Thank you for sharing your valuable experience. I want to add that carrying the Ultimax drums don't look like a picnic either. See attached pic.Carrying the 200 round boxes can be a pain as well.
I think the years of fighting in Iraq speeded the requirement for a different kind of weapon and deployment.I'm not really sure of the value of an IAR per the USMC requirement.
Since for the past few years US military had been fighting insurgents in (still deadly) smaller scale urban actions, moving in/out and around buildings a lot...
You won't be digging in and fending off human wave attacks.
In these situations you would want more mobility in firepower than sustainability. So you're gonna be carrying your auto-fire platform into battle more than using it in a bipod role.
Besides, the role of sustained fire MG is well provided for by the platoon's M240s or from the mounted weapons of the accompanying Humvees or other AFVs.
Jim Sullivan's Ultimax is the perfect example for this role of highly mobile firepower. The 100rd drum still gives a good account, and while no picnic, should be easier to change than belts. It is extremely controllable and highly accurate and has QCB if you do need to burn brass.
And even the short gunners (we're all asians) in my reservist section had no problem climbing through windows etc with the Ultimax cos it is so damn light.