"detonate" on impact airsoft ammo
What about instead of shooting a round that will burst and spray the pellets, just make it more like a shotgun in the first place? maybe have film cans that had the bottoms cut in an x shape, and a paper cap. Just drop them in, fire, and the can would come out. Then replace with another cartridge. Maybe they would fit inside some sort of repair fitting, and be kind of like a shotgun?
I'm weird, I know it, you don't need to tell me.
I was able to make a good fragment round by taking a piece of pvc same size as the barrel, putting a strip of paper around the inside, putting in bbs, and then pouring in melted wax 
. if you need to get melted wax, take a candle or two and chuck them in a glass bottle, put that glass bottle in another glass bottle with some water in it, and stick it all in the microwave for 1-3 minutes. when you take it out you can pour some of the liquid wax into the mold, and the water will continuously melt the candles, so you can make them in bulk (helps if you have multiple PVC molds 
).
. if you need to get melted wax, take a candle or two and chuck them in a glass bottle, put that glass bottle in another glass bottle with some water in it, and stick it all in the microwave for 1-3 minutes. when you take it out you can pour some of the liquid wax into the mold, and the water will continuously melt the candles, so you can make them in bulk (helps if you have multiple PVC molds ).