I apologize in advance if this has been the topic of previous threads.

The plastic tube that silicone caulking (and some other stuff) comes in can be re-purposed as a storage/transport tube for the Atchisson pattern AR-15/M16 .22 conversion unit.  When the caulk has been used and the tube is nominally empty, push the piston back all the way out of the tube.  Make sure you do this as soon as the the caulk is empty and the residual caulk has not set up.  Wipe off/remove the residual caulk from the inside of the tube and nozzle, and the piston.

The chamber adapter section of the .22 conversion fits nicely into the nozzle and the caulk tube piston securely retains the unit inside the tube.  To easily pull the piston out, I put a couple small holes in the flat of the piston and used a small "cable tie" that came from a used clothes washer lint trap to make a pull loop, thereby keeping the storage tube assembly free of cost.  To fit into the 10X7X3 cardboard box that I use to transport the conversion unit and mags, I trimmed about 3/8" off the back of the tube and shortened the nozzle by about 1/4" from where it had been sized for the caulk bead size I had been using, taking care to ensure that the nozzle was still long enough to adequately protect the .22 adapter chamber section.

MHO, YMMV, etc.

ETA pic: