That doesn't make sense to me. It seems to say the gun with that load could only be fired horizontally. I don't get that at all. The more horizontal the barrel is, then the Friction forces will increase. Friction is proportional to the Gravity Force perpendicular to the "Surface" or the inside wall of the barrel.
Also think of it this way. The "ball" spend only a fraction of a second in the barrel. So how could the weight of the ball add more weight forces to the barrel for that short period. I guess I am thinking of a small caliber Blakely gun but I don't see any difference.
Whereas, if he means it adds more stresses to the Carriage or Frame or Axle, then it could cause damage. As an example, if you have a standard field gun and mount the gun on the slope of a hill and fire it vertically, then the load of the recoil could overload and break the structure supporting the gun.