I'm planning to build an upper chambered for an "improved" version of 7.62 x 39, and I'm wondering whether to use a full-length, middy, or carbine length gas system.  Theoretically (based on expansion of the case volume into the barrel) it seems like the carbine length would best emulate the full-length 5.56, and I've seen that carbine uppers are available.  If I recall correctly, though, Colt used to offer full-length 7.62's.

Does anyone know whether the 7.62 x 39 carbines are rough on brass, like the 5.56's sometimes can be?  Or do the full-length uppers tend to be under-gassed?  Or should I just relax and go with a middy? he
Thanks for any info!

My 16in. 7.62x39 has a carbine set up, as recomended by Accuacy systems,the company that supplied my bolt and barrel.No problems with damaged brass.Where are you getting the chamber work done?I want to go with the imp. also,but I can not find a smith that does it.

284win --- Thanks for the reply.  It does seems like carbines should work well.  I just can't figure out how the old Colt 20 inchers had enough gas to cycle.

As for the chamber work, I'll be doing it myself.  There's a smith who posts here from time to time --- Randall Rausch at ar15barrels.com --- maybe he could help you?

Curlew                                                                                                                            Your welcome. Are you going to rent the reamers? If so could you inform me as to who the rental company is? Thanks

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Quoted: I'm planning to build an upper chambered for an "improved" version of 7.62 x 39,

I'd like to know how this goes.  I've thought about this a lot over the years.  It seems to me it should be an answer to the magazine problem with the 7.62 x 39 in the AR with the reduced taper of the improved case.  Not to mention the potential for a little more velocity.  It seems like somebody must have done it before, but I have never heard of anybody who did.

Ofreenbean --- It'll be a while before this gets done, but I'll try to remember to post some results.  In my view, the 7.62x39 Improved is problematic for a couple of reasons.  First, I'd guess most people who want to shoot 7.62x39 are interested in using inexpensive imported ammo, typically with steel cases, and so they would not be very motivated to bother with reloading to get whatever benefits the improved case might offer.  Second, there's the issue of what bore size to use:  .311--.312 like the original military ammo, or .308 like the commonly available US-made bullets.

I'm doing this as a project to make use of a suplus 308 barrel, an unused box of Lapua brass, and a spare bolt.  The chamber will be designed to fit that brass, and it'll probably never see a round of factory ammo.  I'll mainly be using this thing in slow-fire offhand matches, loading one round at a time, but it will be interesting to see how the case feeds from a Grendel magazine.

284Win --- I tried sending an email, but I suspect you may not have received it.  The reamer will be ground to my specs, probably by Pacific Tool and Gauge, so no, not a rental.  FWIW, reamerrentals.com lists a 30 PPC, but I don't know how well that would work as a 7.62x39 Ipmproved.


Anybody here have any experience with a 20" 7.62x39 with the full-length gas system?

All my uppers are 20" tubes(Colt and DPMS) and I haven't had any trouble with them.  Flawless ejection.

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Quoted: In my view, the 7.62x39 Improved is problematic for a couple of reasons.  First, I'd guess most people who want to shoot 7.62x39 are interested in using inexpensive imported ammo, typically with steel cases, and so they would not be very motivated to bother with reloading to get whatever benefits the improved case might offer.  Second, there's the issue of what bore size to use:  .311--.312 like the original military ammo, or .308 like the commonly available US-made bullets. Anybody here have any experience with a 20" 7.62x39 with the full-length gas system?

Thanks, curlew.  I believe you are correct about why the improved 7.62 hasn't received any play.  However, I am one of those weirdos that handloads 7.62 x 39.  I use .310 bullets in my 20" Colt and get MOA accuracy.  I mentioned on another post that I will slug the barrel and report back the results.  I'll get around to it one of these days.  I like the 7.62 in the AR and think an improved 7.62 x 39 would be very interesting to play with.  I'll be interested to hear what you come up with.

To answer your question about the full length rifle, my Colt upper has never failed to function in about 2500 rounds.  I have a few 20 and 30 round frankenmags and have had good luck with them.

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Quoted: I did a preliminary workup on case dimensions for a 7.62x39 improved based on the PPC case body chambered long to rechamber an existing 7.62x39 barrel and still be able to fire factory ammo.

This is very interesting.  I think I will get in touch with you after I finish my current project.  I just bought some Grendel mags for that project, and it seems to me they would work with an improved 7.62.

Greg

The whole gas sytem length vs cartridge expansion ratio vs barrel length interests me too.

It's a balance of port pressure vs dwell time (length of barrel beyond the port)

I'd love to see more info on what is "ideal" for each cartridge and barrel length.

With your planned 23" barrel, the dwell will be so great even with the rifle length system that anything shorter would be overkill.

I'd guess it looks something like this (anyone "in the know" please edit):

____________16"_____18"_____20"_____24"

5.56________Mid_____Mid____rifle_____rifle

6.8 SPC_____Mid______?_____rifle_____rifle

7.63 X 39___Car______?______Mid_____rifle

458 SOCOM_Car______Car_____?_______?

Thanks for the corrections Randall,

With your professional involvement with barrels, I'm sure your "opinions" have more experience behind them than the rest of us can hope for.

I only wish I could access your web site. I work in Saudi and am blocked by their govt. servers from viewing any web pages with reference to "weapons" on the front page. I'll try to take a look at it during my next repat leave in Texas.

Thanks to everyone for the replies.  Apparently the port location isn't too critical for this cartridge, since folks seem happy with both 16" CAR's and 20" rifle-length set-ups.  I'll probably just split the difference and try a middy.

Brazos_Jack --- I agree regarding the importance of dwell time.  The pressure at a CAR-length port is probably close to that at a 5.56 rifle-length port, but as you point out I'd want to move out from there based on how much barrel is in front of it.  It's hard to guess how far, though:  to mid-length or rifle.  I've seen a table that says Colt used a .125-diameter port on their 20" rifles.  That strikes me as kinda huge, so I'm biased toward something shorter.

Randall,

I'm very interested in doing the Improved version. Any idea how much of a velocity increase we could see from this? I would be using the Hornady 110 AMAX and maybe the 155 AMAX in this round. Also, what problems occur when you fire the Russian ammo with the .311 diameter bullet in the .308 diameter barrel? What solutions have you found?

I would specify a 14.5" barrel with a carbine gas system.

Thanks for the reply Randall. I am wondering if you would mind doing a small Quickload workup on the 7.62x39 Improved? Maybe take the Grendel pressure of 52,000 psi and try using the 110 AMAX to 155 AMAX with various projectiles in between to get an idea of the improvements we could expect from this cartridge. Then maybe post it here so others can see it? That would be awesome.

Using a .311 throat with a .308 bore would be kind of like the 6.8 SPC II/Weatherby freebore chamber? No? If so, is that so bad? I mean if I wanted accuracy and velocity I could use my improved reloads, or I could just shotgun some Russian ammo around. You kind of have the best of both worlds with that combo, don't you?

Im quite interested in the idea... a 7.62 cartridge thats made to run in an AR-15.  After playing in the 6.5G world... 7.62x39 brass is super cheap and easy to come by!  If I could use the brass, I could care less about buring the cheap factory ammo.

I'd say a 120 grain bullet, maybe 125 would be sweet for knockin' cows in the head and bears in the lips when they try to eat me and what not

Well, I havn't recieved any feedback. So I guess we are the only ones interested in this idea?!

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Quoted: Well, I havn't recieved any feedback. So I guess we are the only ones interested in this idea?!

Once somebody actually does it and gets the results out, I reckon there would be a lot more interest.

So let me clarify, with teh 7.62x39 Improved, can you still fire brass case ammo?  (Winchester is the only mfg. that I know making some)   Or can you shoot only .308 dia bullets, which again really only means ammo made in the states (Winchester)

I am just trying to bring all the peices of information together to make this work. I have now been told by TWL in another thread that you would have to use a larger bore diameter to fire both commercial ammo and .308 diameter bullets. I don't know what the real scoop is yet. But this is the basic idea. I was hoping to be able to use an improved chamber for accuracy/ballistic performance and also be able to throw surplus ammo around. I had heard on another thread that a modified throat would basicaly swage the bullet down to .308 and enable the use of the surplus ammo. Is this the case? I don't know for sure. Someone who has fired surplus ammo out of their Mini-30 should be able to tell us if this works.

Quoted: I did a preliminary workup on case dimensions for a 7.62x39 improved based on the PPC case body chambered long to rechamber an existing 7.62x39 barrel and still be able to fire factory ammo. www.ar15barrels.com/gfx/762x39imp.gif