Simple and effective - Arisaka Defense Streamlight Protac HL-X Tailcap Adapters

It's very simple and it does it's job. It's pretty much just the two sets of threads with a free floating transfer bar for the current, so there's very little that can go wrong with it. The only issue with it (which is unavoidable) is that it makes the overall light assembly about an inch and a half longer.

I wouldn't recommend that someone buy a streamlight with intention of putting this on there. A light that doesn't require an adapter would be shorter and simpler. But if you already have a streamlight and need to run it with a different tailcap, it will do the job.