If the barrel isn't finished just clamp it with some brass, copper or aluminum in your vise jaws. If it's finished you might want to use some relatively tough wood and orient the grain such that the force in clamping is less likely to cause a split. A big sturdy vise is beneficial here. For a tight fitting plug, it sometimes helps to tighten until almost complete but with a little slack left and pull rather quickly to snap the plub into position. This should work fine, but if if it doesn't , take a bit of metal off wherever it's hitting. No need to buy any special equipment for the job.