Lacerta file says 2-3 center thick bone plate from rear hip to neck. This tracks other sources and my own experiences. I think most common hunting rounds should able to take them down similar to hog hunting. Lots of .308 and up loads, but also 5.56 75-77gr bonded hunting loads which is a caliber commonly owned by American civilians. There's probably even 9mm special loads that could penetrate that.
Farsight Battle of Dulce has RV sketches of reptilians running away from US military firing at them with m16s - given how dated this event was they were probably shooting either m193 or m855 - but if they could survive in open cover after getting shot the ammo isn't effective enough. M855 is inaccurate because the steel core is off-center, particularly the common mil issue ammo. M193 would likely fragment from the jacket and can't be relied upon for a CNS kill.
Here's something for farsight to confirm - do reptilians have a heart in each breast? What are other areas good for a quick CNS kill with either 5.56 or .308.