It is my understanding that a DI AR requires the user to "crack open" the bolt and let the water drain from the gas tube. Firing it without doing that is a major no-no and leads to catastrophic failure. So, in fact, you can not pull an M4 out of the water and start firing. You have to drain the gas tube. How often does anyone in the land of the civilian ever need to do that? I'd say...um...probably never. How often does the special forces teams equipped with the HK416 need to do this? Probably more often than we will ever know about.