.40 S&Weakest?

Handloads.

Since bullet weight is a major factor in recoil, the light 135 gr. loads would probably produce the least recoil.

Muzzle blast and flash are a large part of "perceived recoil" so often the abnormally light bullets seem worse than the heaviest even if their "free recoil energy" is lower.

A 180gr FMJ and 4.7gr Unique is softer shooting than a lot of 9mm loads. With hand-loading you can go as soft as will cycle your action.

I don't think the 40 is known to be a low recoil caliber unless you hand load with the purpose to down load / produce a light target load I have been looking
into this and was advised try using 180GR and a faster burn rate powder but I have no Idea if you reload

A few months ago, I got an M1911 chambered in 40 S&W. It does a good job of taming the 40 S&W recoil.

Of course, there is a weight penalty that goes along with an M1911. But, I'm more interested in shooting 40 S&W these days than with my Beretta M96 or S&W M&P40.

I have a CZ 40P that seems to like only Hornady Critical Defense and Fiocchi Shooting Dynamics 165 gr. FMJ TC, so I have a whole lot of the Fiocchi ammo. They changed the country of origin for that ammo recently, and I noticed the recoil (and likely ballistics) have diminished with the change. I find it a decent recoil impulse not too much worse than 9mm, though probably still noticeably stiffer.

Hopefully you're not asking for a .40 that's as light as a 9mm, because it's my experience that trying to turn a caliber into something that it isn't is often futile. Best wishes.

With factory ammo I always found 180 grain ammo the softest shooting.

A 180 gr. lead bullet with WST was my lightest recoil. Shot some 155 gr. plated using Titegroup the other day. They were a bit snappy.

180gn Winchester White box from Walmart, Wolf, Tul, Perfecta. Basically, WWB and cheap Russian ammo are the easiest to shoot.

I enjoy the challenge of a little recoil form a .40, it's snappy but won't hurt you. You can handload it down to 9mm levels of recoil with the right recoil springs. But that's not for everyone.

Another vote for 180 grain. I down load them just to the point where they cycle reliably.

Going strictly by felt recoil and case ejected i'd say 180gr Fed aluminum from WW

Not exactly an apples to apples comparison, but in 45acp I worked up a light load.

Started with a 200gr LSWC loaded so light that it wouldn't cycle and just kept bumping it up slightly until it did. I'll have to check my notes but it's a small dash of bullseye under that LSWC. The load would barely cycle my 4566TSW I had at the time, the action was moving so slowly you could feel it closing after you were back on target and once it got dirty it would require a light tap to get it into battery. That load functioned perfectly in my 1911's though even when dirty.

If you want a light load, switch to a lead boolit slightly under the standard weight and start from the bottom until it juuuust cycles. Faster powders seem to play better in this pressure range, they burn more consistently and cleanly. I'd be looking for a ~150-160gr LRNFP or LSWC propelled by a tiny amount of bullseye, RedDot or whatever you have on hand with a similar burn rate.

Gun design is a big factor. The Ruger SR40's are awesome because they have a low bore axis and extra mass in the slide. It really tames it down, really enjoy shooting mine.

Would have to say that the 180 gr. loads ran just fine in my Kahr CW40 but some of the faster moving, lighter weight JHPs were definitely a little bit snappier and exhibited a bit more muzzle flip.

180gr bullet at 750 - 800 FPS is a wonderful load.

Since we are talking about 40S&W I, and probably everyone else, was thinking of a semi auto handgun. The felt recoil will be much different from a GP 100. If you ever start hand loading you will be able to make some real powder puff loads. With the weight of your revolver, just about any cheap 40 ammo should be comfortable to shoot. Get some heavy bullets and some lighter ones and see what you and the gun likes.

There's always this...

Might be a little pricey for plinking ammo though.

Although .40S&W is often characterized as "snappy", I've had no issues with felt recoil when shooting 180 gr FMJ from either my RIA all steel Commander sized 1911 or my SA XD40. YMMV

Tula 180gr FMJ