Is an adjustable gas block generally needed on a Grendel?

[LRRPF52]

Yes. Grendel runs an oversize gas port(.125). Makes the whole system very over-gassed with most factory loads. Seekings, Radical, and Anderson make a dirt cheap adjustable gas block that works well. JP makes top of the line variants for all barrel sizes.

No. Not sure where you saw this, but 6.5 Grendel uses several different port sizes depending on barrel length, and .125" is not one of them.

RLGS 20-24" use .094"
MLGS 18" uses .076"
MLGS 16" uses .078"

.125" is the ID of the port in an actual gas block and is huge.

I had to conform it myself. The first Grendel I built was so over gassed it was unshootable. I had to tear it down and measure the entire gas system before I noticed the size of the gas port. I measured it with a drill bit blank at .125 inches(number 30). When I called Black Hole, and was mad as fire, they told me the gas ports on Grendel are oversized to allow for lower chamber pressures, and lower overall gas pressures from the rifle length gas system and the Grendel round. When they changed the gas system, they just assumed that everyone building a SPR would be using an AGB, so they went much larger that the maximum port size for an 5.56. When I ask about wear on the barrel, they said if it starts to open up before 5k rounds they would replace it for free. An Anderson Gas block on the 5th click made the rifle shoot like a dream. Now I use JP's, because they work just a little smother that the cheap ones.

I see where you got the bad info from now. If someone is purposely using .125" gas port diameters in 6.5 Grendel, it is and can only be with the intent of using adjustable gas, because that is so overgassed, it will not work. I didn't know they were doing that.

Gas port diameters are pretty much like 5.56 at the same barrel lengths and gas system lengths. While it operates with much lower chamber pressure, it has way more case volume and capacity than 5.56, so port pressures are very similar.

You absolutely do not need adjustable gas on a correctly-built 6.5 Grendel barrel.

There are quite a few barrels out there in the 18" MLGS combo that are way overgassed with MLGS/.094" ports, and they do need to be tamed somehow.