PROS:

15% more damage per bullet,
30% more health than the default sentry,
10% faster deploy time,
Rockets deal 15% more damage,
Bullets 100% more accuracy,
Spawns with 50 health instead of 1,

CONS:
55% slower firing speed,
5% rocket explosion radius,
15% less ammo,
Fires two rockets instead of four,
While the Arms Race is equipped the engineer loses 25 health.

The Arms Race replaces the secondary slot.