I have found you actually want a bit of weight in a 3gun rifle although personal preference may differ. 3gun courses will test you, they shouldnt be setup to your strengths. A bit of weight in the fore end is a benefit especially when you find yourself trying to hit somthing small at 200 yards while standing without a rest or leaning around a barricade shooting weak hand. Good trigger will be a priority here to.
A light gun may help on short range hoser stages for maneuverability but the trade off will be muzzle jump and getting alot of feedback through the gun which isnt a bad thing but you start getting sight picture deviation. The sr556 would probably make a real nifty ipsc shooter if thats your intention. A little heavy for a hunting tool but its a preference thing.
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