I know one's a CAR and one's M4.  What are the advantages, disadvantages and whats the reason for the larger?

Quoted: I know one's a CAR and one's M4.  What are the advantages, disadvantages and whats the reason for the larger?

It's a matter of preference for us couch commandos. What's more comfortable, the wider guards or the narrower ones? It will all depend on the user.

The M4 has double heat shields while the CAR only has single shields.

Some do not like the double shields as it makes for bad heat dissipation. There are other aftermarket M4 type HGs however that only have single shields.

That being said, I will venture a guess as to why the wider HG's were developed. Maybe to accomodate the double shields .... to have more room inside the guards for more heat dissepation ... to put the shooters hand farther away from the barrel (to be more comfortable).

Hope that helps.

Colt and Surefire are the only ones selling double heat shield guards, but the Colt patent expires in a week or so.