Okay, I'm going to offer some options and alternative designs here, please feel free to weigh in.
1. Drill a 3/16" hole all the way through and let the end user drill the top end to an appropriate diameter to match the caliber.
2. Drill the top end to 1/4" or so and run a tapered reamer into the hole.
3.(a). Drill and ream the top end to 1/2" - 5/8" and include a plastic plug that the end user could drill out to fit or omit for .40+ caliber bullets.
3.(b). Thread the plastic plug and the hole in the die to guarantee it will stay in place.
3.(c). Cut an internal groove in the die for an O-ring to retain the plug with a friction fit.
3.(d). Drill and tap a hole in the side of the top end of the die for a setscrew.
1 is the cheapest and simplest.
2 a little more expensive to make but less work for the end user. I haven't found an appropriate off-the-shelf reamer yet but I'm looking.
3a doubles the number of parts that have to be produced although they are fairly simple parts. If it's a drop-in part will it stay in place?
3b requires drilling and tapping the die and threading the OD of the plug for at least a short distance on one end.
3c would require me to buy a toolholder capable of cutting an internal groove in a 1/2" bore, and adds one more part (a small O-ring) to the parts list. It would make the plug a simpler part than 3b.
3d adds a third part but makes for a simple plug and die hole design. Might want to make plugs out of aluminum. I've never been wild about having setscrews bear against plastic.
Have I missed any alternatives? Any thoughts or preferences? What's more important to you, low cost even if you have to do more work, or a little higher price for a more "sophisticated" product.
If I can get the right reamer 2 would not be much more cost than 1. 3a, 3b, and 3c will cost more than 1 or 2, with cost going up for each in the order listed. 3d requires another machine be used to D/T the side hole but if retention is required this would be the simplest and lowest cost option.
See how many questions there are when deciding even the simplest features and part configuration(s)? I'm trying to include the end user in the decision making process. I read here and elsewhere that people are little perturbed at times when they don't feel like "business" pays attention to their needs or offer the features they want. I feel way that about numerous products and services - now's the chance, I'm listening.