To get back to your original question, is there anything else to check out other than the 870 or 500? Well of coarse there is, but the main difference is price. Winchester model 97, and 12, Ithaca 37, FN police, benelli nova, and a few more big names not mentioned. Also don't forget the different grades of the remington 870, wingmaster, marine magnum, police, express and the new 887. Mossberg also has several models to chose from like the 590, 590 special purpose, marine, and the 500 in everything from plain ol all purpose wood stocked blued barrel to the outragous rolling thunder. And dont forget the maverick 88, its a cheaper version of the 500 and alot of parts interchange between the two including barrels and stocks. It's all about what you like and what you can afford.
So which one is better the 500 or 870? Well there both good quality firearms but it's more about personal preference and needs. I have three old wingmasters two 12ga and a 20ga that I like best mainly because of the quality of wood and blueing, and because they where what I learned to shoot with. I also have a newer 20ga 870 express with a rifled barrel, scoped for deer hunting, it is nice and shoots great but the plain wood and matte finish doesnt look as nice, and the action is not as smooth. Thats possable because it has not been shot enough to be broke in. I also have three older 500's one .410ga and two 12ga. To me they look nicer than the 870 express and were cheaper. The main noticable difference is the saftey. The 870's are right behind the trigger and 500 is on top of the receiver.
As for what type of ammo to run for hd? Well thats another can of worms but I like 00Buck. Some say to alternate buck and slugs. You can google box of truth and see some of the tests they do and make up your own mind. Just remember your living situation, alone, with family, or friends, apartment, house, dorm whatever. I dont use slugs because they penetrate walls pretty easy and I have a two year old son here with me.
As for the surefire. I know you can buy a mount that just clamps to the barrel on one end and the flashlight on the other and its pretty cheap. I don't if its any good or if you can add a pressure switch.
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