OPATRIOT: If your chamber mouths mic to .359 - .360, then you can shoot bullets up to .360 diameter. I checked my older Lyman manual and the diameter of the cast bullets for that caliber run up to .360.
Slugging the barrel means taking a soft lead slug, or fishing sinker or the like close to your suspected bore diameter and pushing it through the barrel so it takes a full imprint of the rifling and measure it to determine the diameter.
A 200 gn 38 caliber round nose soft lead bullet really does not have to be moving much faster than 600'/sec to be effective. Yes, they are slooow but thump surprisingly hard when they hit. I can remember shooting 200 gn rn 38 Specials from a snub-nose and seeing them fly downrange - it was a hoot (but ya learned to call your shots really well - lololol)....
You should not have any problems by going up from a 147 gn to 160 gn slug. The first step down from 200 gns the Brits made was to 177 gn and later due to the liability concerns over the older American made breaktop revolvers the slug was dropped in weight to 147 gns.
Your Colt revolver should be able to handle slugs right up to the 200 gn load without problem, if it is good condition. Even though you have shot it you should still have it checked by a gunsmith to make sure it is sound. HTH. Mikey.