With a manual lathe and floating reamer holder, whatever preboring system you use, the bushing needs to be started in the rifling before the reamer starts cutting.
There are several ways to do it, but the basic idea is to start the reamer coaxial with the inch or so of rifling ahead of the case using the bushing in the rifling on one end and further back on the reamer using the prebore. You're trying to get the body of the reamer to start just behind the shoulder in a concentric hole cut with a single point tool. If the reamer starts straight, it'll tend to run straight.
I started with a straight undersized prebore maybe 50 thou short of the finished shoulder and then taper bored it until the bushing would engage the rifling. That minimized the amount of reaming you had to do. It helped reamer life and the bore 50 thou, clean, lube, repeat process went a lot faster.
After I set up a flush system, I prebore to 1.5" with the next size down end mill, clean up the bore with a single point tool, and bore a short section just over the shoulder diameter so the reamer starts on the end of the prebore just behind the shoulder on the reamer. With the flush system, it only takes about 15 minutes with the reamer to complete the chamber.
This is a manual machine, small shop or hobbyist method. It's very effective.
