Just found out, while finally putting together my Grendel AR upper, that the JP adjustable gas block I had bought a while back is .750 diameter and my new CSS barrel is .875 diameter - duh.

So, I have to send the JP back - they don't make an adjustable block in .875.

Now, do I REALLY need an adjustable gas block for this?

If so, why would I need or want one?  I doubt I'll ever shoot suppressed with this rifle, so that consideration is not one to deal with.

Any recommendations on a gas block, adjustable or otherwise, for the .875 diameter?

Was considering this one:

http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=22733&title=AR-15/M16%20ULTRA%20LOW%20PROFILE%20GAS%20BLOCK&s=48253#48253

Ultra-low profile non-adjustable Badger Ordnance stainless steel.

The barrel I got for my 6.5 grendel didn't come with an adjustable gas block, and I don't see many of the factory builders using them so my guess would be no, you don't need one

You could do an MGI adjustable gas tube and not worry about the block. Just have to make sure you can access the adjustment through your rail if thats the route you go down.