Armor piercing rounds can be used against,
1. Vehicles.
2. Heavies.
3. Turrets.
4. Maximum bodyshot push.

For the tactical XCR, its a primary gun. There are far more better options in the same range. Try Blower and SVDm. Better everything, the mobility won't give you an upper hand.

I like long range guns for long range bases, like for Arakcheyev fortress I wont use an XCR 700. The barret is a wise choice, more fun and max tactical output.