[img]http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=16149[/img]
[center]The gas tube roll pin hole is shown here ~3/4" to the right of the stamped "F" on the gas tube bracket.[/center]
[img]http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=16150[/img]
[center]Here is another view with the pin installed.[/center]
Remove the handguards and bolt carrier ass'y and support the off side of the FSB with an armorer's block or equivalent. Do not try to drive the pin out with the handguard cap resting on your work surface.
Use a starter punch to recess the pin slightly then drive it through with a short pin punch of the correct diameter.
If you cannot remove the gas tube by pulling it to the rear you can use a set of smooth jawed pliers. A vise with smooth jaws or one equipped with vise blocks is another option. Brownells also sells a handy, tho not very well designed, tool for this. Often the gas tube will be locked into the FSB if the rifle has many rounds through it.
Why are you removing the gas tube?