After thats done take your half inch coupler and glue in about a 1/2" pice of 1/2" PVC on the side of the coupler that will be glued to your tank.

Now take a 3" section of 17/32" brass and insert 2 inches of that into a 1ft section of PETG. Glue a section of 1/2" PVC from the base of the PETG upwards. Make sure that 1" of brass is still protruding out and glue an airtight seal to the PVC.

Now when you insert your barrel into the coupler virtually all the dead space is removed.

Then I just added 6 scrap pieces of PETG as dart holders.

Haven't bothered measuring ranges. Slugs at 8 pumps hit close to 140.