If it's going to be rimless with no shoulder you have to headspace on the case mouth and the rimmed reamers cut a taper large enough you have to go in deeper to get the diameter back so you can cut the case mouth step. It also can throw off the taper of you chamber slightly. Much easier to make a reamer. The reamers not hard to make, just tedious. I bought an indexer between back surgeries (second one wasn't planned) so once I'm moving around good again it should be even easier.