Galactic Patrol by E.E. Smith

While servicing and checking over the speedster required only a couple of hours, Kinnison did not leave Earth for almost two days. He had requisitioned much special equipment, the construction of one item of which -- a suit of armor such as had never been seen upon Earth before -- caused almost all of the delay. When it was ready the greatly interested port admiral accompanied the young Lensman out to the steel-lined, sand-filled concrete dugout, in which the suit had already been mounted upon a remote-controlled dummy. Fifty feet from that dummy there was a heavy, water-cooled machine rifle, with its armored crew standing by. As the two approached the crew leaped to attention.

"As you were," Haynes instructed.