Here's what might happen if you make a 410 Double Barrel Pistol in your house.
Test firing guns is always a trial-and-error procedure, and the most important thing to remember (of course) is safety. Check out this video and see if you can predict what makes this 410 Double Barrel Pistol a failure.
Did you see what happened on the second test firing? The gun came completely apart, and it's by design the shooter wasn't holding the weapon at that time.
Here's what the Youtube user had to say about the gun fail, "As predicted, the wooden handle was the weak point of this build, as a result it was the point of failure."
As you just saw this 410 Double Barrel Pistol was a recent case of gun failure, so if you're going to use homemade guns test fire them remotely. Do this a couple of times until you're sure the gun will fire safely.