I am thinking about getting a Mossburg 835 Ulti-Mag 12 gauge turkey gun with a 24" barrel. What is the effective shooting range for this barrel length?

It depends on the choke. With a good tight choke, any good turkey gun will consistently take birds at 40 yd.s. Round shot is a pretty aerodynamically inefficient. After 40 yd. it starts losing velocity fast. Barrel length doesn't figure much into the equation. 24" is a good compromise between handiness in the brush and more velocity.

I have seen 24" barrels outshoot 30" barrels alot. It depends on your gun and choke.

Most of the powder has been burned by the 20" mark, the rest of the bbl will help you with the sight picture, and helps stabelize the shot.

Hurry up and wait.

I have a mossberg 835 with a 24" barrel.
I get very good patterns with #5 shot out to 45 yds.

Take a kid hunting,for they are the future of the sport.