1st, who is selling 30rd 6.8 SPC magazines?
2nd, will 6.8 SPC magazines work with the 7.62x39MM round?
I don't think anyone is selling 30 Round mags for the 6.8. 25 is as hign as they go if your LEO/Mil.
"Proper 6.8SPC magazines are available from two sources:
PRI directly in 10, 15, and 25 round capacities (15 & 25 for LEO/Military only).
Conversion kits consisting of a new magazine body and replacement follower are available from MSTN to convert your old pre-ban AR15 5.56mm magazines to be 6.8-compatible, also in the 15 and 25-round capacities. This works just like Armalite's deal for AR10 conversion kits.
* - There is one report that the cheap 20-round Triple-K 7.62x39 magazines work with 6.8SPC."