Re: POF 308 lower and DPMS upper
Assuming you already have the proprietary front pivot pin that should have came with your POF lower, you will need a sleeve for the upper receiver. The diameter of the front pivot pin on the POF 308 is smaller than the DPMS so you will need to "shim" it, so to speak
Cold War Shooters was the only place I know of that commercially manufactured that sleeve, though I don't see it on their website anymore. You may want to contact them to see if it's still available.
Another option and a much cheaper one is to DIY. Go to the hardware store to see if you can find something within that diameter. Some people have been known to use telescopic radio antennas, .25 ACP cartridge casings, etc.