AR-15 buffer weight - what weight to use full length upper + "carbine" lower.?

As far as I can tell in order for a full length (full length gas system) upper to function correctly with a carbine length buffer tube it needs to be equipped with a heavy or H2 buffer, does anyone have first hand experience with this combination and if so what equipment did you use? Added - Don't tell me it... show more As far as I can tell in order for a full length (full length gas system) upper to function correctly with a carbine length buffer tube it needs to be equipped with a heavy or H2 buffer, does anyone have first hand experience with this combination and if so what equipment did you use?

Added - Don't tell me it can't be done or its not safe, Colt Canada makes all their full length rifles on contract for the Canadian Army with "carbine lowers" and full length uppers.
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