i wanted to shoot some 6mm and 9.5mm steel balls indoors, so here's a project i've just finished.
Step 1: Materials
materials are based on the things that i have available in my room. you can just use this as a reference, make your own adjustments and choose your own materials when building the same.
2. corrugated boxes
3. A3 sized thick paper
5. duct tape
6. cloth or towel (A3 paper sized)
Step 2: Preparing the Target's Body
the main body must be prepared so that it will catch and collect the steel shots inside at each release.
stabilize the bottom part... glue the flaps together so no movement would occur. you may use books or anything for weights to press on the flaps while curing.
Step 3: The Backstop
due to the sling's power, steel shots would just punch right through the back end of the corrugated box. not to mention the noise it will create. installing the backstop will halt the steel projectile and would dampen the impact's noise into just a small thud.
have the box in a portrait position, using a cutter and ruler, slit an opening from one end to another.
insert the towel's end from the inside, adjust the length so that you would have the towel still touch the bottom, but hangs freely.
secure the towel from above using duct tape.
Step 4: Target Face Loader
when the glued flaps are cured, get a ruler and mark a rectacle in the middle having an inch from its sides to the boxes edges.
(again, i based this on an A3-sized paper. you may do your own measurements/adjustments depending on the materials that you have.)
cut off the inner rectangle using a cutter and ruler.
from your scrap boxes, measure and cut away half-inch strips with the exact length of the box's sides. glue them to the edge. the strip would then be higher (raised) afterwards. you only need to do this on three sides, the top side should be open.
from your scrap boxes again, measure and cut away 1 inch strips with the exact length of the box's sides, then glue them on three sides. this is the second raised level.
after this, you have now finished the area where you would easily insert your target faces. very convenient when replacing worn out targets.
Step 5: Target Face
here are some tests shots.
now using a slightly thicker A3-sized paper (just thick enough to pass through a laser-jet printer and not to jam it), print out your targets and slide them in the loader.
at this point, you're done and you can now shoot away.
i didn't make an elaborate setup on this project, just the basic idea (because i'm excited to shoot). you may do your own modifications if you wish.
by the way, i have taped the target onto two more boxes to make its stand, or a sort of tower, and placed some weights on the lowest box for stability.