I think that it's intended for it to take time to use the synths. If there were a hotkey, in theory you could use one during a reload of your last clip and never have to stop shooting. Gives no reason to change position or go pick up ammo at all. I wouldn't mind having the hotkey, I just think it's intentionally not there. PS. Primary ammo should be more plentiful in the Raid. I don't mind having to wait for special and especially heavy, but you should never run into a situation where you run out, use a synth, then run out again. This doesn't happen too often, just the random nature of it leads to a frustrating situation every now and then.