Why are they chopping down the gun barrels?
What effects, positive and negative would that have?
Was it at all officially sanctioned or would they be in trouble for doing so?
Cheers
Well, the advantages are a shorter, and lighter weapon. The disadvantages are a shorter effective range, and worse spread, together with a larger muzzle flash (7.62 NATO can cause quite the fireball in a short barrel). It makes sense if you expect combat to only be up close.
I don't know about the regular Army or Marine units, but concerning the Special Forces, here's a quote from "The M60 Machine Gun" by Kevin Dockery:
"The SOF were given a great deal of leeway in adapting their weapons for operations, and the M60 was no exception." ... "The average engagement range during SOG patrols or a SEAL ambush was 25-30yd maximum. The barrel was too long and the front sight unnecessary, so the barrel was cut back to the gas system."