I recently bought a Winchester 1887 leveraction shotgun. Perhaps it is now time to say that i dont know much of leverguns at all. I let my gunsmith check it and after his blessing i test fired the gun from bench. I was strongly surprised after first test shot (4 grams of BP, 28 grams of pellets), because the lever was opened and shell halfway ejected.

Now im thinking if i did something wrong, or is the gun damaged some way which even gunsmith didnt notice. First question is was it a mistake that i didnt tie the lever close in any way?
I mean... there is fingers closing the lever normally when shooting. But how strong forces are directed to lever during a shot? This is something what i dont really want test with my fingers.

Another possible cause is that it seems that lever doesnt close perfectly, because old stock repair. if i loose screws a bit it the lever closes 1 or 2 millimeters more. This is easy to fix and im gonna try it anyway.

Then i have also heard that M1887 is designed for brass shells, i did test with plastic one.

My gun is original Winchester, 12 gauge, made 1890 if i remember it right. I feel i need some advice from more experient shotgunners before i do more test firing, because i dont want to harm my gun or myself.

If somebody has experienced same problem (if it is even a problem), i would love to hear how to fix it. I can also tell more about my gun and/or loading components if that helps.
But anyways, if not beautiful, that is gun with lot of character and i would love to get it work safely!