Flying Space Cannons!

Hi, sorry for my joke title, I was just being silly. The truth is I want to explore the possibility of performing low cost micro launches into space. I've had this idea for years but for lack of anyone to share it with I gave up on it but that guy wants to freefall/parachute from low orbit via weather ballon so I have decided to give it another go

The procedure is:

1) use a weather balloon to go 36 km up
2) attach a gun (more like a artilliary barrel)
3) point up until ballons about to burst
4) *BANG* fire a microload (~3 or 4 kg)
5) at apogee fire rocket motors on 3~4 kg bullet
6) catch 1 or 2 kilo mass in kevlar net in space
7) recover barrel (it has fins and can glide robotically)

rinse repeat 50 or 60 times a minute (that is have 50 or 60 barrells a minute - - yes a minute - - shooting once each and descending by gliding - so thats thousands in the air at one time all returning to a air strip for new rounds and new balloons)

purpose? To send basically "lego" into space - use computers on earth and robots in space to build (slowly) space ships and fuel them but doing everything in tiny bits

I don't want flames or "lolz" I would genuinely like peoples guestimates on the amount of propellant needed and how large the charge on the gun would have to be