Cutting a QB78 Main Tube ?

Joined: November 14th, 2015, 5:30 pm

January 16th, 2016, 3:22 pm #1

Can a main tube of a Qb78 be cut using a pipe cutter .... the type with the round blade that you twist around the tube slowly tightening till it cut s through ? After which you de-burr the inside and you are ready to drill and install your tank cap replacement !

wll2506
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Joined: June 4th, 2013, 4:42 am

January 16th, 2016, 4:12 pm #2

I did it once but didn't like how it turned out. I prefer to just use a turn of painters tape to establish a square cut line and use a hacksaw and do a little cleanup with a file.

The pipe cutter invariably mars up the bluing even if you apply tape to protect it, and it still needs a similar amount of cleanup.

Just my take on it.

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Joined: November 17th, 2006, 3:51 am

January 16th, 2016, 5:13 pm #3

Can a main tube of a Qb78 be cut using a pipe cutter .... the type with the round blade that you twist around the tube slowly tightening till it cut s through ? After which you de-burr the inside and you are ready to drill and install your tank cap replacement !

wll2506
Been about 7 years and the pictures were missing....but I updated it.

Rifles still working fine, so evidently the process woeked and lasted.


This is geared to HAND TOOL conversion. A lathe, drill press, etc. will make it a lot easier, but if you take care with simple hand tools, can do just as good a job.


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 different sized drill, but 5/32th 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)
12. Lots of rags and cleaning gear (one stray chip of metal can undo a valve in a heartbeat).

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 caerfully 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 section 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.

12. By using a punch, you get nice starting dimples for the drill.

13. 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.
14. Deburr the new holes on the inside of the gas block..they will cut the o-rings of the block unless smooth.

15. 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.

16. 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. Holes will actually be tighter and stronger than a QB79 if you did it right.

17. 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.

18. 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.

19. 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.

20. 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).

21. 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.

22. Should be go to go at this point, just reassembly and have fun.


For a time, I tested a pair of QB carbines, one a standard tube QB79 and one a long tubed QB78 conversion. Pretty much equal in length (both had short barrels).

Same pressure, same barrel length, I even swaped around the valves at one point.



Whatever I did to increase speed in the QB79 also increased speed in the long tube conversion, but got much greater speed in the long tubed version. Just had more “on deck” air, air that had already been regulated, so could maintain a higher pressure “push” for a longer time.
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Joined: November 14th, 2015, 5:30 pm

January 16th, 2016, 8:30 pm #4

Your post was EXTREMELY helpful to me. I had thought the conversion was a lot harder when I first started learning about the QB series of guns, but it really is not.

The adaptation of the block to '78 leaves all types of possibilities, including a backwards, larger tank ;- )

Again thank you very, very much. now comes the decision of, do I do this to a new '78 or get a older used one and use that as a "Starter Learning Gun" If truth be known I thought I had two very, very old Qb78's in boxes, many, many years ago, and they would be perfect for my experimenting with ! I may rip through everything to see if I could find them !

wll2506
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Joined: September 7th, 2001, 3:52 am

January 17th, 2016, 5:18 am #5

Once you've fitted a QB tube with a tank block, we'd like to know if you (then) consider it relatively easy, or a PIA.
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Joined: November 14th, 2015, 5:30 pm

January 17th, 2016, 7:38 pm #6

Looking for an older QB78 to work on ... believe me, I will let you know ;- ) I will mod the heck out of that gun, I have plans for a larger than 13cu/3000 tank and will put it backwards ... so stock modification is in order, Will do some simple mods on the breech air to barrel opening ... will try to get good speed but have a high shot count gun ;- ) ... I have my feelers out now ;- )Thought that 22 caliber would be perfect, but now not so sure ?

wll2506
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Joined: May 25th, 2013, 11:52 pm

January 17th, 2016, 7:39 pm #7

Once you've fitted a QB tube with a tank block, we'd like to know if you (then) consider it relatively easy, or a PIA.
I tried to help him out the other night. I have two QB79 air tubes and was going to offer him one for the price of shipping. Before I could offer he changed his mind and was going to buy the gun after they came in to the dealer. I had looked up the parts from Archer and it was going to cost 81.89 and that was not with the stock barrel band and some of the little parts that come up later in the build. so I thought that his idea to get the whole gun was a good idea but it would be not as much fun to build one from a QB78. So I am like you would like to see what he thinks about the whole thing. I found it a lot of fun and wanted to shoot my self at times Go for it Will2506 and let us know. THANKS Tom Mace
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Joined: November 17th, 2006, 3:51 am

January 17th, 2016, 11:11 pm #8

Once you've fitted a QB tube with a tank block, we'd like to know if you (then) consider it relatively easy, or a PIA.
..so much as outline the easy way to turn a 78 into a tanker (either co2 or HPA)

Opinions (having used both) are:

If using co2, it doesn't make a rat'sarse either way.

If using HPA, either way lets 12 foot pounds happen without much difference in shot count with even 850psi out put.

If looking for more power, the long tube lets that happen with fewer valve mods as it has more available pre-regulated air.

Head to head, short tube to long tube, the long tube runs faster with the same valve mods.

So I'll make it simple.

If looking for as many shots as possible at about 12 foot pounds, would go with a QB79.

If looking for mo'power, would go with the QB78 cut-down-tube.

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Joined: November 14th, 2015, 5:30 pm

January 17th, 2016, 11:34 pm #9

Can a main tube of a Qb78 be cut using a pipe cutter .... the type with the round blade that you twist around the tube slowly tightening till it cut s through ? After which you de-burr the inside and you are ready to drill and install your tank cap replacement !

wll2506
I'm a babe in the woods, and I'm excited about everything I'm learning. Tom was a big help to me and a very nice guy and I decided one thing, then I read another post and had second thoughts.... so I'm all over the place :- )


wll2506
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Joined: June 4th, 2013, 4:42 am

January 18th, 2016, 3:34 pm #10

Can a main tube of a Qb78 be cut using a pipe cutter .... the type with the round blade that you twist around the tube slowly tightening till it cut s through ? After which you de-burr the inside and you are ready to drill and install your tank cap replacement !

wll2506
Cutting and squaring the end is easy enough. It's drilling for the retaining screws that requires patience and attention to detail, particularly if you are to maintain the maximum amount of material between the screws and the end of the tube for safety. Robert's soda can jig works great, I highly recommend it. Beyond that, you just have to be attentive that the drill bit does not walk. It helps to center punch before drilling and start with a small (say, 3/32") drill bit and work your way up.
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