well, especially with something like the Grendel in a rifle length. You surely COULD have an issue like I had with my WSSM. If it were a rifle in a .223 It would be less likely to cause issues.
This was a mistake I made in my 243WSSM build.
I (for some reason) forgot to order a buffer/tube/spring when I built my lower a couple years back.
Cousin happened to have a spare, so we used it and everything seemed OK. Only issue was that about 1-2 out of 10 shots would have the brass fail to eject properly. I didn't think it was a huge issue at the time, thought it would be maybe be due to a new upper, getting the gremlins out, or just the break in. And it seemed like after some load testing and what not, this issue almost completely worked itself out.
I got some slow motion video footage of it somewhere, and the problem was that my bolt was coming back and then forward with the next round SOO fast, that the spent round wasn't even out of the receiver before it was getting caught and jamming.
The problem was that in our haste...we mistakenly installed a rifle length tube, a rifle length spring with a CARBINE buffer on an upper that would be shooting WSSM ammo.
The problem is refered to as "Bolt Bounce" and
Long story short, if you were to go .223 carbine to .223 rifle...i'd say no big deal, give it a whirl, but here is a reason for the two different buffers, best to use the right one.
You wouldn't use 2st gear to drive 50mph right? I'm sure you could, but it wouldn't work very well and wouldn't be very good for the motor. Same thing with the AR.