The turbine core silencers are USFS approved spark arrestors.. it isn't built into the endcap.. its a 2 chamber system or somethign like that. Its actually part of the silencer construction.
EDIT: this is off of FMF's site
Our best selling Turbinecore 2 uses the exclusive "Turbine Dampening System". This top-notch product has well earned its U.S. Forest legal status. The straight through design of the spark arrestor canister and the performance flow stainless steel end cap ensures strength, durability and most importantly, track and trail proven unrestricted power. The Turbinecore 2 is the #1 spark arrestor in the world