You wont get a good measurment with a calipers on your chambers.If you have the calipers cocked to one side just the slightet on a small round hole like the chamber, it will throw the measurment off a good bit. kwhi said to puch a .457 ball in the chamber with the rammer and then push it back out with a rod through the hole after a nipple is removed.Then measure the diameter and it should be real close, just dont put much pressure on the soft lead. Then do the same with another ball through the barrel. Measure that the way rod doc showed,that is the best way to do a 7 groove slug without special measuring tools.I am almost certain your cylinder bores will be bigger than you have came up with so far.It seems to be common from what others here have found,for the chambers to be undersize especially on many newer guns.Have no good idea what is behind that.Optimal would be for the chambers to be the same as barrel groove dia or several thousandths bigger than the barrel groove.Regardless it seems that most people guns still shoot accurate like that.If you want to get everything you can out of your gun, the best thing would be to ream the chambers out to the correct size though.This should give an increase in velocity too.