....
I'm going to make some broad suggestions based on what you said.
You have a handgun.
If you can shoot a consistent cluster about the size of your hand at 25 yards, then you already have marksmanship skills that will carry over to a rifle.
Go for an ar15 type rifle.

If you can't shoot a nice cluster with your pistol, then go for a 22 rifle because you need more practice. Practice takes plenty more ammo.
22 is cheap practice.

Does that make sense?

Thanks, it does. I'm still working in improving with my pistol. Lately I've just been practicing at 50 yds on the Koko Head steel silhouettes because I've been a little lazy to deal with the frame and paper targets.

I'm actually highly considering a 22 rifle because of the economics of ammo that would let me do high volume practicing.

Hey, welcome aboard.
When people tell me they want to buy something, I always ask them what their intentions are.
I ask because:
1. I am nosy
2. Sometimes people want stuff which is inappropriate for their intended purpose
3. You will get better quality advice from forum members

So....what do you want to do with this gun?

Mainly to learn how to handle/shoot rifles and probably get into accuracy/marksmanship. I did try a bunch at the shooting fair and the 223/556 guns felt pretty easy to handle. The 308/762 rifles were a little intimidating and kicked pretty hard.

Not really self defense right now since I live in an apartment. Even my handgun seems ill suited because of the possible ricochet and the noise in a small, enclosed space.

I'm new to firearms and newer to the site. I've been looking around and asking a lot of questions because I'm looking at getting an AR-15/M4 rifle and ran across Aero Precision's M4E1 (http://aeroprecisionusa.com/ap/m4e1-m5e1-carbine/). Has anyone tried know someone who's tried it or any of AP's other rifle parts?

I'm also wondering about the SIGM400 Enhanced that has the flip up front and rear sights. I got to fondle lots of rifles at SEC and it looked pretty good.