I love me some Hk21...
The new M60E6 is a machine of true awesomeness as well -- not being an M60 guy, it has been interesting to learn about it's development (or mis-development) and the issues that where caused due to the manufacturing was done over three companies, not 1.
The 63 is a great tool for its day - the Colt CMG2 is another impressive RVN era gun that unfortunately died a painful death.
While modular is great for some things, the modularity of the 63 would have driven any logistician bonkers - their are around 26 variants of the 63A, from rifleman, to coaxial MG - at a certain point, I think it is important to see the diminishing returns.
I think you'd have ended up with Platoons of folks using the carbine in the belt fed mode, cause it was cool, not due to any requirement.
Also due to its modularity - it is pretty impossible to setup well for current requirements - since the receiver could be flipped over depending on feed method - the gun does not lend itself to modernizations (rail or attachment methods for combat enablers).
The LMG 86, LMG 99 and now LMG A1 (our 2013 and 2014 PIP) use run out - like the Ultimax, the recoiling parts do not impact the rear of the receiver - the moving parts simply are retarded by spring pressure and return into battery -- most LMG's/GPMG's do not have enough receiver space to do this, and the operating system collides with the rear of the receiver (Minimi etc).