Added some pictures and expanded a couple of the steps...but no inletting pictures as of yet.
Got some questions about converting QB78 tubes to tankers. Figured it would be good to put the reply here.
Besides the tube and gas block, will need (or borrow):
1. masking tape
2. tubing cutter
3. hack saw (or Dremel tool)
4. Electric drill and drill bits (can use a closer sized drill, but 3/16th will work).
5. 1/4" wood chisel (or 3/4"..either will work)
6. Empty beer can (or Coke can)
7. strong scissors (or tin snips)
8. fine cut round file (for deburring edges/holes)
9. Silicon grease (can get it at Lowes or other hardware store.."Gunk" brand works fine).
10. Epoxy putty
11. pocket knife (or Exacto knife...or any kind of small knife)
1. Turns out all of my QB 79 gas tubes are cut to an over all length of just about 13 1/4", but anything from 13" to 13 1/2 will remove the treaded section. The idea is to use a full length QB78 stock, with the gas block inletted into the front of that stock, so the exact length isn’t engraved in stone. You just find the length that pleases your eye.
2. Tape up the end at the expected cut with masking tape.
3. Run a TUBING cutter around the masking tape at your chosen length (call it 13 1/2"). Length isn't real important, you are doing one gun, not making a production run. You are NOT cutting the tube with the tubing cutter, are scoring a line evenly all around it (the tape is just to keep the tubing cutter's wheels from scratching the blue).
If you don’t have a tubing cutter to score a line into the tube, can very carfeully apply two strips of tape, with a small space between the strips…that space would act as you guide line.
4. Remove the tape and cut along the scored line. Can use a Dremel and cut off wheels or a hack saw, just STAY ON TOP OF THE LINE ALL THE WAY AROUND and it will be pretty even.
(The actual project pictured was a Crosman tube…but I added a cut off sction of QB tube at the top to give you and idea of how much is getting cut.)
5. FILE the end smooth and even...lot of checking to be sure it is even.
6. DEBURR the inside of the tube. Any lip/burr here and it sill slice the o-rings of the tank block.
7. Cut two 1 1/2" long by 1/4" wide strips of Al. BEER CAN (or Coke can).
8. Punch two small holes in one end of each Al. strip.
9. Lay them so that the punched holes line up with the tube mounting holes on the gas block (can run a nail though the block and through the holes on the Al. strips to hold that end in place) and tape them FIMRLY to the gas block on the other end.
10.Insert the gas block into the gas tube. take CARE that the top of the tube is really "up" and the gas block hangs down....won't do to have the gas block at an angle.
11. The two Al. strips are on the outside of the gas tube...the holes are in the right spot for the threaded holes in the gas block...so mark the tube with punch through those holes in the AL. strips.
11 ½. By using a punch, you get nice starting dimples for the drill.
12. Remove gas block and drill the tube for the mounting screws (want the hole to be close to the screw size). The common size that is a close fit (closer than the factory holes in a QB79) is 5/32.
13. Deburr the new holes on the inside of the gas block..they will cut the o-rings of the block unless smooth.
14. STOP…the tube is done. CLEAN IT…clean it again…really really clean it. One metal shaving is enough to shread the valve stem seal…so be sure everything is clean-clean-clean.
15. Take the tape and Al. strips off the gas block. Lube the gas block's o-rings with silicon grease and insert it into the tube. The holes should line up if you took care with steps #9-#12.
16. The mounting screws are there just to keep the gas block from shooting across the room,,,they don't "seal" anything, so don't crank down on them extra-extra tight our you'll just raise burrs on the inside.
17. I like to inlet the tip of the stock to surround the gas block (as is done on the QB79), which isn't that hard to do as you are basically cutting a square. Can do it with time and a 1/4" wood chisel (although a Dremel tool can help). Get the width close to tight. Don't feel you have to be too exact in depth, are going to epoxy bed the block to the inletting as you want the support the wood can give. No need to add the stock screws unless you really want to.
18. With a little "extra" depth, are going to use epoxy putty to bed the block. GREASE the bottom and sides of the gas block. Kneed a lump of epoxy putty about the size of a dime. Spread it out on the bottom of the gas block inletting cut (make it about 1/8" thick). Put the rifle back into the stock, and press down firmly...want the rifle to sit down in the stock right, and the gas block to squeeze out the extra putty.
19. Let it set until firm, but not rock hard. Cut off the excess the squeezed out with a dull knife (or the edge of a credit card).
20. While still firm, but not quite rock hard, remove the action. The putty will stay in the stock, and can clean the metal work and cut off any epoxy lips. edges with a pocket knife before it turns rock hard.
21. Should be go to go at this point, just reassembly and have fun.
On your own about barrel bands..some use them (I do) and others don't. Best choice is a QB 79 barrel band because of the way the cross screw runs (by design, the QB79 band allows a full diameter screw without needing to slot the barrel).
