All the reloading data I can find says that the COL of a 115 gr FMJ bullet is 1.100". Winchester factory bullets are 1.175". Can anyone tell why the difference. I had about 20% FTF with 5.1 grains of WIN 231 powder @ 1.100" and no problems with factory loads.. Thanks for the help in advance.

Different bullet construction.
It could be different jacket thickness, different lead alloy, or a different bullet ogive. These can all effect bullet length.