No offence but buy a Trap Gun before you develop the dreaded "flinch". The 500 is a decent knock around gun but it will beat you to death shooting Trap. No matter what recoil pad you use. It is just to lite for Trap.
Funny, I guess someone didn't tell my shoulder this. I've shot many rounds of trap with my Mossberg 500 and I can hit clays (and shoot rifle, etc) just fine.
Shoot what you have to get into the sport. Buy 2 3/4 dram loads instead of 3 dram, and try to find light loads of pellets. Be sure the stock fits you, if its too short, add a pad. If it's too long, have it cut down. The correct fit of a shotgun will determine whether or not the gun will beat you up or not.
Practice your hold before shooting, hold the gun tight to your shoulder so that it doesn't smack up against it. Watch the placement of your shooting hand so you don't hit yourself in the nose with your thumb.
Other than that, don't overdo it! If you feel like you can only shoot one round of trap the first couple of times at the range, watch for the rest! I've put over 100 rounds through my Mossy in the span of about two hours, without a sore shoulder afterwards. My shotgun fits, and I hold it properly with my weight on my forward foot.
Now.. Once you start shooting and realize how much fun the sport is, you'll probably be surfing the EE looking for trap guns, anyway.
