Chris Costa Experimental 11.5" SR-16 FSGB | URX4
Not much information on this upper receiver, but we know it came from Chris Costa, and he received it from KAC. in all my years i have never seen an upper configuration such as this come from KAC. you have an 11.5" E3 barrel with a 10" KeyMod URX4 with a machined FSGB underneath.