This is what I used, and is pretty inexpensive. It is meant for plumbing work, but worked great to ream out the Dillon powder funnels to avoid bridging, especially with smaller calibers. It makes a more tapered "funnel". I have done all my rifle calibers, and have had the reamer for 20+ years. If you chuck the reamer up in drill press or mill at low speed, it takes all of 5 minutes to do. I leave about 3/8-1/2" at the bottom of funnel at the original diameter. Use lots of oil.
https://www.amazon.com/General-Tools.../dp/B00004T82J