What are you referring to when you say "single piece" action? Every receiver I've ever seen is a single piece. As far as strength, they're all built strong enough for their intended purpose. Stiffness is a relative term....stiff enough for what? I've never had an issue with an action not being stiff enough.

The current IBS 1 mile 5 shot record was shot by Tim Lambert on 10/04/03 at Pella Ia. The group measured 10.938. Tim also won first place at the NBRSA 1000 yd Nationals by setting a new 6 target agg record of 8.964. That summer he also garnered the VHA longest shot award by hitting a prairie dog at 2155 yds.
And now you're wondering what did he do it with?..............

a 7mm WSM screwed on to a 2002 vintage Savage flat top single shot action.