I have an SD40VE that I'd like to use with 9mm. Can I use a barrel swap, a SD9VE slide assembly swap, maybe a SW9 top end swap? Thanks

Barrel... no, Top end maybe..not worth the trouble IMO..

Cheap enough to just go buy a complete gun.

Barrels are not interchangeable. I already tested it. As bruce said, top end maybe, but not worth the trouble. I have seen the SD9s in the 250 range used.

Rosewood

Unfortunately, it's completely impossible.

Neither the slides nor barrels, frames nor magazines are interchangeable between the SD9 and SD40. Despite its lineage, the SD Series is not a Glock and therefore does not possess the parts interchangeability nor ease of conversion of a Glock.

You'll have to buy a second separate SD pistol chambered in another cartridge if you want access to an alternative cartridge in the same platform. Fortunately, the SD Series is so cheap that the cost of an SD9VE and SD40VE combined are less than a Glock + a cartridge conversion kit.

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Shooting Comfort is bilateral.

Thanks to all for the info. I'll look for a good used one.

As others have noted above, you would be better off just buying an SD9. In my area, new SDs go for $269-$299 on a regular basis. I started out with the SD9VE, added the SD40VE then added the SD9 & SD40 in FDE w/black slide. Plan on adding the SD9/SD40 w/Gray frames as I go along.
I also have an assortment of M&Ps & Shields, and I still enjoy the SD series very much! :-)

Go to ACADEMY SPORTING GOODS website and look for the SD9/SD40 guns. IMHO. these are a better gun than the SDVE series gun.

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How many guns are too many?

The SD is objectively superior to the SDVE due to the Melonite treatment on the slide and high visibility (either a Tritium Night Sight or a Fiber Optic Sight, depending on the model) front sight.

VE stands for Value Enhanced, which signifies that it's the economy class model.

However, unless it's priced well and the Melonite slide is particularly important to you, then it really isn't worth it. The SD model was replaced by the VE models because it was so close in price to the M&P that most folks either opted for the cheaper SWVE or the M&P, but was revived as a distributor exclusive since there is a niche market for the midrange SD.
Personally, I'd sooner opt for the SDVE or the M&P, unless I could find an SD for a really good price. In fact, that was my plan until I happened upon a last-gen SW40VE at my LGS for $199 LNIB.

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Shooting Comfort is bilateral.

My SD40 and SD9 both with melonite (err Armornite) finish and FDE frame both came with standard white dot sites.

Rosewood

Mine did as well, and that's okay, I like the 3 dot white sights.

I upgraded mine to TFOs. Would put TFOs on everything I own if I could afford it and if they made them for my other guns.

Rosewood