I am curious if any of y'all have experience with MGI hosts running 7.62x39, either with the MGI AK magwell or an AR-pattern x39 mag in the AR magwell.  

My MGI host is mostly together using the MGI x39 BCG, but I'm still looking for a barrel.  What is a good barrel in the 8"-10" range?  I am leaning towards one of PSA's KS-47 barrels, but don't know a whole lot about them.  

Who makes these barrels?  

Would an 8.5" pistol gas, or 10.5" carbine gas be preferrable?

Any other barrels I should be looking at?

Been running full auto 7.62x39 setups since around 2003.
My website here: https://c3junkie.com/?page_id=185

No issues using the PSA KS47 upper with my MGI lower either.

It seems to me that the MGI upper is the more useful component for use with a RLL, as opposed to the MGI lower.  Using the upper, all of the rll clearances and dimensions around the rear lug can be set with the MGI upper, and calibers can still be changed without disturbing the link operation.  

My plan was to stick with the same MGI upper and just swap in a x39 barrel and bcg.  The 8.5" KS-47 barrel seems to be the best length aesthetics-wise for the short MGI handguard, but from amphibian's link it looks like adding tungsten would be a good idea for that short of a barrel with pistol gas?  I am planning to use a H3 buffer, but maybe that's not enough weight?

Looks like using a .308 recoil spring is a must?

I have the opposite opinion.  I have no interest in the MGI upper.  The main reason is zero repeatability.  There are very few optics I know of that have programmable zero's for when you swap barrels..
Likewise, I know of only one front site that you can move windage and elevation.  
I also don't have a RLL, just a RDIAS so I can understand your perspective as a RLL owner just wondering what you are doing for zeroing when swapping barrels or maybe you are happy with them being close enough?
I'd rather just swap complete upper.
Now the MGI lower, I absolutely love.  I have most of the magwells and wish someone would tool up to make more like some for the MP5, Scorpion, .308 to be like a DPMS G2 etc.
Don't you disturb the RLL when swapping the BCG anyways?
I think the H3 is a little light for my tastes but I think it will still run.  As you can see from my site, I like cyclic rates in the 600's.  If you don't care about slowing it down, the H3 will be faster than that.  The main thing is to get a firing pin that can deal with the recessed combloc primers (assuming that you are running combloc ammo).  Red-X and Black Rifle Arms both sell them.  
Regarding bolts, as mentioned on my site, the LMT is the strongest and I've never broken one but did break an extractor and those extractors are proprietary and LMT wasn't selling spares last time I checked years ago.
I think that running a weighted carrier helps to keep the bolt locked longer and I haven't broken an extractor since doing so.
I think it depends on your mags / or drums.  The steel drums and steel AK mags have rough feed lips and require more force to strip the rounds off.  If you are running Duramags or P-Mags I would think those are smoother and a 308 spring may not be necessary.

I guess I didn't have much concern about zero shift on what is essentially a blaster.  Although I haven't had enough time with this setup to observe what happens to the zero when swapping barrels around.   If it's overly annoying, I might have to abandon the barrel swap idea.  


Yes, but you have to cut a custom 7.62x39 carrier for the RLL anyways.  As long as that's done to accommodate the same paddle thickness everything should stay consistent.


Do you have any details on how to add tungsten to the carrier?  As in a source for the weights, and how it is attached to the carrier?   Are they silver soldered in or something like that?

How about using one of the ultra heavy PCC buffers?   https://kawvalleyprecision.com/product/kvp-10oz-tungsten-blowback-buffer/    (this one may be too long)

Yes, I am familiar with the RLL and the required carrier (SP1 spec).  However, I am still confused about your comment about wanting the MGI upper so you wouldn't disturb the RLL but then mention swapping the BCG.  If you are swapping the BCG then I don't see why you just don't swap the entire upper.
Only reason not to do so would be if you are swapping between calibers that use the same bolt face like 556 / 300BLK etc.

Regarding carriers, don't know if you aware but the MGI carriers were already clearanced for AK mags/drums and while they weren't SP1 spec they weren't full auto spec either so less material to mill off.
Otherwise, you could get the KS47 carriers that are milled out for the AK mags/drums them mill those off for your RLL.
I have an assortment of carrier weights.  SAW used to offer a ramped bolt service for the Colt 9mm and supplied a custom Tungsten weight which is what I have pictured on my site.
KAK makes one that is identical to the short tungsten I have on my site pictured below (2nd from the top).
The bottom one is a 9mm Colt Steel weight so logically, all my weights are compatible with the Colt 9mm bolt hole location.  So you would need to have a 1/4" hole drilled into the carrier in the same place you will find a Colt (or clone) 9mm bolt drilled for it's bolt weight.


Link to the KAK Tungsten carrier weight: https://www.kakindustry.com/9mm-carrier-weight-extra-heavy
I don't think their weight includes the roll pin.  If KAK doesn't have them, other sources like CMMG have the roll pins.  I bought a big bag of like 50 from MSC years ago made from SS.

So if you look at all the links on my site, you can see that I'm pretty big into tinkering and getting the cyclic rate and smoothness to my liking.
Link to the root of my site.

In the picture below, that same exact lower configuration I happen to use with my 5.56 Shrike Belt fed upper.  Note that I'm using an A5 length buffer tube which is longer....likewise, I'm using a longer RB5007 hydraulic buffer which is the same length compressed as the heavy buffer you linked to.

So you could use an A5 length tube and go with that heavy buffer you linked too.....but I personally like to minimize my changes to the lower and just swap uppers.  The upper is where gas optimization obviously happens for gas operated calibers.
I have a lot of uppers of various calibers and I cannot think of one where I would use a 10oz buffer so I would hate to have some super heavy buffer in the lower for just one configuration.  By having the additional mass in the carrier gives me more flexibility and less things I need to remember to swap out.
In fact, the 308 Tubb spring works in just about everything I run.  If I forgot to swap it, the worse thing that would happen is that the cyclic rate would be faster then I prefer.

I bought a Pickett's Mill complete "pistol" super cheap on gunbroker for less than an upper.
Not really planning to use it much but just to have it around for the 7.62x39 ammo if need be.
https://www.gunbroker.com/item/965124426

Its a machinegun not a precision weapon.
I'll run out of ammo before the barrel burns up even if its a cheapo.

For what its worth the best 7.62x39 host I have found for the RLL is a CMMG MK47 "Mutant".

You get the big fat AR10 style bolt head so there very limited odds of breaking lugs and with the heavier carrier it is just a much smoother/softer shooting gun compared to any of the AR15 sized uppers I own or have shot.

It does takes some mods to the carrier as you have to cut it to SP1 style to engage the paddle.  You also have to mill the hammer ramp back as it will pinch the paddle upon recoil.

I also wanted to use a hydraulic buffer (plus flatwire spring) and had bolt bounce issues so ended up making a custom in carrier tungsten anti-bounce weight system.  (Bolt bounce wasn't an issue with the standard mutant spring and buffer)

I also ended up putting a Strike Ind. adjustable gas block on it for suppressor use.

Carrier from side:



Bottom:



End:

I don’t have any full auto guns but I can speak to the psa ks47 10” barrel.  I have only shot it a few times unsuppressed and it ran fine.  I started to run into problems when I suppressed it.  The first can I was hoping to dedicate to it but it was way too overgassed and was causing bolt over round malfunctions.  I put an adjustable gas block and it helped but didn’t fix the problem.  That was an aac cyclone that didn’t have a home because it’s so heavy.  I had better success with my nomad Ti but Ti is not the best for pistols.  Funny thing I got my nomad 30 out of jail today but with the brace rule I am going to swap to a 16” barrel.  I am not going to sbr this gun because of reasons. I am going to sbr two other pistols.  I would sell you my barrel but I’m not going to pull it until I have a replacement barrel in hand.  
I doubt you care about accuracy but hornady and golden tiger would do 1.25” 5 shot groups at 100 yds.  

I have been looking at the faxon gunner 16” barrel but haven’t found one in stock yet.  Does anyone have experience with that one?  

I also have an odenworks adjustable buffer but haven’t pulled it out yet.