WADR, those BBs made for air rifles are much Harder steel than the Steel BBs made for shotguns. You are making the same mistake that many of us made years ago trying to use the Air Rifle BBs( .177") in shotgun loads. Because we didn't SEE any evidence of damage being done to the bore after the first few shots, we thought we were OK.
The Damage is done over time, However, with repeated firing of these hard pellets in barrels made of much softer Steels. YOU WILL DAMAGE THAT HOWDAH gun in time if you continue to use these BBs, without at least using the very THICK and tough plastic wads( Available from Ballistics Productics, among others, for example) to protect the bore from the pellets.
The Steel shot sold for use in modern guns is both larger in diameter- .18"-- and softer, than the air rifle BBs you are using. However, even these pellets are NOT RECOMMENDED for use in older shotgun barrels, or in shotgun barrels not specifically Designed for shooting Steel shot! Even in the guns that have barrels designed for steel shot, you are told to use special plastic shot cups to protect the bore.
When I was preparing to hunt geese with my shotgun, using Steel Shot, I did a lot of testing of various sizes of shot, and various chokes in my barrel, to determine what size shot, and what size choke produced the best patters. I also CHECKED THE SPENT SHOT CUPS on the ground between my muzzle and the patterning board.
I found that even with the THICK plastic cups, there would be "rub holes" caused by the steel pellets pushing against the wall plastic of the cup, hard enough to completely penetrate the plastic wall. There were corresponding streaks in my barrel from these pellets rubbing against the steel. So, even the " Soft Steel" pellets are NOT soft enough! :shocked2: I ended up using Modified, and skeet 2 choke tubes in my gun, depending on steel shot size, and you could see the beginning of wear grooves in those choke tubes.
Please! If you are going to load any kind of Steel Shot in a MLer, use the proper, thick shotcups made for this kind of thing, and then also use a fine filler to fill the voids between your shot pellets in the cup, like PufLon, or Jiffy brand Corn Bread Mix. Filling the voids will help control movement of the shot pellets in the cup itself, and reduce the ability of the shot to push through the walls of the plastic cup to rub against the bore.
A final note about cost: As expensive as Steel Shot is, the other non-toxic shot types are even more expensive, so far. There is an area in Central California where Lead shot is prohibited for use in all game- a Granola State mistake that will come back to haunt what is left of that state when it goes bankrupt---- but other than that strange state, the only restrictions on hunting with lead shot were imposed years ago by the Federal Fish and Game department on shooting waterfowl. I WOULD NOT USE STEEL SHOT IN ANY GUN if I were not hunting Waterfowl, and I am trying my darnest to get that law repealed.
I lost my waterfowl hunting partner a couple of years ago, so I just don't have the desire to hunt them unless I find a new partner. But, I still have steel shot cartridges. I am very excited that there has been so much research and development work done to find a Non-toxic shot that Can be used in old guns, with more traditional chokes, and hope that as the production of these new shot types improves, that the cost will come down.
In the meanwhile, I am very happy to see that the Waterfowl have invaded the homes and neighbors of the Greenies, making a huge Mess, and Stink, and that these Anti-hunters-- raised to think that meat comes plastic wrapped---- are now demanding government( who else?) DO SOMETHING about all the Goose POOP in their lawns, and in their retention ponds. And they want the geese Humanly "Moved " to some other location, with NO UNDERSTANDING that the Geese will simply fly right back, or how much it costs to traps water fowl to remove them.