IMO accuracy with a 20 is not better than a 16 just because of barrel length unless you plan to use iron sights.
Sounds like run of the mill stuff without knowing what brand, 4140 steel is the softest and can wear out quicker than 4150 but, it is harder than stainless.
"Glacierguards" sounds like DPMS to me, I'd look around a bit more JMO.
hi perf barrels and bolts
The Nextgen of 308 rifles