I got a very different measurement from my Function bolt. I measured 3.8125 from the front of the pin to the end of the bolt.
System X NME LE: Graphite/Black Dust
Bob Long Regulator, Evil Front Block and Alias LPR
FBM Factory Delrin Trigger
Function Bolt, Feedneck and Slider On/Off
UL'ed Evil Minion: Silver Dust
Hybrid Bolt Guide