I recently added a 'red dot' to my Hellcat after much research. It appears there currently are only three sights that will fit without mods. The "Shield RMSc", the "SMSc", and the "RomeoZero". Both the Shields will co-witness with the factory sights, the Romeo will not, even though it does fit the mounting footprint. Of the three, I decided on the SMSc due to it's solid frame/body as opposed to the RMSc and it's lower cost. Prices will vary some across the three by source.
BTW, I'm very satisfied with the SMSc. If I have any complaint at all it would be the 'no on/off' for the sight. It's on all the time, but will go for approximately 2,000 hrs per battery life and really isn't noticeable when holstered or any other time except when sighting.
Along with the fitment of the sight, you must also be concerned with the actual weight of the sight. You may find sights with adapter plates, etc that will fit, but may be too heavy for the slide on the Hellcat to function properly. Oh yeh, and be very careful with the length of mounting screws. Just something to ponder!
regards,
jumpinjoe