If someone is aiming in public, then you should immediately take out your own weapon; Your Desert Eagle as you have no idea what he might do next. You should then tell him, assertively, to conceal his weapon, and holster it. Ask him about three times maximum, and if he doesn't comply, further action can be taken, such as using your pepper spray and nightstick to attempt to get him to comply. If he still doesn't comply, suspecting him can work, as you have enough charges to suspect him with.
Considering the previous scenario, you will be able to not only suspect him for having his weapon out in public, but also for failing to comply with your requests. Also, if he attempts to evade, that's a third charge you may put on him. So, that's where I let you in on a little secret of mine. In cases like these, I usually give the suspect a few chances, and if he fails to comply and listen to my orders, his charges build up automatically. Such as evading, failing to comply, or even assault. Therefore, when I suspect him, there's no reason for anyone to say it was a false suspect, as I have given the suspect enough chances, and I have suspected him for all his charges.
--------------------
If a suspect is assaulting someone, as far as I know, immediate suspecting is possible, and there is no rule against it. You then should use your pepper spray or nightstick to attempt to subdue the subject if he is unarmed. In the case of the suspect being armed, immediately take cover, and open fire, if he poses a high risk threat and/or he already opened fire.
--------------------
I like how you always ask questions when you believe you may be doing something wrong. That's the kind of attitude we like here at the ARPD. Keep up the good work, and make us proud.