I have no line bifocals, and Defariswheel is right, they work pretty well for pistols, but are a real pain in the rear for rifle shooting. As he pointed out, you're looking through the wrong part of the lens, and they also distort the sight picture making it look like your front sight is bent.
If you just got your prescription, your optometrist will probably give you a 'script for single vision shooting glasses like Defaris described at no extra cost.
You can get some good, cheap mail order glasses here. Probably less than $10.
It's where I get my glasses.