AR-15 railed gas blocks are 0.925" above center of bore.
It looks like DPMS and Armalite both use standard AR-15 gas blocks. Armalite receiver rails are 0.1" higher so they require a special gas block front sight that’s 0.1" higher. DPMS–at least in their FAQ–says that AR-15 gas block sights work. This means DPMS receiver rail height is the same as AR-15 rail height.
Fulton receivers are similar to DPMS–so AR-15 height–but Fulton uses a 0.9125" above center of bore gas block. Does this mean one needs to use AR-10 gas block sights on Fulton railed gas blocks?
Edit–if anyone has a Fulton railed gas block, please measure it from center of bore. In the gas block description it says designed for AR-15 gas block front sights. 0.9125" may be a typo.
I’m unsure of SR-25 sizes. I’d guess that Knight uses standard (AR-15) height gas blocks and that the receiver rails are the same height above bore as AR-15.
I called Fulton and was told the website has a typo–the railed gas block height is 0.925 and not 0.9125. It sounded like they’ve had the question before.
It looks like all AR-10 and SR-25 designs use standard mil spec railed gas blocks.
This is off-topic but related. I really want an A2 front sight base on my AR10 flat-top carbine. Since we’re talking about a delta of 0.1" here, I am assuming that a combination of a taller front sight post and a height-adjustable rear sight should work. I don’t have the tools to measure it all and maybe my scratch arithmetic is wrong. Any help appreciated.
It should be the same–both are measured from the center of the barrel. Gas blocks that are for .750" barrels have 0.01" more iron above the barrel than .850" blocks, to make both the same height above center of bore.