Not wishing to be a smartarse (no, really i'm not) but if you've flattened the rings fitting them you're doing something wrong!
When i rebult the calipers on the Rangey, i put the bottom seal in, rubbed a bit of fluid around, put new pistons in and then put the top seal on.
Then, making sure the ring is sitting square, put a 2" square of plywood on top of the ring, attach a g-clamp and slowly compress the ring into the caliper body. Because you've already checked the ring is sitting square, you can't fail but put the ring in the right place without rubbishing it up and it just pops neatly into place.
Because they're a press-fit, blatting it with a hammer definitely isn't the way to go!
My little bit of ply lives in my toolbox because it makes doing caliper seals a doddle.
Also, it's worth running some scotchbrite around the top of the caliper to make sure there aren't any tiny bits of smeg there to stop the retaining ring dropping into place. Careful you don't get it in the bore and scratch it though.
Best of luck, once you've sussed it you'll do 'em for everyone!