Applying logic to how a fellow commits suicide, or suffers homicide, meets more coherence within CSI series than within reality. Such episodes can happen in the most bizarre conditions and circumstances. For example, suppose the man, against all odds, decides to kill himself, as in actual life such unpredictable episodes occur, and the young men tried to prevent him from such desperate act, ending up triggering themselves the shot, as a result of such struggling, giving (i) logic to the odd place & angle in which the projectile entered Van Gogh's guts. The caliber of the bullet doesn't necessarily determine how long one resists to death ... only potentially. There are 'good' and 'bad' shots.
On the other hand, we are hypothetically assuming that the gun auctioned was actually the one used, which could have not. To say that, there also large pinfire (Lefaucheux system) revolvers (i had some), with so large, or even slightly larger caliber than cap & ball system .44 Colts (which i also had); and not only small versions, some even rather tinier than the show off but reportedly clumsy 'Apaches'.
On yet another hand, when you play Wild West, you do it with what you have at hand; even wooden guns. It's imagination that counts, not the caliber; to be (or not) ridiculous is not in the plans, i guess. When i was young, like others, so poor as not able to have a toy revolver, i used my forefinger to order 'hands up' to my foe and bring him into custody. But i am digressing
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