I think the 16" SS410s are likely more accurate. It's ikely that the 16" is stiffer, though the 18" is heavier for longer between the bbl extension and gas block.
Mine is melointed. If I get it pretty hot the groups open up but come right back as its cool again. No shift in any particular direction. Heat always damages the chamber & bore. I do use the VLTOR A5 buffer and a BCM A2 buffer spring in mine with a cut down spacer in the MAGPUL PRS stock.

