I want to put an 18 inch 6.5 Grendel upper with a rifle length gas system on a lower that has a standard collapsible stock and carbine buffer. Will this work or do I need a different buffer?

Yes no problem. Enjoy your soft shooter.

Non issue.  Shoot away.

You should be GTG, but you will need to shoot it to confirm functionality. Swapping buffer weights is likely to be the most that you will need to do, if anything.

I built a carbine lower with three uppers:

•  16" mid-length gas 5.56 NATO
•  12" carbine gas 6.5 Grendel
•  20" rifle gas 6.5 Grendel

Function testing with an H1 buffer, everything was copacetic except for the rifle upper occasionally had a bolt override malfunction.  I hypothesized that it might be slightly overgassed and swapped out to an H2 buffer.  All three uppers run like scalded cats now.

If you have any issues, step up to the next-heaviest buffer weight and see if that fixes the problem.

For a 20” 5.56 the book answer is HH. Can’t say for sure with your Grendel but CAR is probably too light.

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You mean like the standard issue Canadian service rifle? I don't know why this question gets asked so often.

I’m guessing because folks aren’t sure and there isn’t an obvious thread on the first couple pages.

Thanks for the replies everyone!

Both my 18" ar15 and 18" AR10 run rifle gas with carbine buffer with no issues, they shoot nice and smooth.

People get confused between carbine gas length, carbine REs, rifle gas, rifle stocks etc. Not hard to understand why if you're talking someone new to the platform.

Coll stock and m4 buffer are available officially to fit on m16a2/3/4s.