1-First you will need to match your barrel port to your valve port and breech seal ID dimensions. All three holes need to be the same size so the gas can flow smoothly and without restrictions. So find the tubing to be used as a seal and slide drill bits in it till you find one that is a good fit. I use the 1/4 inch poly tube pictured. It can be found in most hardware or building material stores.



2- Drill the barrel port slightly larger. From years of trial and error with hundreds if not thousands of chrony shots I have found that for the .22-.25 caliber guns ports near the range of .160-.169 inch are best. That's a #19,#18 or 11/64ths with a modified valve and the 1/4 OD poly tubing breech seal. Drill slowly into the existing barrel port and do not push to hard at the end as you go through so as to not cause to much of a burr on the inside of the barrel. Be careful not to damage the barrel opposite the port side when drilling. Larger calibers can benefit from slightly larger ports when on co2.



3- Use a 1/4 drill bit to countersink your port just slightly This time be careful not to drill so far through as to enlarge the port or break through with the larger bit. If you are using a mill then make a ¼ inch flat spot with the port hole in the center.




4- Clean up the burrs from the inside of the barrel. This can be done with a small fine file. Then polish the same area at the edges of the hole using some 400-600 sandpaper on the end of an object of your choosing. Do not compromise the bolt sealing surface by removing too much material here or you will have a leak.



5- Make your breech seal. It’s important that the poly tube be cut straight or there will likely be leaks. An easy way to do this is to push a dowel into the tube and roll the tube with a razor knife while applying pressure. Your seal need only be slightly longer than 1/8th inch. Too long of a seal will cause a restriction and loss of power. To short and you will have a leak. If you want a better seal then it can be made from delrin or metal but needs to be very accurately cut to size to fit and seal correctly.