5r has 60 degree bevel lands instead of the usual 90 degree shoulder. It's also 'gain twist', meaning it starts off at a slower twist rate the first part of the barrel then tightens up to 1-8" toward the end. It's my understanding it makes for a cleaner barrel, less copper fouling.
I know the 5r is used in the Thompson Contender barrels, can't comment about 'sniper' versions.
