Thanks tips. I run two FF rails. I'm well aware of their benefits. You indicate that you are building an M4 clone. M4's run stock handguards, the wizzy M4's (SOPMOD) use a quad rail.
Accuracy is a moot point when you're goal is to imitate a short to intermediate range rifle. Any magnified optic outside of an ACOG you mount will not be accurate for an M4 build.
The accuracy gain of a FF on a carbine without magnified optics is a loss due to range, environment, shooter, and ammo. Keep in mind that stock barrels are designed for combat accuracy, not Olympic grade performance.
TDC