M40 without a doubt is superior to the R700. These are the damage difference between M40 with an ACOG versus the R700 (Stats are shown without and with Stopping Power)
M40 values listed first and R700 damage second…
Head 112.5 SP(157.5) 105 SP(147.0)
Neck 112.5 SP(157.5) 105 SP(147.0)
Chest 82.5 SP(115.5) 105 SP(147.0)
Stomach 82.5 SP(115.5) 77 SP(107.

Limbs 75.0 SP(105.0) 70 SP(98.0)
Players stock health is 100. Juggernaut players have 140 points health.
The M40 with Acog is the ONLY sniper weapon that will one shot kill to the limbs. (Assuming you're using Stopping Power and they don't have Juggernaut). The ONLY advantage the R700 has over the M40 relative to damage is shooting and killing Juggernubs in the chest.
I NEVER use the R700. If I'm not using the M40, then I'm using the .50 Cal.