The UK and US forces I encountered in KAF (usually around Timmies) had/have these 50rd bags on their C6/GPMG/M240's... they had few complaints about them, but pointed out that they mainly used the 50rd bags at "teasers" for dismounted patrols, and would switch up to just a long belt when in prolonged contact (I know, preaching to the choir.)
But they also had a belt bag which hooked up like webbing and fed ammo from the gunners back to the weapon... we had these as well. My understanding, and I may be wrong about this, but this is just what I was told when my boots hit the ground; these ammo bags were built/developed/jury-rigged by the PPCLI guys we took over for, as they needed to carry more linked and loaded ammo into fights as they were heading into Helmand Prov. as well as Panjwayi and Pashmul. These rigs were left in our CQ for the handover, from what I understand, and we used them quite a bit. I'm not sure how many we had, or if every platoons dismounted C6 had one, but my platoon did... and I didn't hear much complaints about it.
Anyway, here's a pic;
pic deleted by MG34
As for the 50rd bags... I definitely would like to give it a try. I've heard mixed feelings on it, but when I think about what purpose it's going to serve where I am not (PRes), it can't be that bad.