What is the shortest barrel lenth you can use w/o having to move the g

[kellysguy 1]

How short can you go before you have to move the gas block back to get it to work proprly? Notice I said not MOVE, I didn't say not MODIFY. I know gas system mods will be needed, but I'm talking about shortest barrel without moving the block back.

 

 

What would need to be modified? Thanks

[RDSWriter 5]

I'm sure someone with much more experience will answer more accurately than me. But from what I've read on this board, 11" is about as short as you should go without moving the gas block. Tony indicates that he moves all his gas blocks if he's going shorter than 16"... which effectively means that all his SBS shotguns have the gas block moved. Now keep in mind that Tony keeps his gas ports the original 0.073" to minimize fouling in the gas block. C&S Metall-Werkes indicated that they go to 12.5" with the block in the original location and angle drill all four of the original ports to 0.113" and add a fifth port opposite the plunger in the gas block. Both of the above specify that their S12s will run light bird loads.

 

FYI, Tony is the one who indicated 11"... but he also indicated that at that short of a barrel you'd want to drill four ports at 0.101", angle them way back when drilling and would be capable of only running heavy loads.

 

On a personal basis, I cut my SBS to 13" and opened up all four gas ports to 0.0937". I have yet to get to the range, but this should easily enable me to run heavy bird loads... and maybe even light bird loads if I replace the front recoil spring with a 12# 1911 spring.

 

Start by ensuring that your Saiga12 is THOROUGHLY broken in (over 200rds). Then once you cut and overdrill the holes, you have some additional tuning options that include:

(1) polishing the hammer face and underside of the bolt carrier to ensure more fluid cycling of the action

(2) replacing the front recoil spring with a 1911 spring

(3) adding a fifth gas port and opening up the 'gas hole' in the gas block

(4) re-contouring/polishing the fee ramp portion of the barrel to provide more fluid loading of rounds

(5) use a short Russian choke (medium) or (full) OR use a Polychoke and increase the choke on low power birdshot

Edited March 24, 2008 by RDSWriter

[BobAsh 582]

^ Excellent answer.

[matt 0]

where are the four ports at ? mine appears to only have two in a recess in the gas block chamber plus one larger that ports excess out to the rear of the plunger. Does the block have to be removed from the barrel to open the existing ports or are they opened by going thru opening of the chamber at an angle downward?

[VanKiller 322]

You can use the S-12 cut to 11 1/2 inches with no mods to the gas port. However you will need to use high brass loads or have someone thread the barrel for choke tubes. The weapon will function with lighter loads when it has been restricted with a mod or full choke tube. This was the configuration I used on the "Beast" in the Vankiller video. The orginial configuration of the ports had quite a bit of development at the factory and I am a firm believer in keeping as much crud out of the gas system as possible. This seemed to work for my guns as we fired 2500 rounds at the SWAT challenge in Moyock that year ,without a hitch,without a cleaning, thru the FA S-12. Got a qute little blurb in Maxim magazine and a lot of attention from the LEO guys, but in reality the FA guns are more for show than use. Please note; you will need to do are your ATF paperwork before you cut the unit.....

Edited March 24, 2008 by VanKiller

[TonyRumore 1,332]

This thread comes up over and over again. Time to sticky it up somewhere.

 

BDSWriter/VanKiller covered it well.

Tony

[kellysguy 1]

Yeah, I don't think I could have asked for a better answer. Everything I needed to know. I know I have to get the paperwork first, but good you mentioned and I agree this would make a nice sticky. I'd want to have it rethreaded and choked and would also like it to run off of light (read cheap) birdshot.

 

I've been thinking about the rear angled ports latey. Is this for reducing fouling? It would seem that angling towards the rear would lower gas cylinder pressure creating a "syphon" effect similar to the "rearward" cant of the feed port on a sandblasting gun. Don't get me wrong, I'm not saying it would actually try to suck the puck back, but rather achieve a lower pressure than could otherwise be obtained if drilled forward.

 

I should be (hopefully) going tommorrow to start my NFA paperwork on the 8" Tromix I want to buy, but I'd like to take a stab at modding one myself. Last time I held something that felt that good I married it. I figure with the backlog Tony has it'd be a good idea to go ahead and pick one up if I can. Plus, the wife say I could.

 

My wife ROCKS !!!!!!

Edited March 25, 2008 by kellysguy

[BobAsh 582]

...angling towards the rear would lower gas cylinder pressure creating a "syphon" effect similar to...a sandblasting gun.

 

No, the gas pressure that operates the gun is created when the bullet passes the gas ports, but has not yet left the barrel. It's not the same as air blowing over a hole. This is why you get more gas when you move the gas block rearward, away from the muzzle; the bullet has a longer dwell time in the barrel.

Edited March 25, 2008 by BobAsh

[kellysguy 1]

I understand that, but a majority of the gas charge is still moving past the ports going forward. Making the 180 degree "turn" more gradual then immedatly would lessen restriction requiring less pressure for the charge to make the turn. But I can see where a foreward swept port would allow more crap to go into the chamber and cause fouling quicker though.

 

I can see the argument both ways. Since it's a unilateral expanding gascharge with allegedly equal (should be) pressure in all areas of the barrel, a straight shot back would make sense. I can see that, but since the load it travelling down the barrel (forward), so is a majority of the gas charge, which is passing the rearward angled gas ports, which should (theorhetically) create a lower achieved pressure than if the port was facing forward. The charge is travelling down the barrel at a greater velocity than in the gas chamber, therefore; the barrel charge should be of lower pressure (increase in velocity = decrease in pressure). Especially when you consider how quickly barrel pressure should be dropping because of increasing volume. You have to remeber, at one point, both areas are of equal pressure. (probably just after the load passes it, and then the barrel will start to drop pressure quickly (in relation to the gas chamber) due to increasing volume. The puck is about the same dia as the barrel (probably not a coincidence (sic?)) so it might not drop as quicky as I believe.

 

If it were facing forward, more enegry would be imparted on reg face negating some advantage. But the other way, after the gas hits the puck, energy is imparted to it, but the gass still has to change directions 180 degrees to fill that area first to create pressure. AAAAAAAAAAAAAAAgggghh!!!!!!!!!!!!!!!!!!!!!!

 

I guess the only way to see if my theory was correct, would be to mount an AK gas block backwards (on an AK) and see if it produced greater pressure than if mounted forward. I'd do it to one of mine, but I don't have a way to measure it.

 

Rereading over it and thinking again, it might create an overall greater pressure, but it make take more time to do so as it doesn't impart energy directly to the puck face, which would require a longer barrel to give the gas time to create more pressure. So I guess rearward facing ports employs pulse technology, which is a whole 'nother can of worms. hhhmmmmm.

 

 

I am by no means an expert, It's just a thought. Like I said, it makes sense to me both ways and now I think my brain is smoking. Probably due to rapidly decreasing volume.

 

p.s If y'all sticky this thread, leave this crap out.

Edited March 25, 2008 by kellysguy

[kellysguy 1]

Rethinking once again, a forward facing port would have a cheese grater effect on the cup, wad and shot so if it did work, I don't think it'd be for long.

 

as Rosanna Rosanna Danna would say: "...........Neeeevermind"

Edited March 25, 2008 by kellysguy

[BobAsh 582]

Angling the ports cannot increase or decrease the pressure. Pressure does not drop after the bullet passes since the combustion gasses are expanding.

[RDSWriter 5]

Bob nailed it in that the direction of the holes does not materially impact the pressure of the system. Kellysguy nailed it when he indicated that THEORETICALLY.

 

Increased velocity only equals a drop in pressure under certain conditions. Bernoulli's priciple is based on the conservation of energy and the resulting equation for this relationship specifies that there will be a lowering of pressure in system where the flow velocity is increased. That being said... I think principle also depends on constant temperature and a constant density. While I don't know the specific thermodynamic curves of Temperature and Density for gases from a fired gun... obviously both are changing and as such I don't think Bernoulli's equation holds.

 

Also the burning of the power exponentially increases the volume of the 'gas' in the (closed) system due to the conversion of a solid (The propellant) to a gas state. Since the exponential increase in volume (due to combusted gases and increased temperature) is far greater than the increased volume of the bore as the bullet moves forward, there is an exponential increase in pressure.

 

The best way to understand why the ports should be angle back is that (1) the pressure/energy that is lost due to their angle is inconsequential, and (2) there is a beneficial impact of directional gases. The distance from the starting point (inside the barrel) to the finish point (in the gas block) is too small to materially change any pressures. Per Bob's comment... the expanding gases will overcome any impact. Theoretically (at a micromicromicro level) the angle impacts the transferred energy to the gas block area... but the mathematical impact of the changed angle is probably so small as to need a computer model to even begin to calculate it. Also, this inconsequential loss of energy is probably 1000s of times smaller than the benefit of using angular mementum of the gas velocity and the energy it imparts on the gas piston (puck) due to the directional flow of gas that hits the face of the 'puck'.

 

Finally... if someone wants to ask me some questions on this topic, DON'T. That's about as much engineering as I've remembered in 15 years. While my explanation would probably make a practicing engineer laugh... it should be (overall) correct. If someone else cares to clarify it in easy-to-understand terms... feel free to correct me if I'm wrong.

Edited March 26, 2008 by RDSWriter

[kellysguy 1]

I'm sure someone with much more experience will answer more accurately than me. But from what I've read on this board, 11" is about as short as you should go without moving the gas block. Tony indicates that he moves all his gas blocks if he's going shorter than 16"... which effectively means that all his SBS shotguns have the gas block moved. Now keep in mind that Tony keeps his gas ports the original 0.073" to minimize fouling in the gas block. C&S Metall-Werkes indicated that they go to 12.5" with the block in the original location and angle drill all four of the original ports to 0.113" and add a fifth port opposite the plunger in the gas block. Both of the above specify that their S12s will run light bird loads.

 

FYI, Tony is the one who indicated 11"... but he also indicated that at that short of a barrel you'd want to drill four ports at 0.101", angle them way back when drilling and would be capable of only running heavy loads.

 

On a personal basis, I cut my SBS to 13" and opened up all four gas ports to 0.0937". I have yet to get to the range, but this should easily enable me to run heavy bird loads... and maybe even light bird loads if I replace the front recoil spring with a 12# 1911 spring.

 

Start by ensuring that your Saiga12 is THOROUGHLY broken in (over 200rds). Then once you cut and overdrill the holes, you have some additional tuning options that include:

(1) polishing the hammer face and underside of the bolt carrier to ensure more fluid cycling of the action

(2) replacing the front recoil spring with a 1911 spring

(3) adding a fifth gas port and opening up the 'gas hole' in the gas block

(4) re-contouring/polishing the fee ramp portion of the barrel to provide more fluid loading of rounds

(5) use a short Russian choke (medium) or (full) OR use a Polychoke and increase the choke on low power birdshot

 

 

 

Keep us posted if it'll run light birdshot after you change the spring out.

[VanKiller 322]

Looks like you've pretty well got the theory nailed down........................NOW , for the important information, ,,,Got any pics of your wife, and does she have any sisters.............. .........Sounds like a great gal....

[kellysguy 1]

Hehehehe, I'm not posting ANY pictures of my wife ANYWHERE on the internet. Yes, she is the best person I've ever met and well worth the 34 years I spent looking for her. Unfortunatly, my sweet, perfect and beautiful wife in whom I'm well pleased flaked on me last night when she asked "what do you have planned for tommorrow". I explained I was going to start the paperwork.............. She had know for a week this was going to happen but appartently something from earlier in the day bothered her, planets weren't alligned correctly or something and I got "the look" and several concerned questions. I had been preping her for 7 days and just sold my boat for the purchase. I think the class III stuff concerned her. What can I say?!?!?!?! She's a woman and they have a way of taking ALL the fun out of things sometimes. If you're married, you know what I mean. She killed my buzz enough to turn me off to the idea for the time being. I have a bunch of inventory I need to move and just haven't felt like doing it lately. I guess I'm being punished for it. If only she would have used sex as a weapon I try to include her in everything I do out of love but I have a new policy now. SHE WILL KNOW NOTHING from now on. I informed her of this earlier today. I should have known better then to bring her in the shop when I first looked at it but oh well.

 

I think I'm gonna go downstairs and have a talk with her about this. This was the first time in a LONG WHILE that a gun made me feel this way. I'm sure y'all know what I mean. ................Chics !?!?!?!?!?!?!?!?!?

 

 

And no, no sisters. One of a kind.