I've shot Skeet with my Mossberg 500 w/ 18" barrel. It's possible but not advisable. Don't get me wrong. It's fun and good practice with your HD weapon but you won't score well.
For comparison. I have a Remington 1100 with a 26" skeet barrel. Thousands of rounds thru it. I generally shoot in the 22-23 range with it and have shot a 25 with it many times when I shot skeet a lot. I shot 8 rounds of skeet with my Mossberg 500. Never broke 15. Doubles were hard and the long stations difficult. Plus 30Y shots on the second double with birdshot is challenging from an 18" barrel on a tiny bird. Fun but not easy. Good lesson in humility