Question about gas port with handguard cap removed

AR/M4 Beyond 5.56/.223
1

I hope this isn’t a dumb question, but on the theory that the dumbest questions are the ones you don’t ask out of ego, here goes:

I am putting an Apex free float handguard on my new M & P 10 and I have everything disassembled as far as I need to and am ready to attach the handguard. It occurred to me after I took the gas block off that the gas block didn’t rest snug against the shoulder on the barrel because the handguard cap was in between. The handguard cap obviously won’t be used with the new handguard. When I reattach the gas block, should I go ahead and snug it up against the shoulder or do I need to leave a small gap? My concern is, with the gas tube 1/32" (or whatever) further back because that thickness of sheet metal is absent, is there still enough clearance for the gas tube to go all the way into the gas key? And is the port in the gas block oversized enough that it will still be properly aligned over the gas port in the barrel? It looks to me like the answer is yes to both but I don’t want to take any chances.

Any advice anyone can offer would be greatly appreciated.

2

If you are re-using the original gas block, you will need to stand-off from the shoulder and leave that gap. If you set the gas block all the way against the shoulder the port on the block probably won’t line up right with the port on the barrel and you may end up under gassing the rifle and have cycling issues.

3

The obvious answer is to put the gas block back where you found it.

Moving it around is likely to cause issues and won’t make it “better than factory”.

The purpose of the barrel shoulder is to hold the hand guard cap on, not the gas block.

4

Thanks, that’s what I needed to know!

5

So, if you are really serious about finding the answer to this problem, here is what you do. Measure with some calipers the size of the hole in the gas block, and how far it is from the rear edge of the block. then measure from the shoulder to the barrel gas port.

I think what you will find is that the hole in the gas block is larger, most of the time, than the gas port itself. This is evident if you take the gas block off of a rifle with a few hundred rounds through it, you will see a bigger carbon “hole” around the gas port because the gas block hole is larger.

all of this means that snugging it up against the shoulder may not matter at all and could still entirely encompass the barrel gas port.

6

Good idea but calipers can’t get down inside there in a way that would allow an accurate measurement.

I ended up just attaching it with a small gap between the gas block and the shoulder as those above suggested.

7

For a Mil-Spec barrel the center of the gas port is 0.295" from the shoulder. Most commercial barrels are .300".

If the gas port is .125" in diameter.

And if the gas block was truly centered over the gas port, then the distance from the back of the gas block to the center of the .125" dia is 0.232 to 0.237 (1/16" thick cap).

Seating all the way against the shoulder closes off about 65% to 70% of the gas port doesn’t matter what size port you have. But. if the hole in the block is .156" and placed .263" from the back of the gas block, it will work with or without the cap.

You can often measure where the .gas block hole is and how gig it is on most gas blocks. The hole goes through the bottom, so it is visible, or is the rear set screw hole. Clamp on types you can’t tell as it usually passes through the slot.