When you have no ammo in reserve in your gun in hand, fire all but 3 bullets from your gun.

Usually, this will make your character start complaining about being out of ammo and he will say several of his sayings in a row. If not immediately, just wait for it.

I believe it works on DR and SSN, but not the others.

If done with Nikolai, he often starts shouting several of his audio clips over top of each other, so he is saying multiple things at the same time.

Tank will sometimes say 1 saying, then 2 sayings, then 3, but sometimes it does not work that way, he may yell 6 in a row or keep doing groups of 2-3 sayings at a time.

Some times it does not work, I do not know what makes it not work sometimes, but it works even with PaP'd guns when it does.