Will a standard M4/A2 FSB work on the older thickness (height) DPMS style .308 Uppers?

I am getting to the point of rebuilding my .308 AR (see my problem solving thread a few pages in) and am wanting to start with one of JP Ent. Adjustable Gas Blocks and the one with the Iron sights knocks out two birds with one stone.

Thoughts?

The older LR-308 lo-profile upper receiver is higher than the AP4 or Armalite uppers, and you need a special front sight solution to that upper if you want iron sights.  I would just get a new upper, if you really want irons.  With the DPMS specific quad rail meant for the LR-308, you can have a rail that is the same height, then use a clamp-on iron sight for your front, but I think a new upper is actually cheaper.

The LR-308 uppers seem to have been geared specifically for scope mounting without needing extra-high rings.