I have tried this with 22LR Sub-Sonic, 9MM Sub-Sonic, and 45ACP all shot through suppressors of course.
The quantity of bags of sand used will depend on how deep your wheel barrow is.
You can't use to much duct tape to hold the bags of sand in the wheel barrow when it is stood up against the wall.
Last but not least............DON'T MISS YOUR TARGET! Or you might just be sleeping in the garage for awhile!

As you shoot it the sand will come out. You are going to end up with a hole in you wheel barrow. I would try using a sheet of steel plate angled at a 45 encased in a 2x4 frame. Built a 4-6 inch deep "box in the bottom and add sand.
   ___
  /     l
 /.     l<<<<............................
/ .     l
/_.__ l_________
++++++++++++l

Not sure if my diagram translate very well. But the angles steel (/) will direct the bullet (.) downward into the sand (+) in the bottom of the frame work (l___).

This is true, however I can generally get over 100 rounds off before I start to see the bags of sand "bleed out". You can get even more shots off if the sand is damp. This was thought up  merely to test my 9mm, and 45acp reloads.

I wouldnt discharge a gun in the same structure as the normal living areas due to the lead and heavy metal contamination. In the garage or outside I would use a 55gal drum at a 45deg angle from the floor with a hole cut in the top and filled with sand (same as a clearing barrel). Put a piece of rubber (plan to replace frequently if using hollow points or wad-cutters) across the top to contain the sand.

Kharn