Another "it can be done, and done well".
Did a 25-20 chamber this way. Takes a while and you have to be very careful with how its cut, but it can be done and if done right, just as well as if done on a lathe.
Use the largest tap wrench available. Mount the barrel in a vise and turn the vise on its side (and clamp it down), so that the barrel's muzzle is pointing down. Flood the barrel with a good cutting oil (non-sulphured is what I was told to use) and put a catch bucket/cup under the muzzle to catch the excess oil and cut metal chips. Using the weight of the tap wrench, insert the reamer into the bore and make a turn. Take note, DO NOT turn the reamer like a tap. Use light, downward pressure (not side to side). Twisting the reamer will cut an oblong (wallowed) chamber. Downward pressure with minimal twist can be done using your index and middle finger of each hand, placing them lightly on each tap wrench handle, and (as if you were tracing a circle that is the diameter of the tap wrench) turning the tap wrench/reamer 180*. Remove the reamer, clear the chips out of the chamber and add more cutting oil. Do these half turn/180* cuts until you have the proper depth of the chamber. The last few passes with the reamer should be almost feather light, as this will finish out/clean up the chamber of any tooling marks.
It will take some time to cut a chamber this way (even a small 22LR one), but it can be done and can be done with a very high quality finish. Just take your time, and as they used to say all the time, "pay attention to detail". As you do this, you'll begin to learn to "feel" the chamber being cut, and adjust accordingly. It's a kinesthetic skill, that you learn by sense of touch...like the gunsmiths did a century ago...
JMTCW...