What has been said is correct, the case is not expanding enough to seal up in the chamber.
What you are seeing is a little blow by of burnt powder.
Lil gun is a slow burning powder, You could use a little faster burning powder or bump up the load a little.
If it were me and I liked what I had then I might try a little heavier crimp ( don't go nuts with it )
just crimp a little bit more. That should increase the pressure in the case a little, and may be just enough to help you out. Other than that --as it was said don't worry about it, happens often with light loads and slow powder.
Sam