Building a Semi-Auto SIG 510: How Redleg Manufactured a Swiss Rifle from a Demilled Parts Kit
- Brandon Lolkus
- Jun 18
- 12 min read
Quick Answer: Redleg Company built a functioning semi-automatic SIG 510 (the Swiss StGw 57 pattern) chambered in 7.5x55 Swiss, starting from an 80% receiver and a demilled full-auto parts kit. The work involved machining the receiver to accept the front and rear trunnions, fitting and welding the trunnions, charging handle plate, and hammer stop, dressing the welds, hot salts bluing the receiver, setting headspace, and registering the finished rifle as a manufactured firearm with the ATF under the Redleg Company name. The rifle was built as a true semi-automatic with no full-auto parts in it. This is firearm manufacturing, not assembly.
This post is for collectors, surplus rifle owners, and serious shooters who want to understand what it actually takes to manufacture a roller-delayed Swiss rifle from a parts kit and a blank receiver, and what separates real manufacturing from bolt-together work.
You can buy a parts kit. You can buy an 80% receiver. What you cannot buy is the part where those two things become a rifle that locks up, headspaces correctly, and fires safely under roller-delayed blowback pressure. That gap is where most projects die in a drawer. This semi-automatic SIG 510 build is the story of closing that gap. Redleg Company is a precision rifle gunsmith and custom builder in Chandler, MN. Not a gun store. Every rifle that leaves the shop, including this one, was built and tested by a trained gunsmith.
Before we go further, if you want the broader picture of what goes into a ground-up rifle, start here: 6 PRC Custom Rifle Build: the diagnostic process most shops skip.
What the SIG 510 Is, and Why Building One Is Hard
The Swiss StGw 57, sold commercially as the SIG 510, was the standard service rifle of the Swiss Army, adopted in 1957. It runs on a roller-delayed blowback action, the same operating principle family as the German HK G3 and the Spanish CETME, manufactured by SIG to the exacting standard Swiss small arms are known for. It is chambered in 7.5x55mm Swiss and feeds the legendary GP11 cartridge.
The rifle Redleg built is a semi-automatic version of this design. It fires one round per trigger pull. The kit came in as a demilled full-auto parts kit, but the rifle was built as a true semi-automatic with no full-auto parts in it. The only nod to the rifle's origin is cosmetic, and we will come back to that.
It is a heavy rifle, over 12 pounds loaded, which combined with its muzzle device makes the 7.5x55 shoot smooth with moderate, manageable recoil. It is also known for hard, enthusiastic brass ejection. Cases frequently dent on landing. The action was originally developed and tuned by SIG using GP11 ammunition, so the rifle and the cartridge were engineered together as a matched system.
Here is what makes building one from scratch difficult. A roller-delayed action does not have a mechanically locked bolt. It manages bolt thrust through the geometry of the rollers, the locking piece, and the trunnion surfaces they bear against. If the trunnion is located even slightly wrong in the receiver, or the welds that hold everything together move under pressure, the rifle is at best inaccurate and at worst unsafe. There is no margin for "close enough" on a build like this.
If you want to understand why action fit and geometry matter this much, the science of rifle actions and why fit matters breaks it down on a different platform.
Starting Point: An 80% Receiver and a Demilled Parts Kit
The build began with two things. An 80% receiver, which is a receiver blank that is not yet a firearm and requires machining to become one. And a demilled Swiss parts kit, which is a complete rifle that was torch-cut by the importer to comply with import law. Every part the rifle needs except the serialized receiver comes out of that kit, right down to the original diopter sights, folding carry handle, and a buttstock that still wears a Swiss Schützenfest match sticker from its service life.
The job is to manufacture a new receiver that brings all of those original Swiss parts back into a single functioning rifle, configured for semi-automatic fire. That means the receiver has to accept the front and rear trunnions in exactly the right relationship to each other and to the barrel and bolt group.
Machining and Fitting the Trunnions
The front trunnion and rear trunnion are the structural anchors of the action. We machined the 80% receiver to accept both, then fit each trunnion to the receiver shell. This is the heart of the build. Trunnion placement controls how the barrel sits, how the bolt group travels, and how the roller-delayed action locks up under fire. The fit has to be right before a single weld is laid.
Welding the Trunnions and Internal Features to Hold
Once the trunnions were fit, they had to be welded so the assembly would hold under the forces a roller-delayed action generates. The front trunnion was pulled tight against the receiver face and welded in its located position. The rear trunnion was welded using the rifle's own fire-control components to confirm correct location before the welds went in. From there, the charging handle plate was fit and welded across the top where the charging handle rides, and the hammer stop was fit and held tight before welding so the fire-control system functions exactly as it should in semi-automatic.
These welds are not cosmetic. They carry real load every time the rifle fires. You can see the heat coloring across the receiver in the photos, the signature of welding done where it counts.
What Most Shops Get Wrong: Most shops treat a parts-kit build as an assembly job, where the goal is to make the parts go together and the rifle go bang. That mindset is exactly backwards. On a roller-delayed receiver, the receiver is the precision component, and the kit parts are the fixed reference you have to build to. If you locate the trunnions to make assembly easy instead of locating them to make the action lock up correctly, you get a rifle that headspaces wrong and beats itself apart. The order of priority is not assembly first. It is geometry first, then assembly follows.
Dressing the Welds
Once every load-bearing weld was laid, the welds were dressed down by hand with progressively finer abrasive, taking the joints clean without undercutting the surrounding metal. This is patient work. Cut too far and you weaken the joint you just spent hours getting right. Done correctly, the result is a receiver that holds like one piece of steel and looks the part too.
Finishing and Bluing the Receiver
A bare welded receiver is not a finished receiver. Once the welds were dressed and the machining was clean, the receiver was hot salts blued. Bluing is not just cosmetic. Hot salts bluing puts a controlled, corrosion-resistant black oxide finish on the steel so the receiver holds up to handling, weather, and decades of use, the way the original Swiss arsenals finished these rifles to last.
There is a detail in the finished rifle that tells you it was built for a customer, not for a catalog. The customer chose to blue only the new receiver, leaving the original Swiss parts in their service finish. So the deep, fresh black of the receiver intentionally does not match the older finish on the surrounding parts. That contrast is not an oversight. It is exactly what the owner wanted, the new Redleg-built receiver standing out as the one piece of this rifle that did not come from Switzerland.
Making It Legal: ATF Registration and Marking
Because Redleg machined a non-firearm 80% receiver into a functioning firearm, Redleg is the manufacturer of this rifle under federal law. That carries real obligations. The rifle was properly registered with the ATF as a manufactured firearm, serialized, and marked. In the photographs you will see the receiver marked STGW 57, 7.5x55MM, and the serial number S57-0001, along with the manufacturer marking: Redleg Company Inc., Chandler, Minnesota. That serial is no accident. This is the first StGw 57 Redleg has ever serialized.
One detail collectors notice: the original Swiss selector markings remain on the receiver, a cosmetic reminder of the rifle's service-pattern origin. The rifle itself is semi-automatic only, with no full-auto components in it.
This is where the Type 7 FFL and Class 2 SOT matter. A shop without manufacturing authority cannot legally do this work. Brandon Lolkus owns and runs Redleg Company, a precision rifle gunsmith and custom builder in Chandler, MN, holding a Type 7 FFL and Class 2 SOT. Brandon earned his gunsmithing and machining degree from Pine Technical College, trained under Gordy Gritters, and has spent over a decade running the shop. He is a retired Army National Guard combat veteran, and on precision work chambers barrels to under .0005 inch runout. Manufacturing a serialized roller-delayed rifle from a blank receiver is exactly the kind of work that authority and that training exist for.
Headspace, Function, and Live Fire on GP11
A built rifle is not a finished rifle until it is verified. We set and checked headspace on this SIG 510, which on a roller-delayed action is non-negotiable because the system relies on correct bolt gap and lockup geometry to manage pressure safely. Then we function-checked it and took it to live fire on Swiss GP11, the cartridge the rifle was designed around.
The first two rounds were fouling shots through a clean bore and were excluded from the numbers. A clean barrel shoots differently than a fouled one, and reading velocity off a cold, clean bore would not represent how the rifle actually performs. The figures below are from the rounds fired after the barrel had settled.
GP11 Through This Rifle | Result |
Cartridge | 7.5x55mm Swiss (GP11) |
Bullet | 174 gr FMJ boat-tail |
Shots recorded (after fouling shots) | 6 |
Average velocity | 2,443.2 fps |
Standard deviation | 23.0 fps |
Extreme spread | 69.8 fps |
Headspace | Set and verified |
Function | Confirmed on GP11, semi-automatic |
Velocity measured from this specific rifle on Swiss GP11, June 12, 2026, on the shop chronograph. The first two fouling shots through a clean bore were excluded. This is real recorded data from this rifle, not a published or estimated figure.
A 23 fps standard deviation and sub-70 fps extreme spread from century-old-design surplus ammunition, fired through a freshly manufactured roller-delayed receiver, is a strong result. It tells you the trunnions are located right, the headspace is correct, and the rifle is doing exactly what the Swiss engineered it to do.
The rifle was also sighted in to original Swiss military zeroing protocol rather than a generic 100-yard zero. That is a small detail most builders would skip, and it reflects the same principle that runs through the whole project: measurement and intent over guesswork.
Headspace work is the same discipline whether it is a modern build or a classic, like this .30-30 lever-action headspace repair we brought back to life.
Does Redleg Build and Ship Rifles Like This Nationwide?
Yes. Redleg ships completed builds and gunsmithing work back to customers across the country, working within federal and state transfer law. Whether the project is a modern precision bolt gun, a custom build, or specialty manufacturing work like this Swiss rifle, the rifle can be built in Chandler, MN and returned to a shooter anywhere in the US through the proper FFL channels.
Current build slots are running 8-10 weeks out. Specialty manufacturing work like a roller-delayed build adds to that timeline because of the fitting and verification involved. If you have a project in mind and you want it done before fall, the conversation needs to start now.
👉 Have a parts kit or a build project sitting in a drawer? Call 507-677-6007 or email info@redlegguns.com and tell us what you are trying to build.
Full service list and current pricing at redlegguns.com/services-offered.
What Separates Manufacturing from Assembly
The reason this build matters is not that the parts went together. It is that the precision work, the trunnion fitting, the load-bearing welds, the headspace verification, was done to a measured standard before the rifle ever fired a round. The chronograph numbers are the proof. That is the same discipline behind every Redleg rifle, whether it is a carbon-barreled PRC build or a serialized Swiss roller-delayed rifle. If you want to see how that diagnostic-first mindset plays out on a modern precision build, the Action Blueprinting and Ignition System Work
shows the same principle applied to a bolt action: get the geometry right first, and accuracy follows.
Frequently Asked Questions
Can you legally build a rifle from an 80% receiver?
Yes, with conditions, and the rules are now well settled. In 2022 the ATF redefined what counts as a frame or receiver, the Supreme Court upheld that rule in 2025, and the Department of Justice left it in place in 2026, so this is stable law rather than a moving target. Under it, an individual can still machine an unfinished receiver into a firearm for personal use without a serial number, as long as they are not prohibited from owning firearms and do not transfer it across state lines or to a dealer. When a licensed manufacturer like Redleg Company machines the receiver instead, the rifle is a manufactured firearm that gets marked with the maker, given a serial number, and registered the proper way. State law is the real variable here. The same build can be legal in one state and restricted in another, so the path always depends on who is doing the work and where.
How do you build a semi-auto SIG 510 from a full-auto parts kit?The kit came in as a demilled full-auto parts kit, but the rifle was built as a true semi-automatic with no full-auto parts in it. It fires one round per trigger pull. The Swiss-made parts from the kit supply the barrel, bolt group, and furniture, and the newly manufactured receiver brings them together in a legal semi-automatic configuration. The only nod to the rifle's origin is cosmetic. The original selector markings remain visible on the receiver.
Is the StGw 57 the same as the SIG 510?Yes. The StGw 57 is the Swiss military designation, and the SIG 510 is the commercial name for the same roller-delayed rifle chambered in 7.5x55 Swiss.
What is GP11 ammunition and is it safe to shoot?GP11 is the Swiss military 7.5x55 cartridge, a 174-grain full metal jacket boat-tail load known for excellent accuracy and quality control. Swiss surplus dated from 1950 forward is non-corrosive and safe to shoot with normal cleaning. It is widely regarded as some of the best surplus rifle ammunition ever produced.
Why blue only the new receiver and not the whole rifle?That was the customer's choice. Bluing only the newly manufactured receiver preserves the original service finish on the Swiss parts and lets the new Redleg-built receiver stand on its own. The finishes intentionally do not match, which tells the story of the rifle at a glance.
Why does headspace matter so much on a roller-delayed rifle?A roller-delayed action does not lock mechanically the way a bolt action does. It manages chamber pressure through roller geometry and bolt gap. If headspace and lockup are not correct, the rifle can be inaccurate or unsafe, so verifying them is not optional on a build like this.
What to Read Next
If this build interested you, read the 6 PRC Custom Rifle Build featuring an Impact Precision action and a Bartlein carbon barrel.
It walks through a ground-up modern precision build and the diagnostic process most shops skip, and it shows the same measure-first standard applied to a very different rifle. It is one of the strongest examples of how Redleg approaches a build from the first cut to the final group.
The Redleg Standard
This project started as an 80% receiver and a box of torch-cut Swiss parts. It ended as a serialized, headspaced, function-tested semi-automatic SIG 510 that fires the cartridge it was designed around, with a 23 fps standard deviation to show for it. The finished rifle, blued, marked S57-0001, and zeroed to Swiss protocol, is shown on the bench at the shop in Chandler, MN. The difference between those two states is measurement, fitting, and verification done in the right order. That is what manufacturing a rifle actually means, and it is the same standard behind every build that leaves the shop.
If you are serious about building something the right way, not guessing, reach out.👉 507-677-6007👉 info@redlegguns.com
Which article brought you here? Let us know when you call.
Last updated: June 2026 | Data based on rifles built, tested, and diagnosed by Redleg Company, Chandler, MN.
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