You will need:
Metal supply
-2' of 1.5 in ID - 2.25 in OD .375 in wall DOM    If you can get an end piece it has a nice finish bevel to it. I bought in person at a metal supply but could be bought online (for a little extra cost), this is the cheapest I found online but may be more expensive without combined ship with other items https://www.ballisticfabrication.com/products/1-5-id-2-25-od-375-wall-dom?variant=25412672327
-1.5" of 1.5" round bar stock (inner plug)
-2.25"x2.25"x3/8 steel (rear plug) 2.25x3/8 bar stock or https://www.ebay.com/itm/202065039057?hash=item2f0c03ced1:g:OdAAAOSwT6pVnzrs
-12" aproximently of 3/4 round bar stock (arms)
Hardware store
-a few inches of 1/4 round bar stock (cross pins) optional could also just plug weld and it would probably be fine
-3/8" x 3.5" lag bolts and washers for wheels 
-6" aproximentaly 1/2" threaded rod for elevation
-4pc 2x3x8
-3/4 spade bit
-various screws 
Ammo
- sling shot grape shot https://www.amazon.com/gp/product/B07PMSX9W9/ref=