Considering the factory transfer port appears to be made out of a piece of burnt egg, it seems like just about any material should do.From what I've read here on the GTA it seems like restricting the transfer port is a good way to reduce velocity but not necessarily the best way to increase efficiency. It seems like most people mess with the hammer, hammer spring, and/or valve to increase efficiency. It seems like it would be quicker and easier to shorten the hammer spring than to create a new transfer port, albeit more permanent. I'm certainly not an expert though, so I may be misunderstanding. When I replaced the factory transfer port in mine with the Crosman M-rod one I got a significant increase in velocity with not as much of reduction on shot count as I expected. I'm eventually going to try to tune it for better efficiency, probably starting with the hammer spring. Another future retirement project.
Very good info! It seems like every answer brings new questions, though. I've found 1/4" PTFE Nylon round stock on Amazon..I'm wondering if I could cut it to length and drill the port any size I want? Tons of it for cheap so trial & error mistakes wouldn't be costly.On the BStaley mod, I saw a guy in the UK (YouTube) glue an o-ring directly onto the hammer block. Is that necessary, or will a 113 o-ring just drop down the tube & live happily there? Since there isn't a lot of valve stem exposed for the hammer to strike, will one o-ring do the trick?Lots of interesting new ideas to test out!