Benefits (How this will benefit the server and game as a whole): merely a new attachment that grants marines a new option when using shotgun.
Details (Description of how you think this would work, the benefits, etc): a magazine tube extension is a simple way to expand the ammunition capacity of tube fed shotguns. it is an extended piece of metal tube of correct shape and size and an improved spring to push the extra shells back into the shotgun's workings. i refer you to some relevant videos
https://www.youtube.com/watch?v=fUUpvjWSiCM
https://www.youtube.com/watch?v=GlXrdhibiKo
how many extra shells? thats up to you, i would say 2 at least, up to 6 at most as that would bring the m37s capacity up to 15 (14 in the tube+1 in the chamber)or 3 full handfuls of shells.
any downsides (accuracy loss, etc) are up for debate and testing
i do not have enough experience with any other shotguns in the game to tell whether they would be compatible with tube extensions or not, besides the obvious fact that shotguns without magazine tubes definitely wouldnt
also, one final video for fun

https://www.youtube.com/watch?v=i-koXI1iEK8