I picked up a lower receiver and previous owner spray painted the lower a brown color.  What is the safest and effective way to remove it?  Will paint stripper be safe?

Try some brake parts cleaner.

didn't think about that

If its cheap paint (like krylon) brake cleaner will work great.

Acetone on a rag works great. Do your hands a favor and wear some gloves...

I recently purchased a Daniel Defense Omega rail that was painted dark earth. I was worried about using a paint stripper that would also remove the logos, but a local anodizing shop said any remover would work well and not remove lettering. So I picked up an aerosol paint stripper and it worked great - even with 2 application to get the paint out of all the nooks and crannies.

+100 to the gloves.

Make very sure that any paint stripper specifically says it's safe for use on aluminum and test it on a small area.

x2 on reading labels and testing on a small area first (that goes for any non-gun solution used). Also be very careful with brake cleaner. It does work, but a few years back, I used it to remove Krylon from an old LMT upper, it took off some of the anodizing. The same goes for using any automotive cleaner on your AR parts. I know for a fact that a strong degreaser can turn your black parts an fugly gray.

Plain acetone with a rag like neverenoughII said would be the best way to go. Take your time and work small areas just enough to remove the heavier coated areas. Then gently work the rest. Have an old soft bristle toothbrush handy for the hard to reach parts. If you start to see blackish residue, rinse the part off right away with some warm water.

If anyone is wondering about the LMT upper I f'd up with brake cleaner? I completely stripped and re-finished it with a lye solution and a homebrew anodizing set up. I think I have another beater upper somewhere...if I can find it I'll do a proper write up.