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I saw a moc from here.
But have no idea what the piece it use on the arms of the robot, and on the guns.
It is a Tile 1 x 1 Round, with groove
Edit after clarification in the question:
The arms have something that looks like a bit of Hose, Ribbed cut to the required size.
The gun is, as gev pointed out, based on a standalone ray gun piece that has the ribbed section molded in originally.
The bit held in the mech hand is a Ray gun.
The bit on the forearms is almost certainly a short bit of Hose, Ribbed 7mm D. 2L like zovits said.
Required, but never shown