Question for those that have been there.....When installing a single rail gas block (PRI) should the rear of the block butt up against the handguard cap or should there be some gap/play between the two? When I carefully line up the gas block using the pre-drilled barrel dimple (BCM barrel) there is a resultant gap of close to 1/16th of an inch between the block rear and the handguard cap front with the back side of the cap butted against the barrel shoulder. Assuming that the dimple is doing the correct job of lining up the gas ports in the barrel and the gas block, Is this normal spacing or should I be looking for a closer fit between the two parts in this area?

Thanks,
Steve

1/16" seems excessive with the hand guard cap. Without the cap, between the shoulder and end
of gas block should be closer to 1/32", or the thickness of the metal used for the hand guard cap.
The gas block should actually hold the hand guard cap fairly well. Not vice tight, but so it doesn't move around.

Interesting......So now I guess some careful measurements of gas port alignment are in order. Perhaps my assumption that the dimple position is "correct" is in error and the block may need to be located further back for proper alignment. If the alignment is correct I guess a shim is the way to go to take up the slack.....Either way it's caliper time! Sorta figured that gap was a little wide, makes sense that any GB or FSB should in fact "capture" the handguard cap fairly closely.....Will see where this goes!  Thanks!

On the 4 AR's that have low-profile gas blocks, all are snugged up against the barrel 'shoulder'. The gas port hole in the gas block is large
enough that it is still over the gas port hole in the barrel completely. But if I really wanted them perfectly centered, large hole in block over
smaller gas port hole, there would be a very small gap between the gas block and barrel 'shoulder'.

Guess what I am saying, if everything is within spec, the holes don't have to be perfectly centered over one another to function 100%.

Understood........I know there is a little "wiggle room" on longitudinal placement of the block due to the port in the block being oversized. It is my intention to place the block as far back as possible while still maintaing a clear gas path from the barrel up into the block. This positioning is easily figured with a few precise measurements. I expect that I will likely be able to get away with no shim to take up the gap. Thanks again for taking the time to clarify the details. Appreciate the help!

Steve

It never hurts to contact the manufacture for their suggestion.  Some gb are meant to be snugged up against the shoulder and other are supposed to be spaced.  IIRC the thickness of most caps is .025, and it the gb is properly attached it isn't going anywhere.  Measure twice and cut once an old say my departed brother use to say relentlessly or in this case install once.

I had an extra front handguard cap that I cut in half with a dremel.
I use it as a spacer when installing a gas block on a barrel I intend to free float.

Rearmost setscrew hole is the same hole as the gas path thru the block.  Slide gas block on the journal upside down, look thru set screw hole and  look to see where the barrel port sits in relation to it.  Try it with a HG cap and see if the port is obscured.  Most likely it won't be.

The "short and sweet", Bretshooter was spot on......Everything lined up within limits with the HG cap in place and the GB snugged up against it. All should be fine with final assembly. Thanks again for all the input.

Steve