Ok, so a 100gr lead pistol bullet in the 7.7 Jap ??
Will work. Round will go bang, bullet will exit barrel. Beyond that no guarantees and is dependent on many specific factors.
Take a look in your loading data for the Speer "plinker" or the Hornady version in .30 cal. I've used both in .30-06, .308, and even .30-30.
this will give you some idea what can be done, they can go smokin fast. Accuracy, on the other hand is another matter.
You could drive a light bullet really fast from a 7.7mm Arisaka...
DON"T DO THAT! with the bullet you mention. Lead, no matter what it is coated with, is still lead, and not a copper jacket.
AT lead bullet velocities, the bullet you are considering should upset enough to compensate for a small difference between the bullet and the bore size, so accuracy might be decent. But drive that lead bullet too fast, and it can "strip" through the rifling, resulting in really poor accuracy and likely heavy leading in the barrel (which also ruins accuracy, and is a pain in the butt to clean out).
The short bullets, seated properly generally feed well enough in a bolt action.
In a nutshell, your cast pistol bullet can go through the rifle barrel ok but don't try to exceed cast bullet speeds or you will most likely have disappointing results and some hard chores.
Plinker, "gallery" and short range small game, that bullet should do fine at the proper speed (or less). Don't try to make it into a 3,000fps+ screamin varmint round. While the case has the capacity and rifle has the strength, its just NOT the right bullet to use for that, and I promise you won't be happy with it when you push it beyond its capabilities.
A purpose made jacketed 100gr bullet can take the high speed, a lead pistol bullet should not be put in that situation.