Stocks we make have the inletting finished and glass bedded for a microscopically perfect fit. Below are some suggestions for people inletting some other brand of stock or one of their own creation.
Airgun stocks typically have a round barrel/airtube/compression tube channel cut in the forearm. I don't know how the front and back of these channels are cut at the factory, but I can tell you that cutting the tube channel with a round router bit (ball shaped) can't cut a square corner, so every duplicated inletting stock needs to have the front and back of the channel squared up. I use a 1/4" router bit in the milling machine, and an X-Acto #18 chisel blade in a #1 handle to square it up and I recommend the 1/8" for Dremel tool Saburr Taper or Cylinder cutter in the green coarse size from Chainsaw Sculptors: http://chainsawsculptors.com/saburr/small_dremel/index.htm. They are inexpensive, will last forever, and make effortless work of the adjustments necessary.
I cut the 1/2 circle cut outs on the forearm of break barrel guns (for the pivot bolt) with a forstner bit in the milling machine, but one can use the Saburr carbide cutter in a dremel tool. Using the carbide bit, you can creep up on the perfect size and location for a custom fit.
Sometimes it is difficult to tell where the action is hitting in the inletting. I can tell you with most assurance that the single most common spot you will find to be hitting is at the very back of the tube, at the bottom of the cut. Open that up to make sure the action is not hitting there. If you still can't find where the steel might be hitting the wood, use lamp black to coat the steel in soot which will leave marks on the wood. You can soot the action using a smokey flame from an oil lamp, or more easily with a piece of masking tape. Fold the tape on itself lengthwise, perhaps a piece six inches long or so. Light it and let it burn. It will produce a prodigious amount of black sticky smoke/soot that will easily coat the steel and give you a good indication of where the action is hitting the wood. The soot will easily wipe off with an oil cloth.
If a compression tube slot needs to be widened, a piece of coarse sandpaper hand held can be adequate, but you may also choose to make a barrel channel sanding tool with an appropriately sized piece of PVC pipe. Screw a cheap box handle to it, and use 3M77 spray adhesive to afix a piece of sandpaper to it.