What kind of weight difference am I looking at with 4, 6 or 8 loaded PMAGS over the same amount loaded with 5.56.

30rd of 110gr supersonic = 1650gr heavier than 30rd of 55gr. Brass weight difference should be negligible. 30rd of 220gr subsonics = 4950gr heavier.

7000gr in a pound.

If I read your post correctly, then 8 mags of 300blackout subsonic would weigh about 5lbs heavier than 8 mags of 5.56

lol you expecting a fire fight in the deer stand? ok seriously yes you are correct there will be about 5 lbs difference in 8 magazines but for hunting or range tme you wont notice any difference.

I picked up 200rds Rem 115gr 300 Blk, loaded the 4 30rd mags on my warbelt, (my normal load), Weight difference was negligable, I have 4 more 30rd mags in a chest carrier that usually go along for the ride in case crap goes south in a hurry (so far they havent come out of the BOB).

Only once did I wear my Warbelt and my chest carrier at the same time, also had my Blackhawk plate carrier w/plates, realized I wasn't going to leap any Mazda Miata's any time soon.