Bought a Bushmaster XM15 QRC, It has the cut down front sight with bayonet lug & sling attachment.  I would like to add iron sights, preferably with a floating quad hand guard.
My question is:  the diameter of the barrel where the current gas block is, Mics out at 16mm/.629 in.  It has the thin barrel not the more common .750 in. barrel.  If I purchase a
.625 low profile steel gas block, how will this .004 smaller gas block fit the slightly larger diameter barrel, (.625 GB on .629 Barrel)?  Will machine work be required on the new GB in order to fit and be pinned to the barrel or will a split, set screw type GB be a better option.  I just want some iron sights as back up at the moment.  Any good solutions?

I think you will be just fine if you buy a .625.

.004 is quite the minute measurement.