The connecting plate for the stock on SCAR 20 is polymer.That's a very good question. I also wonder in the SCAR-20 is possibly softer gassed than the SCAR 17 since it's a precision rifle. The outer part that slide over the "ears" of the rear receiver plate looks like anodized aluminum in the pics I've seen.I wonder if the SCAR-20 has a metal plate where the hinge plate would be on the -17? Or is that piece plastic like on the -17?
The 20” barrels for the 17, and the 20 both use a smaller gas jet when compared to the shorter barrels. This is due to dwell time. When I’m not on a phone I can do a better write up.
The idea of going larger on the gas jet is a bad idea. The regulator is vented, however it’s ability to prevent over pressure is limited. Increasing the amount of high pressure gas getting fed in to the gas block will cause problems, not solve them.