Project SA80 L85A2: Trunion Mock Up Part 3

I have been playing around in FreeCAD on the dimensions for the SA80 trunion and have a couple versions.

Version one used the guide rod spacing of the AR180B. As we saw during the mock up phase, this will not leave enough room for ejection. The op rod is lined up with the other airsoft parts. This would require a custom front sight base and piston assembly.

I then decided to make use of the guide rod assembly that came in the Airsoft gun.

Taking measurements I moved and enlarged the holes in the trunion to line up with this assembly.

I also changed the position of the op rod hole. I have been wondering what I would do for a front sight base. Originally I considered a modified AR180B FSB. It may still be up for consideration but it would require the same barrel extension mods that the original 180B needed. The shoulder of the extension would need to be turned down and the locating pin removed. I wanted to avoid that if I could.

Then a new upper purchase lead me down a different path.

I saw that Atlantic Firearms had just released a 12″ Wolf A1 upper. It is a civilian version of the Taiwanese T91 and features a short stroke piston. Since I have been looking for a different upper for my SBR AR15 I took the plunge. As luck would have it, I got one of the first 100 made that are 12″. All remaining uppers will be 12.5″ Instant collectors item? Time will tell.

Of course the first thing I did when I got it was to take it apart. I really dig the self contained gas assembly.

Using the T91 front sight base and piston assembly would allow me to have a very robust set up that is very easy to field strip for cleaning. It is not adjustable but neither are most of my guns. I have had no problem dialing them in to work with different ammo.

The piston uses the same spacing between the bolt and the op rod that the AR15 uses between the bolt and gas tube. This gives more room and allows an unmodified AR15 barrel extension to be used.

I could drive out the roll pin that holds the top half of the front sight and then cut a dovetail. The airsoft front sight could then be attached.

Of course all of this will require a new bolt carrier to be built. I will be making some drawings of that next.

 

 

The operation shown above was done on my lathe with the 4 jaw. It was fun. I haven’t had that thing out from under the lathe in far too long.

Double checked my mill with a dial indicator and my X axis is off over .001″ per revolution of the handle. My holes are off further than that so it is partially my fault as well. Enough of this mock up works that I can feel confident making some adjustments and building a second. I’m also toying with the idea of a second piece bolted to the front of the trunion to hold the barrel in place. Then the barrel would not need to be pressed in and it could be replaced easily if needed.

 

Long day in the shop but I got a lot accomplished. I worked on a mock up trunion so I can use an AR180B bolt carrier to get several dimensions worked out. My goal is to get reliable feeding from the magazine. The dimensions that I plan to work out will be the distance from the center line of the barrel to the center line of the take down pin. The other dimension is how far to the rear the barrel extension will sit.

I started with some 1.5″ x 1.25″ aluminum bar, cut and squared on the mill.

I then started cutting it down to fit within the confines of the Airsoft L85.

A little more work and it will be done and ready for a trial run feeding cartridges.

I need to break out the 4 jaw for the lathe and turn a recess for the shoulder of the barrel extension.

There is more room inside this receiver than on a 180B, which is what I am basing my trunion design off of. Because of this I do not need to modify the AR15 barrel extension at all. On the 180B the barrel extension would need to have the shoulder turned down considerably to fit within the trunion. This prototype has a larger hole for the barrel extension than will the final product. It is a slip fit but the final will have a press fit just like the 180B. The trunion is made to fit very tightly into the upper and lower to increase rigidity. If I have to make a new bolt carrier the guide rods will be higher to help open up the path way for ejection.

I also think found that the dials on my mill table do accurately reflect the true movement. My guide rod holes are off by about .015″. I actually hope it was a mistake on my part and not the machine itself. Time for some more testing.

Project SA80 L85A2

Working out of town all week had me chomping at the bit with this project. I was digging up pictures on the net and making crude drawings in my note book.

These were from a guy on Facebook that goes by the name The SA80 Man. A message has been sent his way and hope he can help me with this project.

Some screen caps from Ian at Forgotten Weapons

Does anyone else get bored when I’m traveling by plane? I have taken to putting my thoughts down on paper.

I was feeling confident and all was well and fine till I got home and hit upon my first major snag.

I had to work out the relationships between the magazine, hammer, and barrel exiting the front of the receiver. In order for all those things to line up I realized that the 180B bolt carrier is not going to work. It places the guide rods too low ( The receiver is upside down  in the above pic) and they will not allow room for ejection. Several thoughts crossed my mind at this stage like cutting out and lowering the ejection port or raising all of the components to work with the existing port. I believe the first is still on the table but the second most definitely is not. The amount of work that would be required is considerable, including raising the barrel. Do that and the hand guards would no longer fit.

I think the simplest solution is to make a new bolt carrier. The SA80 bolt carrier guide rods are spaced further away from the bolt centerline and as a result, would solve all my problems. I am not sure how I would go about cutting the cam pin slot. I envision myself drilling a few well placed holes and finishing it off with hours of hand filing and fitting. Oh goody. Nobody said this would be easy.

The other option is to find more original parts.