What I saw on Reddit was that you get the Riot shield on and have a VTOL or Chopper Gunner target you, it'll block the shots infinitely and somehow eventually make you invulnerable. Haven't tried it, don't care to, but also haven't run into any "invulnerable shield guys," either. I've got around 50 hours in and haven't run into a single cheater that I know of.
In MW 2019 I actually liked using the riot shield to occupy a VTOL. Just find a corner of the map and put yourself in it with the shield on, it'll eventually lock on ya and just stay there until it times out. Easier to just shoot it down, I suppose. I almost always carry a launcher (hate being on UAV and prefer other perks most of the time). Maps like Rust and Shipment, though, you don't really have time to whip out the launcher, and that's when a shield came in handy to occupy the VTOL.
Been working on my AR's. I've got 5/8 gold. Going to get those to platinum before moving on to a different gun type. Been strictly playing Domination - may move over to invasion to do these last three, I hear it's much quicker there. I'll definitely be doing Invasion for the longshots to unlock platinum though. Never been a fan of AKs, going back even to Counter-Strike, but after playing with the two Kastov's I actually like them quite a bit in this game. Someone here recommeded the M16 early on, and I was happy to hear that it was back and effective. My first few matches with it were incredibly painful, but once I got the hang of it, it truly is a good weapon. I think I prefer the FTAC Recon though, that thing is bad ass.
