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Posted: 3/25/2024 3:50:21 AM EDT
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. |
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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. |
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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 |
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Very cool. Thanks for the build thread.
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BikerNut:
Normal people like motorcycles. Real people like motorcycles. People who don't like motorcycles are just... weird. |
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.
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Turns out we’re not in a single dystopian novel. We’re in all of them simultaneously.
All government compels by force, be it economic or judicial |
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. |
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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
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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. |
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Originally Posted By Namsag: 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. View Quote 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. |
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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.... |
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Originally Posted By Namsag: 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.... View Quote 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. |
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Wow! Amazing work.
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