I've modeled a weapon, but in an attempt to get symmetry, I used clone, and spun it 180 degrees.
However, the original half of the weapon and the clone won't attach.
Is there any way to get these two objects to join?

You most likely accidentally made the clone an "instance". Instances must first be reconverted back to either mesh or poly before they can be attached. Hope this helps.

oh *facepalm*
Yeah, that was it.
I just cloned again as copy and that worked. Thanks.

you could have used a mirror modifier to do that, fyi

or, my cool new script you just inspired me to make - symmetry for gmax!

shapesAndGeometry= #()
for o in selection do
	If IsKindOf o GeometryClass Or IsKindOf o Shape then
		append shapesAndGeometry o

if shapesAndGeometry.count > 0 then
(
	disableSceneRedraw()
	
	select shapesAndGeometry
	
	sm= SliceModifier Slice_Type:2 Faces___Polygons_Toggle:0
	mm= Mirror Copy:true Mirror_Axis:2
	
	sc= sm[#Slice_Plane].controller
	mc= mm[#Mirror_Center].controller
	paramWire.connect2Way	sc[#position]	mc[#position]	"position"	"position" 
	paramWire.connect2Way	sc[#rotation]	mc[#rotation]	"rotation"	"rotation" 
	paramWire.connect2Way	sc[#scale]	mc[#scale]	"scale"		"scale" 
	sc[#rotation].value= (quat 0.707107 0 0 -0.707107)
--	sc[#position].value= $.center <- didn't work as I had hoped. 
	
	addModifier $ sm
	addModifier $ mm
	
	enableSceneRedraw()
)

it creates Slice and Mirror modifiers, wires their gizmos together, and adds them to selected shapes and geometry objects. There's no weld modifier for gmax, so you weld manually after converting to poly/mesh