Huh?

What do you mean you can't see?  The target is blurry? You can't see the front post on the rifle? Your groups seem too big?

W/o being a wiseass-do you need glasses or a better prescription?

Are there rifles besides an AR that you CAN shoot accurately at 100 yards?

If you are only worried about people breaking into your house than a shotgun should be fine (ignoring the issue of overpenetration).

smarty_pants-You need glasses that are the proper prescription or contacts. I have worn glasses since junior high school so most of my shooting has been with them on-you should be able to shoot with them fine.

My guess, and I hate to get into shooting here since there are so many people better at it than me here, but here goes (take this with a grain of salt).

First, I think with your shotgun you are instinctive shooting. You have shot enough slugs that you can sort of look down the barrel and estimate where they are going to hit. This is similar to the way you shoot a bow that does not have sights. Also for many years police departments taught their police officers to shoot like this (there are still some shooting schools that teach instinctive shooting)-having read about this- instinctive shooting seems to have gotten a fair number of police officers killed.

My opinion, for what it's worth, is you're never going to get really good until you start using the sights (I heard a Special Forces instructor on the radio saying how a big problem teaching the soldiers in the new Afghanistan army is although they are tough and experienced fighters, they don't use the sights, they just look down the barrel and spray bullets, so they're mean but can't hit shit)

If you use the regular AR sights you are going to want to focus on the front sight so the rear sight and the target are kind of blurry-your eye can only focus on one thing clearly so you can't clearly see your front sight, rear sight and the target all clearly (does your shotgun just have a front bead and no rear sight, just curious?)

I think your main problem is not getting the right glasses/contacts. If they are properly adjusted, and the right prescription, you should easily be able to shoot 100 yards with iron sights. You need to be able to see to shoot-you may get away with plinking with your slug gun w/o glasses, but god forbid you ever get in some kind of shit where you really need to hit something and you'll regret not having the right glasses.

Another answer (not a substitute for getting your glasses situation figured out), which the other guys mentioned, is a scope or aimpoint-this way there is only one sight to focus on, not a front and rear. I was a smart ass about using iron sights for years (I is a real man) but I got thinking about it and a scope is really better (except for issues regarding peripheral vision & the scope breaking) because you only focus on one thing the cross hairs or dot in the scope, not a front and rear sight, I think.

By the way, here's an excellent link to a concise explanation of how to shoot, everyone should print this out.

www.ar15.com/forums/topic.html?id=78439