Let's do the maths again.

In all my calculations, I make the following hypothesis :

Chaos bag of Night of the zealot, on standard difficulty

gives -1

gives gives -2

gives gives -3

gives gives +2

Comparison with .45 Thomson

On Standard difficulty level, if you aim at +2 (you organize your play so your skill value is 2 above the difficulty), the average damage done by the Winchester is 1.25, against 1.5 for the Thomson. If you aim at +4, it's 1.44 versus 1.88. So Winchester makes roughly -20% damage par attempt for a -33% resource cost. Not that bad

(on Easy difficulty level, Thomson makes roughly the same damage. Still for a -33% cost.)

Comparison with .45 Automatic

The comparison with .45 Automatic has to be different because .45 Automatic has a fewer bonus. Let's make raw comparisons, with no strategy in mind.

On standard difficulty level, if the difficulty of the test if 2 above the base skill value of the investigator, Winchester delivers 0.75 damage per attempt on average, while .45 Automatic delivers 0.25. That's +200% for Winchester.

Ok, difficult tests like this one are uncommon. But Winchester is still +113% when the difficulty is 1 above, +11% when equal, -8% when 1 lower and -18% when 2 lower.

(on easy difficulty level, the numbers becomes : +200%, +92%, +18%, -4% and -10%)

Thomson takes two hands but has 1 ammo more.

Conclusion

The first question is : can you afford two hands to fight ? If the answer is yes, Winchester is interesting versus Thomson if you struggle for resources. It is also interesting versus Automatic if you are a good fighter and have a team mate who can take care of the critters, or if you are a mediocre fighter who wants to shine on the battleflied from time to time (Skid ? Jim ? Diana ?)

If the answer is no. Well, you can't take Winchester.