The distance from the center of the gas port to the shoulder on the barrel.

On Armalite, it seems that this distance is 0.3 inches.

On DPMS, is it 0.275 inches?

I am trying to figure out because I may have purchased the wrong gas block.

I bought my barrel and gas block at the same time so I only got around to measuring things when it all arrived today. I am using a 308 barrel based on the DPMS spec. I bought the gas block from a website that states the 0.275" offset is for the AR10, so naturally I bought the 0.3" offset version.

When I install the gas block on the barrel and snug it up to the shoulder as close as possible, not all of the gas port is exposed by the gas block hole. I'd say about 15% of the gas port is covered up by the gas block.

Maybe I'm splitting hairs and it won't matter?

It will matter, and act like a choke for the gas system, reducing how much gas can flow through the ports at a given time.

The offset for AR-10 is .295" to .300".  It should be the same for DPMS, et al.  If your barrel measures .275" is ain't right.

Gas blocks are made to AR-15 standards which are .295" to .300".  The .005" will not occlude the gas port.

The difference between DPMS and Armalite gas systems is the distance from the center of the gas port to the breech.  The Armalite is about .25" further down the barrel.

Usually, with Armalite barrels, there needs to be a shim between gas block and shoulder. The shim is same thickness as the handguard cap. You can potentially cause sort stroking issues of you don't account for the handguard cap.

Some gas blocks have a big enough port to make up that small difference, but not all. All gas blocks are essentially the same, there is no "Armalite" gas block. However, there is an Armalite gas tube that is slightly longer than an ar15 tube, which DPMS uses.

If it helps any the Armalite gas tube is .200 longer than a standard AR15 rifle length tube. My AR10T gas post is .300 from the shoulder.

Curiosity ask?

What size is the barrel port?
Could you elongate the hole in the block .025?

The difference in gas port location in relation to the shoulder is that there are two specs. One is for delta assembly and one without. I can't remember the exact numbers but its something like .265" without delta and .295" with.

The gas port holes on the gas blocks and gas tubes are normally  drilled to .125" or so...

So, there should be just enough distance to compensate for any differences in shoulder to gas port length...