Do you need a heavier recoil spring for the Buffalo Bore and Underwood cast loads, or is the factory spring okay?

You can try a 20lb spring - same spring as glock - and I haveused a 20lb spring but with underwoods 155gr 1300fps load and the 165gr 1200fps load in my M&P 4.25 but changed back to an 18lb recoil spring . I could notice a slight amount of nose down when next cartridge chambers with both ! Gen 1 factory spring was the same as 9mm - 16lb . I have read the 2.0 is also a 16lb spring and 18lb so try the 18lber s and see how it feels . Cast hot loads are for what hogs or bear ! I buy my 18lb or 20lb springs from -
Flat Wound Recoil Spring for Smith and Wesson M&P Full Size Pistols

A good many decades ago while exploring more performance, I realized that if I wanted more power than the mainstream factory loads, the more intelligent choice was a larger caliber.

I've no idea of the recoil forces on the M&P series, but long ago they did tests on the 1911. With ball ammo and the stock 16 lb spring, the impact force on the slide in recoil was 700 Gs. The force going into battery, while stripping and feeding a round, was 750 Gs. When you increase the spring rating, you increase the impact load going into battery.

Back in the 1980's a lot of folks played with springs in competition. A lot of slide assemblies were launched down range after failures of slide stops and/or barrel under lugs. The 18 lb spring in the 1911 turned out to be enough to keep people happy apparently with minimal bad effects, but again, the comparative forces aren't (widely) known.

But, it's your toy.

CORRECTION: I went in search of the source document (by Leupold- published in the American Rifleman) for the figures above and memory (and a lot else) isn't what it used to be. The forces of the slide going into battery are higher, but my figures are off. In the interests of being exact: The force of the slide stopping in recoil is 750 Gs. The force of the slide going into battery is 800 Gs. FWIW, the force recorded for firing a .45 ACP round is 2000 Gs.

That's strikes me as odd as it implies the slide has more velocity going forward than in recoil. I suppose it has something to do with the slide in recoil being slowed by two springs, the recoil spring and the hammer (main) spring. Even so, I would expect the slide velocity going into battery to be less because of the forces involved in pulling the new round out of the magazine.

With a striker fired gun all the recoil mitigation is in the recoil spring. Now you've got me thinking where the forces are going in pistols that use rotating lock-up or gas delay systems.

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Release the Kraken

You're probably right about the effect of the hammer spring. The firing pin retention plate originally had a square back edge, not the rounded one we're familiar with. But, the test mentioned above involved the rounded edge plate. And, the accelerometer doesn't lie.

While the slide needs more force to overcome the drag of feeding the next round, there's also a rebound effect from the slide striking the frame in recoil.

If you really want to scratch your head, consider the roller locking system of the HK rifles & MGs.