If you're going with a different upper, look at a piston driven or buy an Adams Arms piston kit for a DIY. Otherwise for sure go with an adjustable gas block.
I have good results with the 18" rifle gas combination. The last pair I assembled (w/o adjustable blocks)were just barely functional until broken in but after 50 rounds, are running fine un-muffled and don't suffer or require tweeking for suppressor use.
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