Standard QB79 gas block. Archer sells the part.
Rather than change it all out to a QB79 tube, converted the longer QB78 tube that came with the rifle.
It's a bigger job that it seems. Need to start with a bare-empty gas tube and will need to DETAIL clean after the metal work (even one stray bit of metal grit can ruin your valve seal).
1. Cut the 78 gas tube back about 1.9" (2" will work just as well). This will remove the threaded part and enough of the gas tube that the 79 gas block ends up about flush with the stock. (Needs to be cut even..smoothed,,,and a smooth bevel put on the inside edge).
2. CAREFULLY measure and drill for the mounting screws. Get this wrong and you're screwed, so be sure it's the right spot. One "trick" is to cut shims from a Coke can, tape them to the gas block...punch holes exactly over the holes in the gas block's screw holes. When you slip it into the tube, the shims will be outside the tube and you can spot punch your holes through the shims. MAke dead sure the gas block is LEVEL before you drill (cock-eyed would still work, just look stupid).
3. Take it apart, smooth the rough edge where your two screw holes were drilled, clean it (and clean it again).
4. Can put the metal work back together again as we're done with mounting the gas tube. Really helps to use a bit of Silicon grease on the gas block's o-rings. (HINT: the issue 78 barrel band will NOT go over the gas block, so it has to be put on first).
5. Inlet the stock for the gas tube. This is a job you cannot hurry. I'll rough it out with various Dremel cutters, switching to little burrs and a small wood chisel for the final corners. You just keep fitting...spotting the high spots...sanding/burring them down, until it fits into the stock. YOU WILL HAVE TO INLET the BARREL band as well. I didn't use a 79 band, used the 78D's band, just mounted behind the gas block. Got it about deep enough when a little of the gas block mounting screws have to be inletted into the side of the stock.
6. Measure and drill the stock for the stock screws. These are pretty common size screws, many scope rings use this size (so if you have a dead ring or two, give the screws a try). Again, really need to be EXACTING. I'll inlet a larger opening and use brass washers for the screws to go through (the stock is thinner than a 79's, so there is less wood between the screw head and the gas block, which makes a washer a must to strengthen the stock).
7. Glass bed the gas block to the newly inleted stock. This adds strength and support (but be dead sure you've used a good release agent or you'll never get it apart). Use the gunsmithing stock bedding compounds...can color them brown to match teh stock as my inletting skills are all that great.
Here you can still see a pencil line i put n the gas block sfor a meaasureing point (to get the stock screws in the right place):
First one was a converted 78 in .22. Only difference is that I used two plastic barrel bands. Can get and idea of the differnce in stocking from this picture (have to say, for me, being able to grab wood rather than gas-bottle makes the conversion worth doing).
