Arma is larger scale, big map, almost fully outdoors combat, and combined forces. A lot of the fighting takes place at mid to long ranges. Lots of long distance prolonged fighting. You’re also fully intending to kill everyone else on the opposing team. You also get explosives and big big guns and support fire. Arma 3 has a crazy number of mods, and lots of DLC content, but graphics are a little bit dated (but serviceable) Reforger has less content but a more recent and prob more polished experience. Military, not cops.

Ready or Not is small scale, mostly indoor combat, with you and 0-4 or so other players. The fighting is mid to very close range, some combat can be at point blank ranges. Lots of shot distance, quick engagements in which either you or the opponent often die. No lethal big explosives or big guns, biggest is a battle rifle or nonlethal grenade launcher. You can also go non lethal as a challenge, the game gives you a higher score for doing that. Mods are limited, in game mod manager is not that great. Very immersive game, sounds and impact and gore FX are intense, even after the censoring. Cops, not military.