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Posted: 6/16/2003 7:16:06 PM EDT
Hanging with my Class III (Title 2??) MFR friend here in ID and he's 'hired' me (I get paid in beer and board) to work on this Saiga project with him. That said he's pretty much left me to my own devices as he is out quite a bit on various errands around town, so I'm left to seek info elsewhere (after talking to him when I got in last night he said "cool, well, so long as you dont take it off the property, its your baby"). After tearing down a Saiga-12 that we're going to build the prototype out of, and bringing the barrel to contract-specs, I've discovered it wont cycle.

No biggie, expected that. Took the screw out of the top of the gas chamber and am preparing to drill out the little "spigot" at the bottom of the chamber which looks a bit like a tiny LEGO sized manhole cover with three ports in it to let the gas through. My question before I lay titanium to steel here is, will this work? Will opening up the gas port result in a rise in pressure in the gas chamber, thus sending the bolt backwards with more oomph?

My observation today on the test-fire range (the field behind Bob's house) was that the "low impulse" slugs and birdshot (AA, 2&3/4" stuff I bought for shooting clays) will get the action to come back a little, but never far enough eject the shell and stick a new one in. With a full mag it only comes back a quarter-inch, but with out a mag in the gun it comes back far enough to cock the hammer, but not far enough to eject the shell.

Bob wont be back for a couple of hours (over at a significant other's place...) and I'd like to have this thing cycling before he gets home, so any help would be greatly appriciated.
Link Posted: 6/17/2003 2:24:01 AM EDT
OK Reb, lets get you up to speed on the theory. Successful operation has to do with the Time/Pressure curve. (Huh?)
Here's how it works.
Time: The barrel is under pressure while the projectile is in it. When the projectile leaves the barrel the pressure goes to zip in a micro-second. The shorter the distance between the end of the barrel and the gas port, the less time available to pressurize the gas operated mechanisms that cycle the action. (The guys that build short barreled AR-15's have significant problems in this area. Perhaps a look at the AR-15 full auto forum will also help with this).
Pressure: In order to put the correct amount of pressurized gas against the piston to cycle the action, the gas port is a specific size. This restriction prevents excessive gas pressure from the barrel from blowing the bolt out the end of the gun and into your face. As the projectile travels down the barrel, the pressure drops. (reflected in the thickness of the barrel wall at the chamber v.s. the thickness of the barrel wall at the muzzle). The farther down the barrel the gas port, the lower the pressure available at the restriction into the gas system.
It gets more complicated...
The pressurized gas going into the gas port to cycle the action, can flow out of the gas port and back into the barrel just as fast. This is why the Krinkov's have a "pressure chamber" on their short little barrels. It helps preserve the pressure in the barrel to help cycle the action.
Your gun won't cycle because even when you open the port to maximum, the restriction is too small and the time it's pressurized is too short, to admit sufficient gas into the action. Even if you open it up way over size, if the gas port is too close to the end of the barrel, it will simply flow back into the barrel before it moves the action.
The solution is to move the gas block back toward the chamber and shorten the gas piston. You could try to open the gas port a little. However, the shorter the time window, the more sensitive the gun will be to the ammo used. It might work with a deer slug but not with a skeet load. To overcome ammunition variables is why the gas port is adjustable in the first place! You will have to be very careful and think about what you are doing at this point. This project probably can be made to work, but it probably will only work with one type of load. If you tune it to 1 oz. skeet loads the gas system will be way over powered and potentially dangerous with 1 7/8 oz. hunting type loads.
As you shorten the barrel and the time window is smaller, the port opening must be larger. It seems so simple. It's not that simple. Be very careful.
And we haven't even discussed recoil springs yet! This is an advanced project. You are re-engineering the gas system. Get yourself some graph paper and work out the theory, time on the base line and pressure on the vertical scale. Keep track of the number of little squares under the curve as you increase and decrease the time segment, with the pressure constant. This will give you a feel for the energy available to operate your system.
Hope this helps. I can't solve this for you. I just wanted to help you frame the problems.
Link Posted: 6/20/2003 2:53:05 AM EDT
Link Posted: 6/20/2003 11:54:09 PM EDT
I just bored the whole thing out and now it'll cycle buck and above, which is good enough for what we need it for. I made the mistake of shooting some 3" mag slugs through it this morning and have a very sore shoulder. OTOH I am amazed to no end that the SAR2 stock thats on it held up through the pounding.
Link Posted: 6/21/2003 4:00:21 AM EDT
Just one more tip. Check the rear trunion for faint marks left by the bolt. If it is striking the rear trunion, it will eventually drive the trunion out the back of the receiver. You might be wise to put in a buffer as a shock absorber. This is just a suggestion. It happened to me once.
Link Posted: 6/21/2003 9:42:16 AM EDT
MG- theres a huge gunshow today and tommorow but pay is a bit of an issue between me and my employer :) (Defense Finance & Accounting Services) and I dont know if I'll be able to make it down.

I'm way ahead of you on that about the buffer situation though. I've got a feeling this thing will probably require a new buffer every so often....the buffer in combination with a good stock pad will proably help the recoil situation quite a bit too...well, we'll see what I can manage.

Thanks.
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