i am lucky to own and shoot two full sized handguns -- a 5" .357 Magnum from the 1950's and an all stainless steel Sig P226 in .40 S&W.

When in comes to recoil, the .357 is a thump and a boom compared to the snap and crack of the .40.

Anyone else have the same experience?
Any theory why this would be so?

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I am not sure what you are looking for here. The only response sounds like I am a wiseass

You have different cartridges. Loaded with different powders that burn with different expansion rates creating different recoil pulses. In different firearms with different weights and balances. One firearm stores and uses recoil to load/chamber the next round and one does not

How could they possibly feel the same?

There is also the question what 357 Magnums you are shooting. I've found that American Eagle 158 grain SJSP loaded at 1250 fps is not only excessively boomy at an indoor range it also tweaks the arthritis in my right wrist after 35 or 40 rounds. Counter that with 40 caliber Federal range ammo and I can shoot that all day without issues with my Sig P239. Then there is my SD practice ammo which features a 165 grain bullet loaded to 1150 fps that duplicates the ballistics of the Speer Gold Dot I carry with. This load feels and sounds quite similar to that Federal 357 Magnum mentioned previously with slight less boom and perhaps a bit more bark. However, due the slide on the P239 soaking up a bit of the recoil impulse it's a bit easier on my wrist, so I can shoot a full box of 50 without pain. Note, after a box of this hotter load I do start flinching so any range outing with this load is limited to just one box of 50. Finally, I have a distinctly hot load for my 45 ACP that features a 185 grain Hornady XTP and it feels very similar to my 40 caliber SD practice load. Point is, Snap and Recoil isn't always caliber specific as Coltsaa noted, in many respects it is determined by what exactly you are shooting.

The harmonic frequency of the compared handguns is totally different, along with round fired.

My 686 snubby is a loud handful with .158gr, the 4" 686 is calmer and quieter with the same round, and my 24" octagon barrel .357 rifle makes that round quiet and tame.

My cousin brought his long barreled 1973 Colt Trooper to my range, and it was much louder and snappier than my snubby 686.
Everyone on the firing line and the RSO came by to see what was making so much noise.

The type of gun/wt/ammo can create a vast difference in felt recoil. I too have the stainless Sig 239 in 40, & a 4" S&W 681 357. The 40 feels much more snappy to me. (but it is a compact/sub compact vs a full size) When letting novice shooters try different guns/calibers I get more complaints about the 40 cal. Full load 357's are no picnic, but I "generally" don't get the snappy complaint. If guns were = size/wt, who knows. I find the recoil from a 45acp more comfortable than the 40 S&W. Again however I could load specific ammo to make those calibers feel/sound vastly different from each other.

Is the powder in the .40 generally a faster burning powder?

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I prefer 357 in my model 13 to 40. Cal in a glock or m@p

I think I know what the OP is trying to say, but I find my .40 hard to compare to my .357 simply because of the recoil absorption of the semi. Sound-wise, my .40 is more of a "pop" where the .357 is more like ka-wham.

I would compare the difference in recoil between my .40 and my 10mm auto's as being similar to the difference of firing .38's and .357's from my revolver.

Usually, provided you are taliking full house .357 Magnum with a 158gr bullet.

While you can use faster powder in .357 and you can use slow powder in a .40 that's not generally how it's done.

FWIW I've tried lighter bullets with fast powder in .357 & they were pretty brutal. A friend loads .40 with Unique & he really likes it.

To add to Fishslayers response I'll relate what I use for 357 Magnum and 40 S&W reloads. Because for me it really is all about what I intend to use the reload for.

First, some "rules of thumb" in regards to burn rate. Generally faster powder use lower charge weights, produce less velocity, and feature less flash and boom. Slower powders can in some cases completely fill a case, typically have more weight of powder used per load, and feature more flash and boom. However, the barrel length used for a particular load can have a HUGELY GIANT impact on the amount of muzzle flash and report.

For 357 Magnum I reserve my use of H110 for use in loads intended for use in one of my 357 Magnum caliber rifles. In these rifles I can expect muzzle velocities ranging between 1600 and 2200 fps depending on bullet weight with no daylight observable flash and a report that is rather mild. If I happen to try one of these loads in my 4 inch 620 the resulting muzzle flash is extreme and the muzzle report sounds like a 500 Magnum. For my late model 357 Magnum's I generally load with Accurate #9 and only "lose" about 50 fps in produced velocity with a huge reduction in flash and a rather typical muzzle report. Then for my model 19-3, out of respect for it's age and beauty, plus concern for the forcing cone, I load up Magnum Lites using Accurate #5 and get almost no flash out of the 2 1/2 inch barrel and a report that is only a bit louder than a 38 special. I can also report that this 158 grain, 900-1000 fps load feels very much like a 9mm from that model 19.

Then there is the 40 S&W, for which I've developed two loads. One is a real powder puff clocking 960 fps and featuring 5.6 grains of SR 7625. The other is a bit of a Dragon featuring a 165 grain plated HP at 1150 fps and using Longshot that duplicates the Speer Glod Dot loading I carry. This load is distinctly snappy and sounds enough like a 357 Magnum that I've been asked about my 357 Magnum Semi Automatic.

Point is, different powders, different charge levels, and vast differences in platforms can have a drastic effect in how a particular "caliber" actually shoots.

Back in the "Cowboy" days, the .38-40 (which is identical to the .40 S&W with 180gr load) was said to Shoot Hard. Maybe they knew something and things have not changed that much

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Firing a 357 Magnum revolver is a done deal when the primer is struck.................

The Pistol has the gas going down the barrel and then unlocking the barrel lugs, then the springs or blow back system goes into action, ejecting the case with the rear movement of the slide. At the stop, the slide starts forward loading the next round...............
if things work out......
and the slide comes to rest with the pistol muzzle back or near the POA.

Apples to Oranges..............

Also through into the mix that you could give 10 people that 357 mag and you would get 10 very different descriptions of recoil.