I've got a SiCo Omega and four Dead Air Sandmans so with a KeyMo mount, I can run any on the 553's. The 553R with Omega and direct thread and four position piston was very soft-shooting when I tried last fall. I run a dedicated Sig SRD556 on one 553...didn't want to use a mount for it due to clearance on the gas system on my 9".

You should be able to clear with any mount on a 553-LB though.