You will need to print four wheels, two wheel-axles, and one each of the other parts.
Print parts in the orientation shown. No supports are needed, but a raft is helpful.
Fix wheels to axles with a dab of superglue after the axles have been inserted through holes in carriage. Set barrel into position with small ‘fuse hole’ on top.
PRINT TIMES (on my Makerbot Replicator +):
Carriage - 39m.
Barrel - 26m.
Wheel - 3m.
Barrel axle - 2m.
Wheel axle - 3m.