Author Topic: Ejection angle / gas tube modifcation  (Read 179 times)

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Ejection angle / gas tube modifcation
« on: June 27, 2020, 04:47:45 PM »
Has anyone ever tried the ejection mod where you drill 3 relief holes in the front of the gas tube? I know that it tames the ejection distance down to a normal rate instead of 20ft away from you, and it also cuts down on recoil.

My question is, does it also change the ejection angle? I'm tinkering with the SKS/Tavor bullpup project and trying to get the rifle to eject more sideways that upwards. I've already done the rear ejection port reshape, but the issue I'm having is the cases hit the top of the stock before they even make it to the receiver opening.

I could remove more of the stock and make a larger window, but would prefer not to. If the gas tube mod helps kick the rounds more sideways, that would be awesome.

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Re: Ejection angle / gas tube modifcation
« Reply #1 on: June 27, 2020, 07:18:00 PM »
Hi Sal!

While I don't know the mods specifically you speak of, I was pondering this the other day shooting my Classic and noticing it was the most reasonable SKS I have ever had regarding finding your cases. It is also the most heavily fired SKS I have ever seen...much less owned.

The majority of the cases landed within an 18 inchish circle about five feet to the right, and a yard or so behind me. A couple shot straight up, and a few feet in front of me. I want to say they were the first and last.  I only did a couple of strippers as this was a function test after rehabilitating the weapon to being safe to fire.  Two different types of ammo both yielded similar launch and landing results.

It got me to contemplating the forces involved in cycling the weapon, and what might have affected it to do this.  Unfortunately, I am leaning towards a lot of it being a number of factors, that might be hard to sync.  Based on the sound of the gun functioning while firing, I kind of suspect that the recoil spring might be a little less forceful...which I suspect would influence timing.  As far as the upward push of the next round happening simultaneous with the extraction, the follower spring force AND which side of the stack the next round is coming from would influence things as well. Additionally, the force the extractor puts on could come into play also, both with the spring strength and the shape of the mating surfaces between the shell and its grip. 

I was forced to use a replacment stock and a borrowed gas tube, which while a little wobbly, I don't think affected pressure.  A slightly longer handguard has since tightened it to normal, but I haven't fired to confirm any difference that may have made.

Not knowing about the holes in the gas tube you are talking about adding, it does make me wonder if you could expand the vent holes behind the piston head in the tube to reduce back pressure....but then you also could gain the same effect with a weak op rod spring.

All of this of course could also be adjusted by the size of the gas vent in the barrel. A variable gas valve might be the easiest way to fine tune for results....although I think the concert of spring activity involved might have more of an impact on results more than simply adjusting the pressure initiating the whole mess.

Now I know why I stopped trying to think about it, when I should have been happy coming home with 18 out of 20 fired shells.   chuckles1

Your circumstance sounds as if it's more mission critical for function, hopefully my pondering this lends some insight towards a solution.
Good to see you posting...it seems like it's been awhile.  :)


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Re: Ejection angle / gas tube modifcation
« Reply #2 on: June 27, 2020, 07:57:21 PM »
I think if a guy had a spare gas tube, it would be worth a try.
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Re: Ejection angle / gas tube modifcation
« Reply #3 on: June 27, 2020, 08:29:07 PM »
Hi Sal!

While I don't know the mods specifically you speak of, I was pondering this the other day shooting my Classic and noticing it was the most reasonable SKS I have ever had regarding finding your cases. It is also the most heavily fired SKS I have ever seen...much less owned.

The majority of the cases landed within an 18 inchish circle about five feet to the right, and a yard or so behind me. A couple shot straight up, and a few feet in front of me. I want to say they were the first and last.  I only did a couple of strippers as this was a function test after rehabilitating the weapon to being safe to fire.  Two different types of ammo both yielded similar launch and landing results.

It got me to contemplating the forces involved in cycling the weapon, and what might have affected it to do this.  Unfortunately, I am leaning towards a lot of it being a number of factors, that might be hard to sync.  Based on the sound of the gun functioning while firing, I kind of suspect that the recoil spring might be a little less forceful...which I suspect would influence timing.  As far as the upward push of the next round happening simultaneous with the extraction, the follower spring force AND which side of the stack the next round is coming from would influence things as well. Additionally, the force the extractor puts on could come into play also, both with the spring strength and the shape of the mating surfaces between the shell and its grip. 

I was forced to use a replacment stock and a borrowed gas tube, which while a little wobbly, I don't think affected pressure.  A slightly longer handguard has since tightened it to normal, but I haven't fired to confirm any difference that may have made.

Not knowing about the holes in the gas tube you are talking about adding, it does make me wonder if you could expand the vent holes behind the piston head in the tube to reduce back pressure....but then you also could gain the same effect with a weak op rod spring.

All of this of course could also be adjusted by the size of the gas vent in the barrel. A variable gas valve might be the easiest way to fine tune for results....although I think the concert of spring activity involved might have more of an impact on results more than simply adjusting the pressure initiating the whole mess.

Now I know why I stopped trying to think about it, when I should have been happy coming home with 18 out of 20 fired shells.   chuckles1

Your circumstance sounds as if it's more mission critical for function, hopefully my pondering this lends some insight towards a solution.
Good to see you posting...it seems like it's been awhile.  :)


Yeah....it's been a while since I've posted. I still lurk from time to time.

The gas tube mod puts three new holes at the front of the gas tube, which vents some of the pressure and makes the gas piston hit easier. You start with three #45 drill holes in three very specific places, and then fine tune the system to not throw cases so far, but still cycle the action.

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Re: New 9/30/20 Ejection angle / gas tube modifcation
« Reply #4 on: September 30, 2020, 12:40:28 AM »
Hi - I know this thread is sorta old, but there is not a positive answer that I can see.  I did this mod last year on my 1951 Tula (HA 18xx) light refurb. (Forced #s on stock and mag- Canada pinned)   I actually made the third hole 5/32 rather than 64ths, and ended up with cases landing within 10 feet.  and still cycles Barnaul commercial ammo as well as mil.
    As well, there is a mod that opens the ejection port, basically making the back edge square rather than radiused.  Again, I extended beyond the advised mod, and cut it about 1/4" back from the OEM port edge.  This changed the direction to almost 3-o'clock - direct right.  Easy to pick up all the cases thumb1  I left it in the white cause I plan to take it back a bit further to make it 3-o'clock  or even 4.  I always have to take the right end of the shooting line ;-0

« Last Edit: September 30, 2020, 12:49:29 AM by Fasteddie01 »
51 Tula refurb ( so far )

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Re: Ejection angle / gas tube modifcation
« Reply #5 on: September 30, 2020, 11:53:25 PM »
Please post your pics of the gas tube holes and location.

Years ago Bushmaster did this mod to their rifles as they were patterned after the AK gas system to improve cycling.They called it the "Galil Mod".
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Offline Fasteddie01

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Re: Ejection angle / gas tube modifcation
« Reply #6 on: October 01, 2020, 12:05:31 AM »
Hey Bubba - I sold the rifle with those mods and don't have a pic of the  tube holes.  Here's a detailed description of that one (#2) and other mods.  I have a post further down on that thread, too.  Canada has 5-round mag limits for all centre-fire rifles, so there are several ways to do that too.
Ed
http s://www.canadiangunnutz.com/forum/showthread.php/603359-Helpful-hints-to-DIY-for-Red-Rifles
51 Tula refurb ( so far )