Arnold Rowland is quite specific about the rifle he sees being held by a man on the sixth floor at around 12:15. It's a type of rifle he seems familiar with having used one before:
"In proportion to the scope it appeared to me to be a .30-odd size 6, a deer rifle with a fairly large or powerful scope."
Is he describing a Carcano or something more like a sporterized Mauser?
IIRC, he described it as being "a deer rifle," which could imply a sporterized rifle, assuming that the rifle wasn't an actual hunting rifle rather than a conversion. Then again, he was about 300 feet from it. How well could he see it to describe it.
I'm not particularly enthusiastic about his statements. He places the guy holding the gun 15 feet inside the window. However, if you look at photos of the TSBD exterior, you can see the boxes stacked up in the southeast corner the sixth floor windows. Those are not more than two feet from the wall. On the west side of the sixth floor, there are man-high stacks of boxes, but all you see in the photos is darkness behind the windows. Those stacks are several feet inside the building, too far in to be lit by the high afternoon sun. Someone standing 15 feet inside the windows isn't exactly what I'd expect to see in that situation, especially at 100 yards.
And Rowland's background, as described in his and his wife's WC testimony doesn't exactly lend the ring of truth to the story.