The wings are the strongest major part of any aircraft — their whole reason for being is carrying its entire weight, after all
These two need answering together: 425 rounds per gun, making 3400 in all. The weight of .50-calibre ammo varies per type, but
apparently, one round with link weighs 5.184 oz, or 147 grammes. That means 3400 weigh 500 kg, making 250 kg per wing.
Incidentally, the roll rate of these aircraft increased as they shot off more of their ammo, because of the decreasing the mass of the ammunition in the outer wings.
Rate of fire per gun is 750–850 rounds per minute for an AN/M2 .50-cal. Browning, so each gun has about half a minute's worth of ammo. Since, IIRC, you can only fire all of them or none of them at a time, that means a P-47 has about 30 to 34 seconds' of firing time, depending on the actual rate of fire of the guns.
I assume it would open once the wind catches it. The pilot had better hope this happens while getting up to speed on the runway, I suppose