Enlarge Gas Port vs Lighten Buffer/BCG... What's the difference?

There are 9mm ARs that run reliably. 7.62x39 has way more gas.

Its not the round. Its the build quality.
Those reliable 9mms are blowback.

762x39 barrels ned very different gas tube lengths than 556. While a carbine length gas tube on a 16" 556 barrel is often overgassed, a 16" 762x39 barrel with that same carbine gas is just about right. Anything less than 16" and the dwell is reduced to not enough to cycle properly. your 10.5" 7.62x39 barrel probably should have been made with a pistol length, or even shorter, gas system. Remember, there is a 30 cal hole that openes when the bullet exits the barrel, not 22 cal, thats almost 100% larger gas scavenging hole.
 
If your 95 year old grandma drives her 1970 El Dorado into a farmer’s market she probably won’t notice a thing. Once that big heavy pig (the car not grandma) gets moving nothing’s stopping it. Objects in motion tend to stay in motion so mass smooths things out. On the flip side objects at rest tend to stay at rest so that Caddy needs a 460 big block to get moving.

Same deal with your AR. A standard mass bolt carrier / buffer combo will run over lots of feeding problems but you need to make sure it’s getting enough power to cycle it all the way back so that buffer spring can be fully compressed and build up enough stored energy to handle stripping that next round.

So two choices:
1. Stop using pussy ammo
2. Make sure that enough (all) the gas pressure from your pussy ammo gets fed into the gas system. (Basically this means use a longer gas systems or if that’s not possible a larger gas port*.)

*If #2 isn’t happening for some reason make sure everything is aligned, sealed, clean, and unbent before you start drilling. Then go slow when opening up the gas port. Too much gas will cause problems too.
 
On a 10.5" barrel where is the gas port located? How many inches back from tbe muzzle?
Anyone have any idea what range the powder burn rate is in todays x39 ammo.?
I cant see why a AR cant function on even shit ammo. I just think of how big the piston/gas cyl is on a SkS or Ak and those tend to function even on light cast loads.?
Fun stuff
 
@Atlantis

this build has many flaws, the major one being 7.62x39 in an AR and then more specifically the carbine gas on a short .30 cal barrel. there isn’t enough pressure to cycle well. It would need a pistol length gas which in turn will amplify the inherent 7.62x39 problems even more. Short barreled 7.62x39 Is going to shred bolt lugs all day long. the 7.62x39 AR is inherently problematic from magazines to bolt face/lugs. Shorten the Barrel and problems will only worsten Because it will either need a shorter gas system or a larger gas port both of which are bad. I would avoid such a build Completely.

in terms of your cycling problems, i Would try a Nemo 300 blk spring then avoid your barrel manufacturer for good. Grid defense?? who? Jesus just read their website disclaimer they don’t ensure function w steel cased ammo....there you go they know their 10.5 is undergassed so they blame it on ammo to allow Continued sales of garbage product. Stay the F away.

I understand the bolt lug issue due to the case diameter. If I open up the gas port on a carbine length system, you're saying that will have the affect of increasing the stress on the bolt lugs?

ALWAYS ARMED 10.5" 7.62x39 PISTOL KIT - MAGPUL FDE This was the vendor, and while they are not tier 1 or even close, they ship anywhere legal, which is more than you can say for some other companies. I believe the use Faxon barrels, and from what I've read, Faxon used to ship 10.5 carbine length barrels with like a .07 gas port a la the 5.56 counterpart. They have since addressed this and apparently ship with 0.091 gas ports and most customers are quite happy with the results.

I am wondering if I have some "stackup" of various drag/issues. Weak ammo starts with low gas pressure, gas system is already on the hairy edge of being properly sized/lengths, slight drag of the gas tube in the gas key, slight drag in the chamber due to rough finish/steel ammo being tight.

I'm thinking I'll give the chamber a nice cleaning and maybe a polish with some flitz, go through the gas key/gas tube alignment and get it perfect, and see how that runs.

If not I'll go ahead and open up the gas port a little more.

If your 95 year old grandma drives her 1970 El Dorado into a farmer’s market she probably won’t notice a thing. Once that big heavy pig (the car not grandma) gets moving nothing’s stopping it. Objects in motion tend to stay in motion so mass smooths things out. On the flip side objects at rest tend to stay at rest so that Caddy needs a 460 big block to get moving.

Same deal with your AR. A standard mass bolt carrier / buffer combo will run over lots of feeding problems but you need to make sure it’s getting enough power to cycle it all the way back so that buffer spring can be fully compressed and build up enough stored energy to handle stripping that next round.

So two choices:
1. Stop using pussy ammo
2. Make sure that enough (all) the gas pressure from your pussy ammo gets fed into the gas system. (Basically this means use a longer gas systems or if that’s not possible a larger gas port*.)

*If #2 isn’t happening for some reason make sure everything is aligned, sealed, clean, and unbent before you start drilling. Then go slow when opening up the gas port. Too much gas will cause problems too.

Yep, will do, although worst case you can dial gas down with an adjustable, so that's comforting. I think I'm going to try and eliminate all possible causes of drag first though.
On a 10.5" barrel where is the gas port located? How many inches back from tbe muzzle?
Anyone have any idea what range the powder burn rate is in todays x39 ammo.?
I cant see why a AR cant function on even shit ammo. I just think of how big the piston/gas cyl is on a SkS or Ak and those tend to function even on light cast loads.?
Fun stuff

I believe its 2.7" from gas port to muzzle. No idea on powder burn rates
 
I understand the bolt lug issue due to the case diameter. If I open up the gas port on a carbine length system, you're saying that will have the affect of increasing the stress on the bolt lugs?

ALWAYS ARMED 10.5" 7.62x39 PISTOL KIT - MAGPUL FDE This was the vendor, and while they are not tier 1 or even close, they ship anywhere legal, which is more than you can say for some other companies. I believe the use Faxon barrels, and from what I've read, Faxon used to ship 10.5 carbine length barrels with like a .07 gas port a la the 5.56 counterpart. They have since addressed this and apparently ship with 0.091 gas ports and most customers are quite happy with the results.

I am wondering if I have some "stackup" of various drag/issues. Weak ammo starts with low gas pressure, gas system is already on the hairy edge of being properly sized/lengths, slight drag of the gas tube in the gas key, slight drag in the chamber due to rough finish/steel ammo being tight.

I'm thinking I'll give the chamber a nice cleaning and maybe a polish with some flitz, go through the gas key/gas tube alignment and get it perfect, and see how that runs.

If not I'll go ahead and open up the gas port a little more.



Yep, will do, although worst case you can dial gas down with an adjustable, so that's comforting. I think I'm going to try and eliminate all possible causes of drag first though.


I believe its 2.7" from gas port to muzzle. No idea on powder burn rates
Mid size “large” cal cartridges can be problematic when you need a gas volume/pressure to provide a secondary use.
X39 has low case capacity 35 gn water or so. Making slow powders almost usless and faster powders dont produce the gas volume. So shit ammo can be a problem also. Not to mention the over all low 45k psi chamber pressure.
BUT if I can get cast loads to run in a SKS/AK AR 15 , M1 garand and M1a
Theres a solution to getting fmj x39 running
 
Mid size “large” cal cartridges can be problematic when you need a gas volume/pressure to provide a secondary use.
X39 has low case capacity 35 gn water or so. Making slow powders almost usless and faster powders dont produce the gas volume. So shit ammo can be a problem also. Not to mention the over all low 45k psi chamber pressure.
BUT if I can get cast loads to run in a SKS/AK AR 15 , M1 garand and M1a
Theres a solution to getting fmj x39 running

Yeah I am positive I can get this thing running well.

The more I am reading the more I wonder if maybe its a BLESSING that this thing is a touch under gassed. In the sense that if I get it running well on this level of gas/pressure, with such a short dwell time, then maybe I'm saving the bolt from failure? Might just mean I gotta keep this gun super clean at all times, which is sort of what I do anyways...
 
OK, I've got a similar problem and looking for advice on a Smithy that can fix it.

Bushmaster CArbon15. Gotta be circa 2000 or so vintage.

It FTF's something fierce. Every several rounds.

The problem is Carbon15 is not technically an AR. The whole upper is wonky.

I'm gonna assume it's also a gassing problem. I'm not SURE (it's been a while since I pulled it apart) but I don't think you can swap buffers in it.

Any local-to-SE mASS gunsmiths you'd recommend to at least call about it???? I'm not inclined enough to fix this and I'd love to get it fixed.

Thx.
 
Aren't there diffrent buffer springs to boot to add to the issue.
its a dynamic system.
1) amount of gas, pressure of gas and dwell time of gas at pressure.
2) weight of bolt and buffer weight. (need to add these together)
3) Spring rate, any preload on it etc.
all have to be tuned and work with each other.

for short pulse of gas i would assume you need lightest weight so you capture the energy and get moving with a spring soft enough to allow to travel all the way back and have enough spring pressure to strip and load the next round.
For a pistol i would assume the gas pulse is shorter than standard due to its port location, so high pressure (hotter ammo) or lighter buffer is need to make it work.
 
OK, I've got a similar problem and looking for advice on a Smithy that can fix it.

Bushmaster CArbon15. Gotta be circa 2000 or so vintage.

It FTF's something fierce. Every several rounds.

The problem is Carbon15 is not technically an AR. The whole upper is wonky.

I'm gonna assume it's also a gassing problem. I'm not SURE (it's been a while since I pulled it apart) but I don't think you can swap buffers in it.

Any local-to-SE mASS gunsmiths you'd recommend to at least call about it???? I'm not inclined enough to fix this and I'd love to get it fixed.

Thx.
what type of FTF , is the round not coming up and out of the mag ? Thery also have a plastic feed ramp that gets beat to shit.
like any AR type rifles magazines are a source of a lot of problems
 
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