Don't know of any that fully sit inside the bottle but Altaros (and maybe others) have one that sits between the bottle and gun. But it does add maybe 2 inches. Just watch thread sizes as it would need to thread to both bottle and rifle.
I put an Alteros reg in my Bantam. Works well. No tools required. Mine needed a spacer oring between the reg and rifle to make everything line up properly. I'm very happy with it. Buy it on eBay. Search for an eBay coupon. You can save 10 to 20%.