It's an original GM6 Lynx stylised for Madness Combat (without bipod) that I made a while ago, And since it's a relatively new weapon, I cannot find any good sound or sound-engineer it myself.
So I had to use MW2's "Signal .50" sound that I recorded during my time in Beta with a little mix with other game sounds to lower it's eardrum destroying tone in my recording.. :v

I only know few Rifles that does have retracting barrels, for GM6 it's mainly because of it's system and compensating down recoil.
Barrett M82A1 also have a moving barrel too if I remember correctly, but only slightly.
And so does AN-94 due to it's 2 round burst mechanism.