Found a used KKM 4.25" .40-9mm conversion barrel at a local gun shop for $95 (they retail for double that + shipping).  Was thinking of picking it up for my M&P 357 Sub compact which has a stock 3.6" barrel for more versatility.  Will it fit / function without issue aside for extending past the slide?  I've read conflicting reports about this.  Also, would I be better buying a cheaper factory 9mm barrel to just swap in instead of the more expensive (even at discount) kkm "conversion" barrel?  I have also read that the stock 9mm barrels will fit and feed just fine in the 40/357 guns.  

9mm will feed and function fine out of 40 / 357 mags I imagine as they do in most pistols.  Will regular full size 9mm mags also fit / lock into the 357 sig sub compact pistol?

No one knows, huh?

I have the m&p compact 1.0 longer barrels will fit like 40 to 9 but not sure on the 357.

I don't think the KKM full sized 4.25 barrel will work well for what you want (dependable accurate shooting) in a subcompact M&P.

They are subtly flared at the end of the barrel to fit tight to the end of the slide to reduce end play.

The barrel shaft has to be thinner in the middle to allow the barrel to slide at an angle as the slide moves back across the barrel to eject & feed.  I think you would be better to just run a factory S&W 9mm barrel. I do this for some .40 4.25" M&Ps without any issue.

Feeding 9mm out of forty mags is hit or miss (in my experience).
 It "usually" works - but not 100% of the time. I just use 9mm mags as used 9mm M&P magazines are inexpensive & easy to find.

The longer full sized magazines WILL fit in the subcompact frames. The.bottom of the magazines will hang down past the bottom but there are grip sleeves that make it seem like the hang down of the magazine is part of the grip.  

I'll post pictures in a bit.

Hope this helps!

Bigger_Hammer

I wouldn't use the longer barrel in the compact. That is a full size barrel on a compact frame and more importantly a compact recoil spring. You may have the slide cycle harder than normal.

I bought the 1.0 M&P compact 40 twelve years ago. I later bought a 357 Sig Storm Lake, a factory 9mm and an 9mm Apex threaded barrel. All were made for the compact, and all work well. I found the .40 magazines failed frequently with 9mm so I bought 9mm magazines.

Sorry Blain - hectic couple of days (in a good way!! )

M&P "9C" with 12 round factory shorty mag



M&P "9C" with 12 round factory 'pinky' mag (with extension due to Big Manly Hands)



M&P "9C" will fit the full sized factory 17 round magazines, but they hang out the bottom



M&P "9C" with full sized factory 17 round magazine + "Grip Sleeve" on magazine body so it feels like a full sized grip & capacity on the small "C" frame.



Obviously the .40 mags are slightly less capacity, but exactly the same sizes.

Hope this helps!

Best Regards!

Bigger_Hammer

Stormlake barrel chart
Attached File


I prefer the 15 round mag with X-Grip for the 9C 1.0
Attached File

That is a nice "splits the difference" between the longer 17 round and the shorter 12 round on the Subcompact.

Ive got a M&P1.0 compact with a fullsize 4.25" barrel that was threaded by a previous owner. Slightly looser than a stock barrel would be , but at the time no one was offering a threaded compact barrel. It's doable, might not benchrest accurate, but it works just fine on 10" paper plate targets.