Well, since the Gunco WV shoot is next weekend, I figured I'd better get some more work done. This project has been simmering slowly for over a year and a half, so the heat is now being turned up! This project began in fall of 2003. A lot of early discussion with HCPookie, Vis, Geodykt, Daewoo, Golovko, U.S.M.C.Man, and a few others, helped me develop my initial design. A lot of that time was spent calculating the clearances and cut dimensions. A lot more time was spent using Photoshop and MSPaint to move portions of photos around to get an idea of what problems would evolve and what to do about them. Here's where I'm at, right now.
The gray paint makes it easier to see the areas that were machined. The raw machined surfaces wash out with the camera flash and are difficult to see.
This is the basic machining done to the bolt carrier. The cuts were made before any welding because I wanted to make sure all the interferences were accounted for. The cocking handle has been removed, the clearance slot for the bolt lug is cut, the ejection clearance cut is made, and the firing pin access hole is drilled. Portions of the cuts will be filled with weld once the primary machine work is completed. The rotation channel will be welded up and a notch for the cocking handle will be cut.
This shows the position of the bolt in the carrier. The bolt has been cut to the proper length. The triangular tab on the back of the carrier has been cut flush with the back surface. The bolt will eventually be welded into place.
This is a close up of the cut bolt and carrier. I considered filling the 2 upper bolt grooves with weld and reshaping the bolt, but I think I'll just silver-solder the back end and leave all the welding at the front...
This pic shows the clearance channel for the bolt lug. This is what allows the bolt to slide straight in before final welding. There are several large gaps that will be filled with weld and ground to shape before final assembly. This will allow for a more solid attachment, and prevent the possibility of cracking.
The angled cut for ejection clearance can be clearly seen in this pic. It's tough to see in this picture, but this is the bolt that I opened up to fit the .45ACP cartridge. The extractor only required minor grinding with a dremel stone to get the groove to match the cup diameter.
With the bolt in this position, the ejector lines up correctly with the slot in the bolt. The 7.62 ejector rail will work perfectly with the conversion.
This pic shows the hammer engagement with the modified bolt and carrier. The striking face of the hammer is perfectly flat against the back of the carrier. Because I designed this for blowback operation, the bolt cannot be positioned in the forward position. For proper operation, the bolt must be shortened, moved closer to the back of the carrier, and rotated. The bolt is shortened 0.800". Because of the bolt being shortened, the hammer will not make contact. My design calls for moving the trunnion back the same distance that the bolt was shortened. This will position the back of the carrier in line with the hammer face in the same place that the original bolt stem was positioned. Hammer function is smooth and the carrier easily pushes it back down to engage the trigger and disconnector.
I performed the alignment checks using a standard OOW receiver. This pic shows the carrier moved back exactly 0.800" from the postive stop of the trunnion. This lines the carrier up perfectly with the hammer. I will be using an IBE battle blank for the .45 receiver. The dimples in a standard receiver prevent the magazine from fitting into the receiver far enough. The thicker blank is a little easier to weld on, as well. By only shortening the receiver by 0.800", standard handguards can be used without modification.
This pic shows the carrier seated against the positive stop of the trunnion. When the barrel is installed, the breech face will be right up against the face of the bolt. This gap between the carrier and trunnion is unavoidable, because of the position of the bolt. The barrel will take up most of the gap, but I will add a small piece of sheet steel to the top cover to keep debris out of the receiver. It will also look a little cleaner. This gap is present on the Bizon, too. The Bizon top cover is stamped to cover the gap.
This is one modification that early on I realized was necessary after deciding to weld the bolt in the carrier. The firing pin retaining pin cannot be removed once installed, unless a clearance hole is drilled. Once the bolt location is determined, the clearance hole was marked and then milled. I used 1/8" carbide end mills for this step. I started with a ball end mill to make the hole in the underside of the carrier. Then I switched to a standard end mill to punch through the top of the carrier. The rounded surface inside the recoil spring tube prevents drilling through the material. The sharp corners of the mill bit easily cut into the curved surface and allowed the holes to line up without breaking a drill or screwing up the hole.
My next steps will be to fill in the carrier slots with weld, attach the cocking handle, and then permanently attach the bolt. After that is completed, I can work on the receiver and barrel. After that, I can work on the magwell and think about test firing. WHEW!!!!