If you need glasses to see things up close, bifocals or no line bifocals don't work well with iron sighted rifles.
They work with pistols, since you simply tilt your head back until the front sight is sharp and clearly in focus.
The problem with rifles is that the upper area of the lenses are for distance vision. You look at the sights through the upper inner corner of the lens and can't focus on the front sight, which will be fuzzy.
The best option for this is to have special single vision shooting glasses made.
An optometrist can prescribe a lens that will focus at the exact distance of the front sight. You'll need to tell him how far from your eye the front sight is.
When I still had prescription shooting glasses, my optometrist had me hold a ruler about the thickness of the front sight at the correct distance and he adjusted things until the edge was sharp and clear.