Using the turret of my choice, I traced it out on to the foam mat.
Since my barreling material is PETG for this particular blaster, I simply used a piece of PETG to core out the barrel holes.
Next I just cut it out.
Doing it in the above order makes it easier to core out all the barrel holes since there's more material to hold on to.
These are very easy to make and always have a perfect fit. Since these are foam, barrel tapping will be a little less painful.
I made both of these barrel spacers in under 20 minutes from start to finish.
I could have used a Dremel to create the holes but finding the 'exact' measured bit for the barrels was not available to me. Plus, coring them out like this produced a useful by product.
I call them, Fruit Loop Slugs.
*EDIT - added video of testing these Fruit Loop slugs.
My Bro-in-Law is filming using my iPhone4. He's not an experienced videographer. I'm at the far end while he is standing right at the 150' mark. I've put down pylons every 50' feet, aiming directly at my Bro-in-Law. The first shot lands just to the right of him near the bottom of the frame. It's hard to catch but you can hear it land. it bounced a good 15' past him. You can see where it rested when he turns around to look for it. The second shot is easier to see, landing just to the left of him in the grass. Considering that he's 150' away and these are realistic outside conditions, these Fruit Loop Slugs are pretty accurate.
This wasn't a range test so much as it was an accuracy test. But for those who want to know, the darts landed at around the 140' and 145' mark in 7 degree Celcius (44.6 degrees Fahrenheit) weather.