The new Keeppower USB rechargeable R123s' constant 3V output effectively turns its voltage-based boost into current regulation.
https://www.keeppower.com.cn/products_detail.php?id=664 I am wondering how these are working with your MDC? On the site Malkoff answered a question, saying he doesn't trust the 3V rechargeables to stay at 3V. I am hoping they are working for you still.
I had no idea those existed. I ordered a few of them to try them in some of my lights. As expected, they work just fine in the 6p and M6. Unfortunately, they are too long to work in the A2, it won't even turn on. I hoped they might work. There should be no problems running them in any single cell light, but I think they will probably be too long for anything taking 3 cells or more (the extra length accumulates). They barely fit in the Surefire Fury, but I don't think the tailcap likes it very much. Still very useful and good to have around.