It is but if the lower accepts AR-15 uppers then it's considered an AR or AR variant. Also note the Stag 10 is built around the .308 and larger so matching an AR-15 upper would be to small(width,distance between pins).
Canada Ammo sold/sell a single shot .50 upper that fits an AR-15 lower which makes it restricted. A Canadian company offered a lower which required permanently modifying the upper so it no longer worked with an AR-15 lower. Under those conditions the rifle now became a non-restricted.
There are a number of companies that manufacture a proprietary upper/lower that use a large number of AR parts but don't fit AR uppers/lowers. The Stag 10 is an example of this as to the number of DPMS 308 AR parts it uses but is not considered a 308 AR or variant.
DuhBoredShooter (07-26-2018)
I think my handguard from brownells is at the post office. If so I'll be able to put it together tonight.
“A harmless man is not a good man. A good man is a very dangerous man who has that under voluntary control.”
-Jordan Peterson
Lee Enfield (01-01-2019)