If you are curious about including early type firearms into a fantasy campaign here are some videos for how they work and effects.
[video=youtube_share;OSxFY917UH8]https://youtu.be/OSxFY917UH8[/youtube]
[video=youtube_share;mrIQ59vRlvY]https://youtu.be/mrIQ59vRlvY[/youtube]
I have decided to include a matchlock musket for use in my BECMI campaign. I decided on giving it the same range as a crossbow, d12 damage and make it effectively one shot. It would be loaded prior to an encounter but since the reload process is so slow reloading during combat is not done, rather a reload is done during post combat rest period.