If you have or have access to a drill press, you can chuck up a field point, screw on a 50gr brass insert, turn the press on, and draw file down to fit inside the shaft. Not hard to do, just take your time and check the fit often. Best I recall, you are real close when you get down to the bottom of the grooves machined into the insert. And, best I recall, 50gr inserts come out in the low 40s and 100 gr inserts turn down to about 84gr (+/-).
Once I got on a roll, it took me about five minutes to complete one...with a portion of time spent weighing it on my reloading scales.
Reckon I should have read JRY's reply b4 posting.