Diamonds into protective barrier
I'm not a gun owner or military expert, but from what I understand the requirements for the armor material are more or less uniform; it should:


Be able to withstand an impact and quickly dissipate the kinetic energy from a projectile. Diamonds are are hard, but very fragile. They cannot redistribute incoming impact from a projectile in the opposite or perpendicular direction. Once they crack, there is no more protection, and they cannot self-heal.    
Have low weight. The density of diamond is $\pu{3.51 g cm-3}$, which is nearly 2.5 times heavier than kevlar (ca. $\pu{1.44 g cm-3}$).  
Be easily molded in desired form, and/or be flexible (relevant for body armor). Process of crystal growing is rarely precisely controlled and often requires mechanical post-processing, which is ineffective due to hardness of the diamond and can potentially weaken the structure.
Have adequate thermal interface. Due to exceptionally high phonon thermal conductivity, the person wearing this armor will suffer from cold in winter and from heat in summer time, not to mention what damage an opponent with a flamethrower can cause.   
Be modular and easily repairable. Diamond plates can only be replaced, so one needs to carry a stockpile of the shards.     
Be suitable for camouflage (absorb light and other EM radiation, optionally). Diamond surface is hard to modify, it has low adhesion, so there is no easy way to tarnish the object if the diamond layer is the outer one.  
Be optically transparent and don't cast any distortions (relevant for the bullet-proof windows). Due to high refraction index it's a questionable choice for a transparent shield.   
Be inexpensive in terms of production and maintenance. No comments here.  


To wrap it all up, for now classical macro-scale diamonds seem to be a very expensive and ineffective material for use as an armor material due to their physical properties. However, at a nanoscale the situation can be different, as nanodiamonds show somewhat different properties, yet not nearly fully investigated for military applications.

Also, you might want to check this topic on Worldbuilding.SE: Is diamond armor better than traditional armor?