I have a M&P 40 with a KKM barrel converting to
.357 Sig. love the way it shoots, is actually more accurate
than the .40 setup.
Well anyway the question is I have been loading with Extreme 124 gr Flat points that is the type bullet the new rounds come with. I load my .9's with the same brand only 115 gr round nose, can I use the round nose in the sig rounds? I want to consolidate my lead stock to a more useful supply BTW I have shot the flatty's out of my .9 with no consequences

I don't think RN profile would work in 357 Sig. The neck is very short so if you seat RN to recommended OAL the neck would be at tapered part of the bullet and you won't be able to get any tension. I've used X-Treme 124 HP in my loads for 357 Sig.

Maybe, but doubtful. As noted, you need as much bearing surface inside the neck as possible. IME, only bullets under 115gr in a RN will fit, but your individual gun/mag will determine that. In my 1911, I can make a 115gr, even a 124gr RN work seating longer.

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NRA Cert. Inst. IDPA CSO

I loaded 10 rounds using the 115 gr rn they were a little over the recommended OAL and were contacting the magazine slightly . shortened them up a few thousand they shot with no problems. I want to try and consolidate my ammo stock to use in my .9's and .357 Sig,

Do they pass the "push" test ?