no its not a glitch, u also dont need oldschool just create a class and use over kill with a G3 and an R700 and u will have a lot of R700 ammo

this is the artical that was found in Cheatcc a long time ago

# Extra ammunition for R700 sniper in Multiplayer mode

At level 38 in Multiplayer mode, you get the Overkill Perk Class 2 (located in the Perk 2 or Red Perks). This allows you to carry two primary weapons rather than a pistol. Some of the guns in online play can share ammunition. If you are a skilled sniper and prefer the bolt action to a semi-auto sniper, you can share ammo between the R700 (unlocked at level 34) and the G3 assault rifle (unlocked at level 25). Instead of having 40 rounds, you will have 152 rounds for the sniper, effectively giving you an endless supply of ammo for the R700. If you are a G3 user, you will gain an extra 40 rounds (two clips). This works both ways; since you share ammunition, you might not want to use too much ammo with the G3. Also, there are more combinations than just this one; experiment with different guns to see which ones can share ammo.

here are more guns that share ammo:

Uzi/ M9

M16/ M4/ G36c

M40/ G3/ M14

W1200/ M1014