Can anybody tell me the distance from center bore to the center of the gas block hole on a DI upper? I’ve looked around online a bit, but I keep finding conflicting data, which makes me wonder whether it even is a standardized specification. For context, the reason I ask is for a free float KeyMod/M-LOK handguard design that dispenses with the 12 o’clock rail in favor of another modular attachment position, and so I need to know how small and what sort of profile I can get away with for the inner diameter. Hopefully I’m not asking something someone’s already conclusively answered elsewhere; if so, chalk it up to me being a noob. 
Are you worried a gas block won’t fit under a rail? If it’s a set screw block, I’d say there’s about a 99% chance it will fit. If it’s a clamp on block they aren’t quite as low profile.
Have you bought it yet or not? That would help to know.
Is this what you are looking for?
SixEight,
I might not have explained myself properly in my original post; I’m interested in finding out the distance from center bore to the center of the gas tube hole on an upper receiver. On most handguards, the integral 12 o’clock rail forms something of a channel through which the gas tube passes into the upper, but since the handguard I’m designing has no 12 o’clock rail, I have a minimum “ceiling” for the inside of the handguard that I need to take into account, lest it touches the gas tube and renders it non-free float. Gas blocks are a little less of a concern for me, since there are some really low-pro offerings out there; hell, KAC made the URX 4 compatible with only their own block… :blink:
Mr. Rausch,
I’d seen that diagram once before, but I also saw some other diagrams that contradicted it, so I wasn’t sure which to trust. 
I trust you know what you’re talking about of course, but is the distance from center bore to the gas tube even a standardized spec, or does it vary from manufacturer to manufacturer?
Thanks guys!
Normal gas tubes have a bend in them as you can see from the dimensions on my drawing above.
Normal gas blocks put the gas tube bore 5/8" above barrel bore center.
My first drawing was off on the 308 dimensions, but this one should be the corrected version now.
If you find the drawings for an A2 front sight base, they should verify the 5/8" dimension.
The ceiling you are concerned about will also need to account for the height of the top of the gas tube, which is due to the bend in the gas tube and the gas key bore height being about 5/32" higher going into the receiver than the 5/8" gas tube bore height at the gas block.
You could confirm the gas tube hole height above receiver bore by adding the dimensions up from the gas key and bolt carrier drawings.
Those parts are QUITE standardized.
Thanks Mr. Barrel Guru, sir! That helps a lot! 