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There would be a few decisions I would make first. If you are going to shoot subsonic you would want a pistol gas port. Then you would decide between a "oly" or "standard" pistol port. Nothing wrong with oly parts but they do things their own way and their tubes and gas blocks are not standard size. The location of the port will depend on whos parts you aquire. If you don't mind doing a second setup then you can put the extension on and fit the barrel to the receiver. Put the assembled block and tube up next to the barreled upper and where the rear of the bell on the tube falls in the middle of the milled circle in the receiver you are very close and you will know which dimensions are correct. I would not use any dimension gotten off the internet without varifying. Even the location of the gas port drilled in the block varies slightly between some brands. Get all your stuff together and take some measurements and you will be fine.
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