barrel up to a legal 16", especially if the "work" this rifle will be doing is all within 150 yards or if the OP wants this as a HD gun.

The advantage with doing this on the 7.62 Soviet is that the round isn't as velocity dependent as say, the 5.56. An 80-100fps loss isn't as drastic as it would be for a smaller caliber round, and one that is velocity dependent, at that.
Link Posted: 1/30/2011 7:40:10 AM EDT
Originally Posted By AEnemaBay:
Originally Posted By unseenSNIPER:
Originally Posted By AEnemaBay:
What's your reason for doing so?


Easier/faster handling of rifle without having to go through the NFA hoop. Same reason I'm considering doing so to my next AK.


16" is 16". But now the rifle will have less velocity. If it's for a look then that's fine, but I don't see the point if the rifle is for any type of work.


It really does make a noticeable difference when you do what he wants to do. I have a 14.5" AR with a a2x flash hider and a 16" with an a2 flash hider. There is a very noticeable difference in the length which helps a lot when you are shooting IPDA stuff around obstacles. Which makes me want a SBR, but I can't bring myself to do the paper work and get the sign off.
Link Posted: 1/30/2011 11:02:00 AM EDT
Originally Posted By Accomplice1:
Originally Posted By POLYTHENEPAM:
The claimed velocity of mil surp ammunition from an AMD-65 (12.6" barrel) was 80 fps slower, IIRC.


Wow, that is a pretty small drop from 16". I guess 13.5" would only lose 60 or so. With a 9.5X1 twist rate, will the bullet have enough barrel to get good stability?


There aren't widespread reports of loss of accuracy in short barreled 7.62 rifles.
Link Posted: 1/30/2011 12:06:50 PM EDT
[Last Edit: 1/30/2011 12:08:45 PM EDT by Gunplumber]
in .308 it is about 30 fps/in from 21 to 16, then the drop-off becomes greater per inch. But other than a faster drop at longer ranges, I don't see it as being significant. You'll probably find a wider velocity difference in ammo brands. I know in .308 some surplus is 100-150 fps slower than the USGI.

I understand shortening a barrel for SBR purposes. I don't see much point in shortening barrels and sight radius if nits not going to be an SBR.

Makes cleaning fouling out of the comp a lot harder.

As mentioned above, a heavier bullet still has a lot of umph out of a shorter barrel.

I saw a chart once with velosity tests cutting the barrel one inch at a time on both the 762 and 545 and was mildly surprised tha tthe velocity loss wasn't a lot greater.
Link Posted: 1/30/2011 2:00:53 PM EDT
Originally Posted By unseenSNIPER:
Originally Posted By AEnemaBay:
What's your reason for doing so?


Easier/faster handling of rifle without having to go through the NFA hoop. Same reason I'm considering doing so to my next AK.


Exactly-thanks. I had this done on a Sig 556 in 5.56 and it made a very positive difference in handling (and it looked better too). The change will be a little more dramatic in the 7.62X39 556 because the PWS muzzle brake going on is almost 3" so the barrel will look even shorter.
Link Posted: 1/30/2011 2:01:53 PM EDT
Originally Posted By Gunplumber:
in .308 it is about 30 fps/in from 21 to 16, then the drop-off becomes greater per inch. But other than a faster drop at longer ranges, I don't see it as being significant. You'll probably find a wider velocity difference in ammo brands. I know in .308 some surplus is 100-150 fps slower than the USGI.

I understand shortening a barrel for SBR purposes. I don't see much point in shortening barrels and sight radius if nits not going to be an SBR.

Makes cleaning fouling out of the comp a lot harder.

As mentioned above, a heavier bullet still has a lot of umph out of a shorter barrel.

I saw a chart once with velosity tests cutting the barrel one inch at a time on both the 762 and 545 and was mildly surprised tha tthe velocity loss wasn't a lot greater.


Thanks for the info. Glad to hear there shouldn't be much difference in performance.
Link Posted: 1/30/2011 4:49:39 PM EDT
Indeed, heavier, slower bullets are less affected by velocity loss from shorter barrels than lighter, faster bullets are. This is particularly true of the 7.62x39 - it is a great caliber for SBRs, much better than the 5.56 or 5.45 in my opinion.

Originally Posted By Gunplumber:

I saw a chart once with velosity tests cutting the barrel one inch at a time on both the 762 and 545 and was mildly surprised tha tthe velocity loss wasn't a lot greater.


Any idea where you saw that chart? I'd be real interested to see the numbers they came up with. I'm buying a Chrony soon and the first thing I'm going to do with it is see what the difference in fps is between a 20.5" SKS and my 11.5" Draco SBR, all other things being the same.
Link Posted: 1/30/2011 7:53:34 PM EDT
Originally Posted By 0uTkAsT:
Indeed, heavier, slower bullets are less affected by velocity loss from shorter barrels than lighter, faster bullets are. This is particularly true of the 7.62x39 - it is a great caliber for SBRs, much better than the 5.56 or 5.45 in my opinion.


Good point... 16" barrels and 7.62x39 go well together.

Link Posted: 1/30/2011 8:39:51 PM EDT
Originally Posted By AEnemaBay:
Originally Posted By unseenSNIPER:
Originally Posted By AEnemaBay:
What's your reason for doing so?


Easier/faster handling of rifle without having to go through the NFA hoop. Same reason I'm considering doing so to my next AK.


16" is 16". But now the rifle will have less velocity. If it's for a look then that's fine, but I don't see the point if the rifle is for any type of work.


but the same comp on a 16" makes it 18.5". Now he can have his comp and a 16" tube too!(legal wise). As stated, barrel length is not really a factor with 7.62x39. Krinks throw lead 100fps slower than a 16", at close range, you wont notice it.
Link Posted: 2/1/2011 12:12:18 PM EDT
[Last Edit: 2/1/2011 12:12:58 PM EDT by mouthpiece]
AMD has a 12.5" barrel. I think it is more accurate than my Romy G 16" barrel.
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