As with generic sniper rifles, the Reservist's Rifle has a very high critical multiplier of x5.
Players with ten Luck, Finesse (+5%), and Survival Guru (+3%), will be able to critical nine out of ten shots! ([10+5+3]*5)=90%. During VATS shots, with the flat +15% VATS bonus for targeted shots, you'll critical 100% of the time.
Important note: while the above math is strictly correct, your chance to cause a critical hit is multiplied by the weapon's condition percentage. That is to say, under those conditions a Reservist's Rifle at 50% condition will only cause a critical hit ([10+5+3]*5)*.5 = 45% of the time outside of VATS, and 60% of the time in VATS. Since Sniper Rifles have very low hit points compared to other weapons, there will only be a short window where VATS critical hits can be guaranteed.
This weapon has +50% more durability (Item HP) than the regular Sniper Rifle, preserving the damage and critical rate more effectively. It also has a slightly higher rate of fire in exchange for a lower magazine capacity.
Uses 3 rounds instead of 5 in each magazine.
Achieving 18%MDPS will give the Reservist's Rifle an effective DPS of 122.
Costs 6 AP less to shoot in V.A.T.S. compared to other Sniper Rifles (32AP vs 38AP), so if you have 10 Agility and the Action Boy/Girl Perk you have a max of 110 AP, allowing you to fire the Reservist's Rifle 3 times in a row (96AP) while the other Sniper Rifles use 114 AP and therefore cannot.
The "Reservist's" title might refer to the US Army Reserve, making a reference to some of Fallout 2's weaponry descriptions which give background information about weapons retired by the regular Army and then given to the National Guard or Reserves for their training and use.