I recently added red dot sights to a couple more of my carry guns. I am now up to 8 carry guns and 2 competition guns with red dot sights (all with 3-4 MOA sized dots). This time I went with the Shield Sights RMSc optic as it's smaller footprint was perfect for the narrower slides of my CZ P-01 and my Glock 43X. Now that I have first hand experience with three different brands of red dot sights on carry guns, I thought I would give my thoughts on each of them.
Burris Fastfire III
I started out with the Burris FF3 on a CZ P-07 because it was relatively inexpensive, and I didn't want to spend a lot of money to try it out. I even purchased an older Gen1 slide that was pre-milled by CZ Custom to accept multiple red dot sights so that I could switch to a different optic if I wanted to. The FF3 worked fine for me, but the battery life is only a couple of months if you carry it every day. This isn't too big of a deal because you can replace the battery without having to remove the optic from the gun. I ended up moving the FF3 to a competition gun (CZ P-09) and have since acquired 3 more of them. 1 for another competition gun (CZ Shadow 2) and 2 more for some carry gun projects (a Gen1 P-07 and a Gen4 Glock 19 MOS). I don't carry either of those two guns every single day, so the battery life and durability of the FF3 isn't a big deal to me. I believe the FF3 is an excellent optic in it's price point (around $200), and the warranty and customer service from Burris is outstanding. I have had to send in 3 of my optics to them for various reasons (yes, I've had some failures), and every time I send one in, they send me a brand new replacement within a week! 😲
I have 4 Trijicon RMRs on guns that I carry more frequently (two CZ P-07s and two Polymer80 Glock 19s). In my opinion, the RMR is the best red dot sight for daily concealed carry because it can withstand the abuse of accidental knocks against various things like doorknobs, chairs, desks, etc. The RMR's window is relatively small, but it looks even smaller due to the thicker frame around the window. Some people find the thicker frame more difficult to use, but if you are training with and using an RDS properly, the thicker frame is really not an issue. All of my RMRs are the auto-adjusting versions that are always on, as I didn't want to mess with any buttons on a carry gun. Two of them are the older Type 1 version (installed on both of the Glocks) and the other two are the newer Type 2 version (installed on both of the P-07s). The auto-adjusting brightness isn't that great in low light conditions, especially with the use of a bright weapon mounted light as the dot gets washed out, but the Type 2 auto-adjust works significantly better than the Type 1 in that scenario. I recommend having usable backup iron sights installed on the gun for those types of situations. The RMR battery life is really good. They are supposed to go 2 or more years on a battery, depending on usage, but I just change the battery and re-zero once a year (you have to remove the optic from the gun to change the battery).
I recently chose the Shield RMSc for my P-01 and my Glock 43X simply because of its smaller footprint for those two narrower slides. I have not had any issues with either of my two RMSc so far, and I have been very impressed with them. They have the clearest window (no color tint) and the best auto-adjusting brightness feature. The window is made of clear polycarbonate with a scratch resistant glass coating. I know the polycarbonate window on the original RMS was easily scratched (no glass coating), but I haven't had any issues with these two RMSc that have the newer upgraded window. The RMS/RMSc is so small and low that you can typically co-witness the optic with standard height iron sights. I chose to completely eliminate the rear sight dovetail on my CZ P-01 when having it milled for the optic and utilize the built-in notch on the rear of the optic body as my backup rear iron sight. I've only had these two RMSc installed for a few months, so I'm not sure how the battery life is going to be, but I really like this optic. I may even install one on a compact 1911 handgun in the future. The original RMS battery can be changed without removing the optic from the gun, but the smaller RMSc and the new RMSw have to be removed from the gun and then re-zeroed after installation just like the Trijicon RMR. The new RMSw is the same size as the original RMS, but has a more robust hood over the window, has the upgraded scratch resistant lens, and is also water resistant, so it may be the best of all of them.
The RMSc and the FF3 are certainly not the most durable red dot sights by the nature of their construction when compared to the RMR, but I don't carry the guns that I have those mounted on every day. If you are looking to install an RDS on only one gun, and you carry that gun 365 days a year, then I would highly recommend the Trijicon RMR simply for its durability. If you have several guns that you rotate through, then the durability of the RMR may not be as important to you, and you should consider some of the other popular options like the Burris FF3, Vortex Venom, Shield RMSw or even one of the Holosun optics that have the same mounting footprint as the Trijicon RMR.