If you say it's light, it's probably a 6 lug Mark V in a synthetic stock. They haven't been built for quite a few years now.
What chambering is it?
If it's anything based on the 30-06 or the 308 case it will be a 6 lug. The only Weatherby chambering made in the late 6 lug action is the 240.
Actually, the 6-lug action is current production in "standard" (non-magnum) calibers. The 6-lug ultra lightweight is advertised as weighing 5-3/4 lbs. They are very nice rifles. Here is a link:
http://www.weatherby.com/product/rifles/markv/ultralight
