Tom, I love CMRs (have three), but the I.D. is likely too small for an adj gas block. The SLR has an extra .25”.
Is this the Vltor RE you are using? What spring? Standard sized A5 buffer?
https://www.vltor.com/shop/featured/...ver-extension/
EDIT: I have a Sprinco orange extra power DPMS spring, but it requires the std 7” carbine tube and short buffer. Opinions on which setup would have better felt recoil? Remember this will be for an 18-20” Creedmoor.