Only a one gun test but some years back I acquired an H grade "Parts" Lefever. The stock was broken, barrels heavily pitted & the gun would not close & bolt up. Invetigation revealed the screw which holds the trigger plate had been replace & the new one protruded inside the frame & the cocking hook would strike it preventing the barrels from fully seating. Shortening this screw cured that problem & the gun in spite of obviously heavy use &/or abuse was actually quite tight. It was further noted that at 14" from the breech the left barrel had a crack in it. It appeared as if it had been hit against a sharp object giving a dent & then had subquentlly been fired with thw result being that part of the dent was raised in the form of a flap & part was still dented inward. The crack ran along a weld seam & was about ¼" long & open enough by shining a light down the barrel you could look through the crack & observe the opposite bore wall.
I an an expanding dent plug down under it, raised the inward part & hammered down the raised part. The crack closed up until it became very difficult to see. I then taped up the stock & pout the gun in the ol Firestone Proof House. With me behind a large walnut tree & a cord tied to the trigger I proceeded to fire several factory 3¼-1 1/8 loads through it with no visible results. further proceeded to fire several 3 3/4-1¼ factory loads with again no visible results. I then fired some handloads containg 1 3/8oz shot with about a 3-3¼ dram equivalent of Unique powder. Again "Nothing". The crack did not open in the slightest & there was not even a smudge of residue which made it to the outside. In my mind at least this definitely laid to rest the old "Fact" that smokeless stressed a barrel somewhere down around the left hand position. I feel certain the Remington Express factory load would have had more pressure in the area of the crack than did my handload with more shot but a lighter powder charge & most likely a faster powder besides. This gun was barreled with what Lefever described as "Best London Twist".
