I have a FX-2 Big Bore 3x with the heavy duplex on my Ruger RSI 308. Plenty of eye relief and has the best resolution of all my scopes.
I have a Big Bore 3x on my Ruger RSI 275 Rigby. That scope has enough eye relief on that hard-to-fit rifle. I could have mounted it with two regular rings but I used an offset ring in the rear just to have the mount point a little further back on the tube. I saw somewhere that the eye relief was listed as 3.9" / 100mm while I've seen some hunters guesstimating it was 3.5" / 90mm. One feature of the FX-II Big Bore 3x from their custom shop was that it was a few millimeters longer so it was easy to mount on larger receivers with bigger ejection ports that were often used on African hunting rifles. For an apples-to-oranges comparison, the current 2.5x20 is 8" long while the Big Bore was 10".
The earliest mention I've seen of the 3x20 Big Bore was around 2011. They stopped producing them in 2018. My FX-II is matte. Most of the earlier M8 models that I've seen have been glossy.