Hi lenno11,
I couldnt find anything on ShareCG but the search engine on this site isnt perfect so there might be something I've missed.
I found a few items on TurboSquid that might be useful for you.
http://www.turbosquid.com/3d-models/free-rem-870-shotgun/249506
http://www.turbosquid.com/3d-models/gun-shot-max-free/744353
http://www.turbosquid.com/3d-models/free-musket-3d-model/732042
http://www.turbosquid.com/3d-models/double-barrel-shotgun-dgn-s-free/336778
http://www.turbosquid.com/3d-models/volcanic-gun-obj-free/688208
http://www.turbosquid.com/3d-models/lever-action-rifle-3d-max/301583
There are couple of winchester look-alikes in there but they're in Max format. If you are not concerned with price then there are more options -
http://www.turbosquid.com/3d-models/3d-model-winchester-carbine-rifle-pzcarbine/255437
and
http://www.turbosquid.com/3d-models/3ds-1866-winchester-rifle/459707
Let me know how you get on with that and I'll have a hunt around for more.
Maf