Jump to content
Sign in to follow this  
BobAsh

SBS Technical Info Thread

Recommended Posts

Due to all the interest in building your own SBS, I have compiled this thread based on expert testimony. Feel free to add anything that you feel I have missed.

 

How Tromix makes an SBS

I finished up the micro S-12 with the 8" barrel today and took it to the range. The thing ran like a scalded dog with low base target ammo as well as the heavy stuff. The gas system is backed up 4". It cycles perfectly with low base Walmart shells, both Remington and Federal. I also ran some S&B double ought buck through it as well. It runs great.
On NoAim's 12" gun, I moved the entire gas system rearward 2". On the 8" gun, I moved the gas system rearward 4".
There were several guys before me that made S-12's around 12", but all of the examples that I saw, they had left the gas ports in the stock position. I run a single gas port and modify the gas regulator to work like an RPD machine gun. Albeit, the RPD has three gas positions and the S-12 only two. Obviously, the guys got their 12" guns to work, but I have not been able to confirm that they would run on super light 1oz discount ammo. You need a hell of a lot more gas to run those, then the big 3" mags. So, my gut feel is that it will work just fine with high powered shells, but probably will not with the super light stuff, even if a 4th gas port is drilled. The 19" guns have 4 ports anyway. It's the 22" guns that only have 3ea; .113" is about as big as I would want to go before the holes get close to breaking out. 3ea .113 holes will run a 16.5" gun with super light shells.
My personal gun is a 12". I don't care for the blast that comes off those 8" tubes right in front of my face. There is no definitive answer as to how short you can cut the barrel, since the power level of the shell makes such a huge difference as to whether the gun will cycle or not. If you want to shoot bird shot and not have enormous gas ports, the gas block needs to be moved back 2" for a 12" gun and 3.75" for an 8" gun. With that gas port position, you can use the factory .073" size gas port holes. However, if you don't mind running over-size gas port holes and only shoot magnum shells, you can cut the barrel back to 11" and leave your factory gas system as-is.
Yes, the gas system on that gun was moved rearward about 3".That particular gun is a bit of an oddball however. Since then, I have standardized the gas block positions to either 3.75" back on 8"- 10" guns, and 2" back on 11-14" guns.
Move the gas system back 2" for the 12" gun. Then you can run the standard gas port configuration of 4ea .073" gas ports.
Holes are welded up on the 10" and 12" guns. On the 8" guns, the barrel is actually cut off BEHIND the old gas ports.
Edited by BobAsh
  • Like 2

Share this post


Link to post
Share on other sites

Max gas port size?

 

...you can pretty much cut the port(s) as big as you want. However, the larger you go, the more fouling you will get.

 

I can tell you though; the largest factory port I have ever seen was on the early .410 guns.....they used a single .169" port from the factory.

 

My personal 12" gun uses a single .145" port and it has worked well for years.....and I never clean it. I rebuilt the gun recently and went with a mag well and SRT, and contemplated welding it up and going with a 4 port setup, but decided against it, since the gun has been working flawlessly since the day I built it with the large single port setup.

How short can you go without mods?

 

All Saiga shotguns are on the ragged edge of running, right out of the box. I wouldn't cut it down at all if you want it to run light loads guaranteed. Of course with SOME guns, you can get away with it, but who knows with yours. They are all different.
Jeez.....I don't know how many times I'm going to have to answer this question......The Saiga shotguns, of all gauges, are on the ragged edge of not running with standard off-the-shelf U.S. birdshot loads right out of the box. Some guns may work cut down, but most will not. Cutting it down ANY is not a good idea unless other changes are made to the gas system.
I don't like to go below 16", without moving the gas block rearward. The factory gas ports are about .073". I would re-drill them .101" and angle them WAY back, so you have a bit of "trough" rather then a straight-in hole. There is no magic with the gas ports, you can drill them as large as you want, but the trade off is increased wad fouling in the manifold, so you will have to clean it more often. On the Tromix 8" guns, the four gas ports remain at the factory .073" size and still cycle light loads without a problem. Moving the gas system rearward is a lot of work, and not really necessary in your case, unless you want to shoot birdshot type loads.
There's no silver bullet here. The shorter you go, the larger the parts need to be and that will increase your fouling. Or you can just heavy loads and leave the ports smaller. You can actually go down to 11" with the stock gas system, if you don't mind having large gas ports (higher fouling) and only fire heavy loads.

SBS and Top Cover

 

You need to weld an extension hood back out over the top cover or it will eventually pop out of its track in the front.

You can get away without it about 50% of the time, but the other 50% of the time the gun will be like a boomerang and come right back to you for rework. About 10% of the guns have bolt/carriers that will ride up so high when the carrier comes out of the gas tube that the bolt is pulled away from the ejector and misses it. So....on those guns, you need a shim under the front of the top cover to keep it down in the pathway of the ejector. There is no way to tell which guns will be affected until you shoot them. You can leave the shim out too....but you'll have about a 10% boomerang rate with that as well. Now you're at about a 60% return rate, calling me, and asking me what to do. Just print off this post and keep it handy.

 

On the 8" guns, the bolt carrier comes completely out of the gas tube and if the carrier fit in the rails is sloppy, it will fly up and strike the top cover. I weld an extension out over the top of the top cover to keep it from popping loose on the 8" guns.

 

On a standard gun, bending the rear of the top cover out to "lengthen" it a bit will usually fix the problem. I have seen some guns come from the factory with the top cover barely long enough to even engage the groove in the front.

SBS and piston rod length

 

Just install the gas block in its new position, then install the gas piston (puck) and screw the regulator in all the way and back out to the first position 1 or 2. Then install your bolt carrier and slide it forward until is stops. Measure the remaining distance between the trunion and the carrier on the L/H side. That is how much you need to cut off the carrier's extension rod to get it to work.

 

Keep in mind; if you have moved your gas block back more than about 3", then you will also have to open up the gas block for the carrier to slide through it. Normally just the skinny extension shaft passes through it, but when you move the block way back, the front of the carrier itself has to slide through the opening as well.

SBS and handguard length

 

You can go with an 8" or 10" barrel with the shorty handguards. The 12" uses a different gas block position, a new gas tube, new carrier extension, longer hand guard etc....It is much more work to make, and has been discontinued.

 

If you want a longer handguard on your 8-10" gun, I can do that, but you will end up with a big-ass ugly gap in front of your gas block, where the hand guard extends past it. Additonally, you can't get to the detent to make adjustments to the gas regulator without pulling the hand guard off. I will tell you, almost every time someone deviates from a Tromix design and goes with what they "think" is better...the customer ends up getting a complete piece of shit (that they designed) and then complains to me about it. If you think you are a better gun designer then myself, that's fine, but don't bitch when I deliver you, your own custom POS that you designed.

Edited by BobAsh
  • Like 3

Share this post


Link to post
Share on other sites

Thanks Bob, I have sifted through all that before as I am planning my SBS now. But it's great to have it all in one spot.

 

 

 

Juggs/Indy can we get a sticky on this?

  • Like 1

Share this post


Link to post
Share on other sites
Thanks Bob, I have sifted through all that before as I am planning my SBS now. But it's great to have it all in one spot.

 

 

 

Juggs/Indy can we get a sticky on this?

already done, bro!

Share this post


Link to post
Share on other sites

I would add that when I did the 8" S12, I moved the gas block back approx. 3 1/4". This was because at that point I could totally remove the bolt carrier extension rod, bore out the threaded socket, and weld in a very short extension. It also allowed me to remove the fluted portion of the gas tube, and simply reinstall it. I used a basic factory 4 port pattern, drilled at a an angle very close to, or just a bit more than factory, the same sized as factory (.073). It worked fine with the low brass light game loads. The extension on the rear of the gas tube, where the cover goes in was added. Also, a shim was added inside the cover to keep the carrier in line, since, as mentioned, it does come clear of the gas tube.

 

At the time, I was thinking that the bolt carrier was cut, and a short extension added, meaning it would be cut through the clearance section, the thin part. I didn't want to do this, although I thought it was what Tony was doing. Today, reading this thread, seeing how the gas block entrance hole is bored out for the carrier, tells me that this isn't so, and the end of the carrier is what will contact the puck directly.

Based on this, I may do the next one at 4" shorter, instead of 3 1/4". Also, it's a real bear to get the bolt carrier set up in the lathe for the machining work. Without a fixture, it would be a real bear to set it up in a mill also.

 

I didn't much like the blast from the real short barrel, either. Plus, without a VFG, it would seem easy to get your hand in front of the muzzle during rapid fire. I think if it were for me, I would want a 12"-14" barrel. I seem to remember reading somewhere that the Coast Guard armorers had conducted testing on barrel length for their 870 pumps, as the tight confines of boats and ships made for tough boarding with a shotgun. They cut off an inch at a time and tested, and got the best velocity from a 14" barrel. Since shotguns use pistol speed powders, this is supported. So, if you want a shorty with good terminal performance, and absolute compactness isn't the issue, a 12"-14" barrel is about optimum.

 

Just my .02.

  • Like 1

Share this post


Link to post
Share on other sites

No problem guys!

 

While I don't think that very many guys are capable of building a SBS, it will at least satisfy their curiosity.

 

For those few who can, there are a lot valuable hints in there. If nothing else it will keep us from having to answer the same questions over and over, right?

  • Like 4

Share this post


Link to post
Share on other sites

Excellent info!

 

Thanks for compiling this Bob. :up: It will hopefully keep people from asking the same questions over and over. But I know that's just wishful thinking. There's always going to be people that do that. At least we can point them here.

 

I've considered eventually having my gas block set back a couple inches, but keeping my barrel length stock. To my way of thinking (which isn't always correct), having it set up like that would probably tend to over-gas the system and require me to set my Gunfixer plug to +1 or something. BUT, once Michigan allows for SBSs, it should be a simple matter to cut the barrel down to 10 or 12" and adjust the Gunfixer plug accordingly.

 

Am I correct here, or would making a Saiga SBS-ready be a bad idea? I think some competition guns are like that.

 

 

Corbin

Share this post


Link to post
Share on other sites
...would making a Saiga SBS-ready be a bad idea? I think some competition guns are like that.

 

Yes, some competition guns are made that way. It allows use of very low-power ammo, barrel porting, or both.

 

It's a lot of trouble to go through for a full length gun though.

  • Like 1

Share this post


Link to post
Share on other sites

Who is available and technically qualified to do a 8-10" SBS conversion that can do it fairly soon (like before the Aztec calendar runs out in 2012 :rolleyes: ?

 

eseaton

Share this post


Link to post
Share on other sites

My Form 1 stamp arrived today :super: and I have gotten ahold of Gunfixer to quote me a price on a SBS conversion. In case he is covered up, is anyone else out there doing quality work that might have an opening sometime soon? I am only needing the barrel cut to 10 inches, rethreaded, HK sights welded on and Duracoat.

 

Thanks for the help.

 

eseaton

Share this post


Link to post
Share on other sites

Got my smith lined up. Now looking to see what my options are for a rail on top. I have a Kross Hellion coming for the 8" build but am looking for a solid rail, hopefully to sit over the gas block area. Any ideas?

 

eseaton

Share this post


Link to post
Share on other sites

There is a lot of great information in this thread, but one thing I am trying to find out is:

 

I am working on building a 6" Saiga 12. The barrel on the Saiga is tapered, so what is the best way to move the gas block back? Grind out the inside of the gas block or turn down the S12 barrel behind the original gas block mounting location?

Share this post


Link to post
Share on other sites
There is a lot of great information in this thread, but one thing I am trying to find out is:

 

I am working on building a 6" Saiga 12. The barrel on the Saiga is tapered, so what is the best way to move the gas block back? Grind out the inside of the gas block or turn down the S12 barrel behind the original gas block mounting location?

 

You won't succeed at 6", trust me.

  • Like 1

Share this post


Link to post
Share on other sites
There is a lot of great information in this thread, but one thing I am trying to find out is:

 

I am working on building a 6" Saiga 12. The barrel on the Saiga is tapered, so what is the best way to move the gas block back? Grind out the inside of the gas block or turn down the S12 barrel behind the original gas block mounting location?

 

You won't succeed at 6", trust me.

OK, so 8". What is the best way to reposition the gas block?

Share this post


Link to post
Share on other sites

We turn down the barrel. Making an 8" gun run correctly is a lot harder than it looks.

 

post-2775-1247520582_thumb.jpg

  • Like 2

Share this post


Link to post
Share on other sites
how critical is the distance from the gas ports to the muzzle?

 

Well, it's not an exact science; dwell time, port size, and ammo all affect the volume of gas produced.

 

It's not my intention to make a tutorial for building SBS, just giving some insight from the most knowledgable guys on the subject.

  • Like 1

Share this post


Link to post
Share on other sites

Very very good info. Bob is there any where I could find pics of the different sbs barrel lengths. I am really wanting to see a 12in gun but it seems there aren't many out there. Tromix should post pics of each barrel length( 8in 10in 12in 14in)on their web site so we the people could get a better idea of how they look.

Share this post


Link to post
Share on other sites

Good idea.

8" Standard Tromix.
TromixShorty.jpg

8" W/ Long Tromix Handguard


8" W/ Tapco Galil Handguard
F2.jpg


12" Halo Handguard
8inchB.jpg

12" Factory Handguard

post-2775-12543250754616_thumb.jpg

post-2775-12543251877528_thumb.jpg

Edited by Juggernaut
  • Like 1

Share this post


Link to post
Share on other sites

Hey Bob,

 

In your above post #26

in the very last weapon pictured ....

 

Could you please tell me what folding stock that is? It looks different than the usual AK100 series poly folder.

 

Thanx,

HarvKY

 

 

 

 

...just delete this post if youre wanting to keep this a specs only thread

 

 

ThAnX MaX

Edited by HarvKY

Share this post


Link to post
Share on other sites

SPAS-15 is correct. Custom job done by Tone for JeauxE.

 

Here's some velocity stuff...interesting

 

OK so I finally took most of the shotguns out for a spin on the old Chronograph. What I found is that, basically, those of you chuckleheads with no empirical data who have been yapping your cockholsters about how short barrels on shotguns seriously affect ballistics are fucking wrong. Lots of wrong. Like I said long ago, shotgun powder IS pistol powder- it burns fast and quick, and barrel length doesn't fucking matter that much.

 

I can't .PDF a spreadsheet on this computer so I'll get really slow and dumb like those who talk out their ass about ballistics with no basis in reality and list the average FPS for the shot strings.

 

First, using shitass birdshot- 12 gauge, 2 3/4 inches, 1 oz shot

12" barrel average FPS- 1040

14" barrel average FPS- 1069

18" barrel average FPS- 1114

19.5" barrel average FPS- 1114

20" barrel average FPS- 1108

28" barrel average FPS- 1159

 

Then using Remington 00 Buck-

12" barrel average FPS- 1121

14" barrel average FPS- 1127

18" barrel average FPS- 1216

19.5" barrel average FPS- 1205

20" barrel average FPS- 1182

28" barrel average FPS- 1250

 

Wow- 119-129 FPS velocity loss in 16 inches of barrel. That's devastating. Oh wait- no, it's actually not, it's fucking nothing at all and is actually about the same velocity variation in factory loaded ammo. I will feel as comfortable fucking up a bear using a shotgun with a 12 inch barrel as I would using one with a 28 inch barrel. Those of you clammering over 20 gauge Serbu Super Shortys rather than the biting sting of a 12 gauge to dust up a gravel pit should stick with the shortest of shotshells. I wouldn't want you to drop your purse while shooting.

  • Like 4

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
Sign in to follow this  

Tromix - Lead Delivery Systems
Dinzag Arms
CHAOS, Inc
Mississippi Auto Arms, Inc
Cobra's Custom
Carolina Shooters Supply
R & R Targets
LONE STAR ARMS
SGM Tactical
Mach 1 Arsenal
K-VAR
C&S Metall-Werkes
American Specialty Ammo
Csspecs Magazines
Phoenix Technology
Evlutionz LLC


  • Chatbox

    Load More
    You don't have permission to chat.
×
×
  • Create New...