I think of "space guns" as being the NRA match rifle configuration with iron sights and when they've built an adjustable stock all off the buffer tube but have excluded the typical A2 plastic buttstock, they also typically have the requesite handstop built into the freefloat tube, plus they've got the bubble levels and high dollar match iron sights. The things definitely look like some sort of Frankenrifle.
But I've also heard of space guns as being described as just really accurate full tilt AR15s with the standard freefloat tube and high dollar match barrel. Basically what everyone's seen with respects to scoped flattop and freefloated AR15s, wicked looking but atleast they retain some beauty where as the NRA match rifles can just get DOWNRIGHT FUGLY!
But generally the space guns are your more, or most, accurate AR15s.