This question comes up from time to time. Seems like 416, 416r, or 410 could be good guesses. I am not an expert, but looking at it on a bore scope, it seems to wear away more like a stainless barrel does vice a non-stainless or chrome barrel that is losing its throat. Non-stainless seems to look more pitted or cratered as it wears away whereas the stainless has a more smooth appearance.
Below compares an in-house LaRue barrel with a known stainless (douglas 416). Both are at similar round counts (~1000). The douglas has been mine since new and shot almost exclusively suppressed heavy 77gr at MK262 velocities. I do not know what the LaRue has been used with. The LaRue tube has a little more copper in it and appears slightly tinted as a result.
LaRue 5.56 barrel (16" PTAR)
Throat where the rifling begins
Port- seems to have an interesting lump on the muzzle side.
The other side of the port. Good image of how sharp/deep the LaRue rifling is:
What's hard to picture is that the LaRue has a pretty short throat in comparison.
Douglas 18" 5.56 barrel (known 416)
Throat where rifling starts
Port (rifle gas has a huge gas port)
I tried scoping an LW50 (lothar walter-made) OBR barrel, but the rounded off rifling they use makes it difficult to get good pictures through a cheap scope.