What weight bullet is used? Assuming .45.

The .410 slug load of a 100 or 110 gr slug is simply pathetic. So is a 125 gr slug load.

The .45 Long Colt of a 225 or 230 gr bullet is better, but still for a Howdah, rather pathetic.

One of these however as I have said could be merely a useful fun pistol for a fun Howdah target competition.

So I wonder what or if a .45 bullet could be used in a .410 brass shotshell? What weight bullet? What velocity?

As Huvius said in an early post, a Howdah cartridge is not about velocity for long range pistol shooting. It's about bullet weight and thumping power up close. For target shooting some range might be useful.

.45 Long Colt, inclucing +P
Bullet weights 200 to 360 grs
Velocity 700 to 1500 fps
http://www.ballistics101.com/45_colt.php

.410 Brennecke
Bullet weight 110 grs
Velocity, barrel length ?, 1750 fps

https://gundigest.com/gear-ammo/ammunition/410-slugs-whats-the-use/amp