As someone has suggested, use a .22 LR bullet and squeeze it a little in a vise to make it slightly fatter. Put some extra lube on it and start it in the muzzle end of the barrel. Then, it'll have the rifling engraved on it, just push it all the way through to the breech.

Measure the widest points, which will be the groove diameter you're trying to fill up. .001"-.002" over that diameter is usually what we try for.