Naturally there was more then one revolver used during the series of movies
I do not recall any movie scenes where an 8 3/8" Model 29 was used, however there was a series of Magnum Force movie posters that have the appearance of a longer barrel.

As a photographer, I am certain this is simply the natural perspective distortion effect from a very wide angle lens.

I believe these posters are the origin of the various 8 3/8" stories.