you don't really NEED to slug the barrel, but because the K31 is an older gun (I'm assuming yours isn't new production if there are any), you're more likely to have variations in the bore diameter. These variations will accumulate lead and hurt accuracy. If you find that your bore theoretically should prefer an uncommon diameter bullet, you should cast them yourself, that way you can also play around with the hardness, bullet weights, etc etc. Join the casting addiction!
bottom line though, is no. BUT you'll get better results if you do.
Rev 5:13 "To him who sits on the throne and the Lamb be praise honor and glory and power, for ever and ever"