Since the 8.5" Dragn12 SBS was a successs, its time to build a short one. This one is at 7 inches.
I plan to document the process, changes made, new componets, with some pictures and some dimensions.
I still don't have a lathe or mill set up so I am paying a machine shop to do some of the work.
I hope to have my mill up and running some time this summer. Lathe to follow shortly after.
The work that needs to be done is not difficult. I hold my tollerances to the tight side but there is a little wiggle room.
This is a new barrel so it was never in a firearm. A used barrel would net the same results.
So the obvious is first up. Where to cut the barrel. I cut off the original gas ports taking the split ring grooves with it.
Thats makes the barrel about 7 1/16 inches long. Then faced the barrel muzzle in the lathe just enough to clean it up.
My brother has a small lathe for doing pool cues so I set up the barrel, clamping at the largest diameter (receiver diameter of 1.260) in his 6 jaw chuck.
Used a known good live center in the tail stock picking up the ID of the barrel. This to keep all the barrel changes true to the bore of the barrel if possible.
The barrel was not concentric. Run out more than .005 so if you do this mod, be aware you might be faced with this issue. I'll solve this as I go.
The barrel will be made to accept Benelli Mobil Chokes, as the VR80 comes with, but I will tighten up the choke bore in the barrel for a better fit. This is custom not production.
This barrel will also get 7/8-32 external threads at the muzzle for the JK Armament adapter for the 195 series suppressors. I want to make a muzzle brake to fit those threads as well.
What needs to be changed?  Next post. NOTE: All dimensions are SAE decimal, not mm.

Topic Moved

The barrel has 4 diameters after cutting it off.
1.004 is the first dimension and the final OD of the barrel. Its about 1.9 inches long before turning the rest of the barrel. That diameter will extend from the muzzle to the slide stop near the chamber, about 5.150 inches long.
Part of the original barrel is 1.044. That will be turned to match the 1.004 diameter so its that diameter all the way to the slide stop, the 1.142 diameter.
This is because the ID of the piston (1.008) will be using most of the barrels length right up to the slide stop. Not a lot of clearance once the barrel gets hot but the piston gets hot as well.
The slide stop is roughly 1.142 diameter and roughly 1.100 in length. The slide actually stops on the largest diameter 1.260 which seems consistant, where it enters the upper receiver.
I have not found two barrels the same at this point in diameter or length of this slide stop detail. I'm blueprinting these barrels so they have known dimensions.
The slide is the part with the two ears to drive the bolt carrier. I goes over the OD of the slide stop and stops on the receiver diameter of 1.260. The part of the barrel that fits in the upper receiver.

I shortened the slide stop from 1.500 inches to 1.400 on the muzzle side. I then set the slide up in the lathe using the chamber side ID on an arbor to keep it true to its center.
Then I bored the muzzle side ID to 1.180 diameter deep enough to accept a bushing with 1.181 OD for a light to medium press fit. The bushing has a 1.181 diameter, that is .300 long, that presses into the slide stop.
When assembled, it creates two stops for the slide. The OD of the stop contacts the receiver diameter and the bushing (the other end) stops on the face of the slide stop. Much better than the original design using only the OD of the slide.
The bushing was made from the heavy load piston, leaving the shoulder of the piston at its original diameter but reducing the shoulder thickness from .150 to .100. The OD of the piston is turned to a press fit in the slide. 1.181 diameter.
Trying not to confuse, the bushing length is .400 total with .300 of it in the slide. Makes with a nice interface and keeps it all nicely aligned. Final length of slide is at 1.500, as the original but now with a guide bushing.

Note: The stroke on this guns gas system is .933. Pretty much pre-determined by the length of the slides drive posts operating the bolt carrier.

Here is where we start. The large diameter at the receiver is the slide stop.


7" barrel with gas system showing location on the barrel.


This is the gas slide with the modification to the bore and the flanged guide bushing. The slide, shortened .100, to allow for the .100 thick flange on the guide bushing. 1.500 final length.


Barrel at 1.004 diameter. This shows the split ring groove/gas ports I cut from the barrel showing the location of this detail to be machined on the barrel.

03/27/24  Quick update: Decided to give a shot to keeping the original handguard. I cut about 6 1/8 inches out of the middle.
I turned an arbor to locate each half on the lathe, one at a time. Turned the cut face. This to make sure the two parts butt together nice and square.
There is very little clearance between the handguard and the moving parts of the gas system. This was to make sure there was equal clearance all around.
I also wanted to keep the original retainer in the muzzle end to keep the handguard secure without using the biscuit I used on the 8.5 inch version.
It will end up a flattop so that V in the middle will not be there. The triangular holes will be blended.

Nice project.  I love the SBS VR80s.

I did one and have been experimenting with various barrels and lengths using existing gas system (but opening up ports)  I have it running great at 10.5", good at 9" and only running hot loads at 8" but 8" is with no choke

Is it difficult to thread internally for chokes as the barrel is chromed?

A modified choke makes a big difference in reliability on an SBS for aiding lighter loads.

Do you have some specific drawing dimensions?, I have a Spare VR80 I would love to do a gas system that is shorter

Very cool. Thanks for the build thread.

Have you thought of threading the barrel for Saiga chokes?  There are a number of brakes and suppressors that can use it.  That is an external threading of M22 x 0.75, I believe.

I have done that at 12.5" and 10.5" and it works well and will allow the gun to work with bulk Federal ammo which is nice.  I use a Saiga modified shoke, but don't like the look of the unit - I would prefer an internal choke and add the wide fake muzzle fat device over the muzzle.  Sort of how Derya Arms Factory Shorty 9.5" unit looks but in 10.5"

I have a few old Polychokes  as well I am considering adding to the shorter ones, but I think once you go shorter than 10" its really pushing the limit of the existing gas system - I love how the OP did the barrel conversion - very clever.
Here is mine, I have two spare barrels and picked up another VR80 an auction for under $300 from Rock Island's scratch & dent sale.

Option one. To thread internally, you have to use a carbide choke reamer. High speed reamers won't survive the chrome washed barrels.
Once reamed, you can cut the threads with a guided choke tap as the chrome has been removed in the reaming process.
This info is with using the Beretta Mobile choke system which came on the VR80.
Second option is to bore the barrel on the lathe to the diameter desired and then use the guided choke tap.
At this time I'm going with .003 over the choke outside diameter. This is tighter/closer that the factsory diameter which is loose IMO.
That seems to be where I'm headed with this one.
I have the threading dies for the 22 x.75 mm used on the Saiga/Vepr's but this one gets the .875 X 32 external threads for the JK Armament suppressor adapter.
Pacific Tool and Gauge makes the reamer and the tap used for the Beretta Mobil Choke system.
The carbide reamer is pushing $500 to your door vs $220 for the high speed steel version. (non chrome barrel)
The barrel tap is the same price. Both have guide bushings. I think the reamer bushing is .725 diameter, not sure on the tap bushing diameter.

Beretta Mobil Choke Reamer


Beretta Mobil Choke Tap

This pic to give some reference to the VR80 barrel and its gas system location.
The green tape shows where the 7" cut line is and its reference to the factory gas system.
You can see that the gas system gets cut completely off at 7", ports and split ring groove. It has to be re-created closer to the chamber.
I also marked a point at 8 inches. Do not cut your barrel here (8 inches) unless you are willing to give up your gun cycling with any ammo.
At 7" to 10" you must move the gas port/split ring detail. In this case, about 2.7 inches closer to the chamber.

Reference image.


This is my 8.5 inch gun barrel. This to show you about where you will end up with the gas system. I threaded the OD of the barrel 1"-20 for my custom external choke adapter.
Final cut to the barrel was at the left face of the round nut at the muzzle.


This is the 8.5" barrel after its OD has been turned and gas port/split ring detail machined. No gas ports yet. Barrel turned so the piston can travel the distance required, all the way to the next shoulder.
Gas ports will be drilled at 52.5 degrees. There will be 4 instead of 3, at a reduced diameter of course.

Nice work!!!  Thanks for sharing the information.  I'd be interested to see how it works and what port sizes you will be using -

I've never seen any information or videos where people disassemble the Derya Arms MK12 short guns to see how the factory did it on their short gas versions- Its probably very similar to your work.

Keep up the great work.

That’s pretty awesome. And since I bought a couple of them when Rural King had them on sale I now have an idea what to do with the spare

While I was in the process of doing the 8.5 inch, I saw the Derya arms short barreled version, but no details. That was conformation I was going down the right path.
There is no black art here, just physics. I've decided to move the gas system a little closer than originally thought on this one.
Dwell time is reliability so every bit of barrel length after the ports is important on a semi auto. Maximize is the word.
I've have modified several parts, removed a part, and added to the reliability of the system. One part I did not alter. Gas system still functions the same, and I believe more reliably.
I wanted short, really short. I also didn't want to deal with the original gas ports as I did on the 8.5" as they were not eliminated but blocked by the muzzle device.
I don't like enlarging gas ports which allows more wad contamination and carbon fouling. Thus, 4 smaller ports on this one.
Its a project for sure and part of this was to challange myself and part to give some insight to others as to what works and what not to do.
Going to a highly rated (by review) gunsmith today to see if he can do what I want done. Too many are just armorers and not gunsmiths.
I might be better off just going to a machine shop who still has manual machines and talented machinists, not just people pushing buttons on CNC's.

4 ports!  Why didn't I think of that?! - I did open mine up a little and yes there is more carbon etc. gets dirty quick.

Well I went to the rated gunsmith Saturday hoping to get some work done on a barrel and another item.
They didn't have any machines at all, not even a drill press that I could see. Gunsmith said we're not a full service gunsmith shop.
Just as I expected so the level of dissapointment was minor.
I found another shop says they can do about anything. Call them Monday....

Yes actual hands-on machining gunsmithing is becoming a less common art.  

In regards to your cutting at 7" vs 8 or longer (as the factory gas port area there at 7-8" allows excessive bleeding off)  - had you considered placing some ring around it to seal it off?  similar to the half-round locking rings ?  I can see where that it becomes far cleaner project going to 7"  

This is really impressive - looking forward to more.

Wow! Amazing work.

On the 8.5 inch barreled gun, (Dragn12) the old gas system and split ring grooves are still present. I needed to stop any gas from escaping before the projectile left the barrel.
Planning ahead, I turned both the external choke adapter and the plain muzzle device with shoulders that would effectively prevent the gas from escaping thru the old ports.
The shoulder on each the choke adapter and the muzzle device has two functions
They both are handguard retainers. And they both have close fitting shoulders, to the barrel, that extend further into the handguard and they close off the old gas ports.
All of this required some forethought and some close machining to seal off those ports. About .001 of an inch clearance to seal the ports.
The Dragn12 was a fun project and a learning experience. I was quite excited for that first day out with it.
I did single shots to check out the whole gun, its feel, how it points, its recoil, and if it would eject the spent round. I had a huge smile on the face that took awhile to go away.
I had set the pop up sights by eye for 25 to 35 yards. Used 7.5 shot, 3 dram load with 1 1/8 oz of shot. Pretty much a standard Trap load or bird shot load.
It was dead on with a pretty tight, round shot pattern on a pattern plate with a Cylinder bore choke. (4'X8' greased 1/4 inch steel plate) I was a bit surprised at how tight the pattern was.

This 7" will be different.

Off to a gunsmith tomorrow to discuss some special barrel work on a 10" 1100 Remington. Definately a challenge still adhearing to my gas port theorys.

The now modified choke adapter. Needs bead blasted yet.




The plain muzzle device