I bought both a core & a zci barrel from Ak2M4 when they had them in stock (6.03 & 6.02 respectively).
The core worked really well originally, not actually far off my Prometheus 6.03 (up until the gearbox started playing up) whilst the zci was decent, but when put side by side with the Prom was a bit lacking.
(Note; all AK length barrels, used with G&G .28 ammo and G&G green buckings).
At present I have two AKs with Prometheus barrels (one G3 length though) and one with the Core.
The zci is in a drawer until I scrounge up enough parts to build the rest of this other AK.