I recall my first day of basic training. The drill sergeants made us stand with our arms outstretched holding our canteens for five minutes. Canteens are about two pounds when full. There wasn't one person there who wasn't in agony after minute #4. Holding two to three pounds of handgun out away from your body for extended periods is going to wear on you after awhile. I would suggest some shoulder exercises to make it less exhausting.
"If ye love wealth better than liberty, the tranquility of servitude than the animated contest of freedom, go from us in peace. We ask not your counsels or arms. Crouch down and lick the hands which feed you. May your chains sit lightly upon you, and may posterity forget that you were our countrymen!” - Samuel Adams