The basic strength of the Martini action and steel used in Brit barrels are legend for the times and most gut reactions would be, of course it can handle the pressure....at least the gun will. However most smoothbored .303 for .410 (actually a .303 case converted to shot cartridge vice a true western .410) have quite a choke already and not a very gentle one unless the throat was further relieved from that of the .303. While smoothboring varied, the only examples I have shot are the RFI SMLE rifles converted to smoothbore, which are basically a .330 smoothbore. Slugs for the .410 vary in width and design, with some being softer and more readily swagged down by design than others. If it was converted to take regular .410 shells vice the .303 shotshell, that is a start. However slugging the bore and determining how much the particular slug you are using will have to be slugged down will give you a better feel for what pressure spike may be encountered. Again, this isn't so much a fear for the action or barrel, but the typical .410 shotshell head is not the height of design strength. Also some if not most of the current slug designs don't take kindly to severely being swagged down....leading to inaccuracy even by slug standards. Opinions i'm sure will vary as much as they do in politics, but if I was to use a smoothbore .303 with slugs, I'd start with a Foster designed slug in close to pure lead. This old standby design was created prior to the development of "slug guns" and tolerates severe choking while maintaining form and accuracy and is one of the few designs seen in those shooting older/weaker shotguns with moderate or greater chokes.
Rich in West Virginia, savoring life one cartridge at a time.