To build an AR-15 in .45, the bolt must be opened up to accept the larger case. This also requires some work on the extractor.
To begin, strip the bolt of extractor, ejector and springs. Every time I take a bolt apart, I end up improvising something to get the ejector pin out. All the manuals say a 1/16″ punch is the right thing, but they’ve always been a bit too big, in my experience. This time I made a punch using 0.055″ music wire and a steel handle.
Once the bolt is bored, it can be set up in the lathe to bore the bolt face (rim, actually) open. I push the bolt all the way back against the chuck jaws – there are several different diameters on an AR bolt, so watch where you clamp!
Because the section to be bored is shallow, an endmill may serve as a convenient boring bar. In this case I used carbide due to the hardness of the bolt. Set the endmill in a toolholder so that the cutting edge closest to you is horizontal.
Now simply bore out the bolt, about 0.020″ at a time until you have a diameter of ~-0.485″. Be careful not to cut into the bolt face by more than a tiny bit. (I just touch the bolt face, to avoid leaving a ‘step’ in the inside corner of the bolt).
Note that there is still a ‘web’ left between the bolt lugs.
Next, we need to cut the extractor to match. This involves shaping it to match the new bolt contour. The hardest part is getting a cutter that’s just right. I use a thick metal-cutting abrasive disk, the type used for Dremel tools. I first wore it down by cutting on a steel block until the diameter was approximately 0.485″.
If you are more dextrous than I, you might be able to freehand the extractor cuts, but I don’t trust myself. So, I square up the extractor in a grinding vise…
Then, with the vise floating free on the drill press table, I set the table height so the extractor groove is lined up with the cutting wheel, and begin to remove material – slowly.
Don’t forget to profile the extractor hook – it needs to be able to snap over a .45 case rim. I repeat and periodically re-assemble until a .45 round will sit in the bolt, and the extractor can lay flush with the bolt sides.
Once done, remove the extractor again, and set aside the parts. No point in re-assembling yet. You’ll want the bolt stripped for checking headspace when you chamber the barrel.