7.62 x 39 in a 5.5 inch barrel

So a buddy of mine and I were having this conversation after seeing a Maxim PDW kit go up for sale on one of the discount sites. After doing some research the only velocity tests I could find stopped at 16 inch. However the spread between a 16 inch and 24 inch barrels for this round was documented at 100 ish fps. 2400 at the top and 2300 at the bottom (approximate numbers.) The next data point I found was "dude on a forum" who said he was getting between 1950 and 2050 fps out of his 7.5 inch barrel. Now on the Maxim web site they say out of their 5.5 inch barrel they average 1675 fps. Only based on the data I found this number seems inside the realm of possibility but I could not find any hard data to support it. Also a wide range of info out there concerning minimum velocity for expansion. I hear anything between 1600 and 1800. Both numbers would effectively mean the 5.5 inch barrel is an expensive flame thrower / range toy with no real world application. Interested to hear the opinions of the masses here. What is the shortest you would go on a 7.62 barrel and why? And to be clear, neither of us are actually considering buying or building a 5.5 inch 7.62. However I may build another 7.62 and am wondering "how short is too short."

I would suggest not to go under 8". I prefer something a little longer, then went with an 11" KAK barrel on my 7.62x39 pistol.

Their stated numbers sound reasonable, it's possibly a little on the high side of what I would expect. Most 122-124 grain ammo other than some hot Uly is gonna be in the low to mid 1900s at 7 to 7.5 inches. 5.5 should be effective, but it's getting toward the bottom end. I would only use it in an AR style system. 7 is as short as you can go in an AK without issues (micro draco be damned). 7 inch is a bloody flamethrower, see my pics in my "other semi" thread.

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Originally Posted by Red*Lion

I would suggest not to go under 8". I prefer something a little longer, then when with an 11" KAK barrel on my 7.62x39 pistol.

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How do you like that KAK barrel? I have been on the "notify when back in stock" list for one of those for months now. I have one of their 300 blk barrels (Mr Blonde) on one of my pistols and its a shooter.

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Originally Posted by utahjeepr

Their stated numbers sound reasonable, it's possibly a little on the high side of what I would expect. Most 122-124 grain ammo other than some hot Uly is gonna be in the low to mid 1900s at 7 to 7.5 inches. 5.5 should be effective, but it's getting toward the bottom end. I would only use it in an AR style system. 7 is as short as you can go in an AK without issues (micro draco be damned). 7 inch is a bloody flamethrower, see my pics in my "other semi" thread.

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Yeah this would definitely be in an AR platform. Wonder how short you can go and still put a can on these things without voiding the warranty :D

I have one of the KAK value line 11 inch barrels. For the money, they're hard to beat. That said, like most others, KAK uses M4 feed ramps. If you're not scared to work on the feed ramps, it's a good barrel.

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Originally Posted by casualreclaimer

I have one of the KAK value line 11 inch barrels. For the money, they're hard to beat. That said, like most others, KAK uses M4 feed ramps. If you're not scared to work on the feed ramps, it's a good barrel.

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Did yours require blending?

I wouldn't call it blending, more like sizing for the round. M4 ramps are for 5.56 diameter bullets. Plus, it's a value line barrel for a reason, there were burrs that needed to be cleaned up.
Without changing any angles, I just widened the ramps toward the center a little. It narrows that center locking lug of the extension, but I didn't cut them as wide as the AAC 300 blackout ramps.
AAC ramp cuts are what all 7.62x39 AR feed ramps should look like, or at least ones that use AR type magazines. I have a Bear Creek 14.5 upper that I pieced together a few years ago that has the AAC cuts. That thing has never once failed to feed.

As far as how short a barrel for 7.62x39, I wouldn't go less than 10.5. Anything less, look hard at 300 Blackout, even at the price difference of the rounds. I have a 8.5 inch 300 black upper that runs rings around the 11 inch x39 upper.

i also have a KAK Value Line 7.62x39 barrel, 16" shortened to 14.5". anyways, regarding the feed ramps, I smoothed out the sharp edges and then polished the edges, ramps. didn't really change the shape, just smoothed it so that it's not gouging the copper jacket or the steel case, for smooth feeding. works fine. steel case ammo only. FWIW.

Yeah I already have a 300 blk using the KAK Mr Blonde 8 3/4 inch barrel. It's a great barrel. It's just sat once ammo went north of a buck a round. I built a 16 inch 7.62 x .39 a few months back using a Dez Arms barrel and that thing shoots. Now I want to go shorter and do a poor man's 300 blk so to speak. Unfortunately 16 inches is the shortest barrel Dez makes in that clambering or I'd buy another. Good to know about the Value Line feed ramps. Think I'll wait for one of the KAK stainless ones to come back in stock. Thanks for the input, Gents