A .223 isn't that loud I don't think (having never shot one) - I mean, not like a .308 or a shotgun or anything. It's only slightly bigger than a .22. A good set of earplugs and/or shooting earmuffs inside of a truck would probably be quite effective in protecting hearing. And shooting out of a small window I imagine would do quite a bit toward sound suppression, not to mention flash (which isn't going to be too visible in daylight anyway), as long as the muzzle is inside of the vehicle. No. The .223 Rem or 5.45X45mm is a high pressure round. Out of a short barrel like a Ruger Mini-14 the blast can be painful to be near, even with ear plugs. I'd rather be next to a M-1 Garand shooting .30-06 on the firing line.
Ok. thanks. Then I have to wonder if the guy is using a silencer.