So I bought a fiberglass reinforced HG off of Fulton to replace the polymer brown upper HG that comes standard on Springfield Armory's M1A Standard, but then I noticed that it feels like the polymer SA HG seems more rigid; like not as flexible as the fiberglass one. To my understanding, the fitment and rigidity of the upper HG plays a critical role in the accuracy and consistency of an M1A. However, I always hear that a USGI spec fiberglass reinforced upper HG is better than the polymer ones SA uses?

The only thing I can think of is maybe the fiberglass HGs are less prone to drastic changes in rigidity when exposed to the heat emitted from the barrel and gas system than the polymer SA HG?

Well, I guess I'm going to slap the FA Reinforced upper HG and see what happens; hopefully this weekend.

The only way the upper handguard improves accuracy is if it does not come in contact with the stock, receiver or op-rod, from what I understand.   Match armorers supposedly relieve the under sides of them to not make contact outside of just the ferrule and clip engagement.