As Axxe stated there could be a lot of reasons. Do you have a skinny barrel. Most Stags like Rock River and others have barrels that are actually heavier under the hand guard.
If the rifle uses an auto carrier instead of one that has been almost totally cut out on the bottom like some of the Colt Carriers and Bushmaster Carriers. Then Buffer weight, there are four different weights normally in them Standard, H-1, H-2, H-3. Your Friend no doubt has a Mil-Spec rated Buffer Spring in it. Also there are piston type buffers. Some Stock Kits have a weaker Buffer Springs which can generate more recoil (Slap). Then what handguard does the rifle have.
Small CAR Handguard, CAR M-4 Handguard, Free Float Aluminum or Quad Rail all varies the weight as Axxe stated. So a picture of his weapon and yours would be great if you can post them.
Finally ammunition can make a slight difference. Military ammunition is hotter than commercial ammunition as a rule.
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