I believe that the grooves were milled with a rotating mill mounted in a hand held power tool or the rotating mill was mounted in a fixed drilling/milling machine, and the blade was moved along the machine table.
That's why the inconsistecy in the depth of the grooves and that's why they are not straight. Last, not least, that's why you have the round entry points of the milling tool at the beginning of the grooves.
It is very easy to chisel straight grooves, and it is very easy to control their depth if you chisel them by hand. The natural movement of the chisel is in straight lines, and you can repeat the chiselling operation until you reach the desired depth.
Try doing the same with a small ball-point end-mill mounted in a hand held power tool and you will know what I mean!
I may not know much about antique weapons but I certainly know something about mechanical machining.
