That's a find!
SF, those wonderful Colt Lightning rifles were made back in the day when all of the metallic cartridges in use were black powder loads. I don't know if I'd try a handload with even the smallest charge of modern smokeless powder in it, though I've heard that all factory-assembled loads these days are charged to the 19th century pressure standard (and are thus supposed to be safe in your older firearm).
Luckily, there's a way to confidently shoot these beautiful classics safely...some mfgr's now build black powder loads in the .32-20 WCF (Winchester Centerfire) chambering for your Colt. There seems to be a resurgence in using old .32-20 and .32-40 arms for cowboy action shooting these days. Of course, I'd do just as you mentioned earlier and have a gunsmith or other expert give it a good look, first.
In case nobody has brought this up, the ".32-20" designation means two things. First, the bore size (32 calibre), and second, the original charge of black powder in the case (20 grains).
I'm envious. That is quite a find. I've never seen one before, and the heavy octagonal barrel with that tang-mounted vernier rear sight are not something I've seen often except on rifles used for longer ranges.
v/r, OF
