I have bought nearly a half dozen guns since November

[Pandaz3]

Okay Storm Lake had M&P40C 357 Sig barrels for $169, but I back ordered one from Brownell's for $123 with military discount.

 

[uberguy]

I noted the magazines were stamped .40 S&W .357 Sig. I' know you can do that with most 40's, I'll look for a barrel for my 3.6", that could be harder.

(edit - hadn't seen your post about Brownels when I posted this below)

Midway USA has the Storm Lake @ $160 in stock (guess prices have gone up since 2016) - or you might consider picking up a factory (S&W) 357 SIG in 4.25" at $88 (both linked below); These were drop in for me, but some gunsmith/fitment may be required. Interestingly, the compact barrel is no longer listed on the Storm Lake website.

KKM barrels are well made. I recently picked up one in 9mm for an M&P 2.0 - 5" and expect to pick up one of theirs in 356 SIG also, later this month. Apex are probably top of the line at around $200. KKMs retail for $165 in matte finish.

Apex, Wilson and KKM say their barrels fit in 1.0 and 2.0 models. Not sure about the SL. Mine went in 1.0 guns.

Storm Lake Barrel S&W M&P Compact 357 Sig 1 in 16" Twist 3.58"
Smith & Wesson Barrel S&W M&P 357 Sig 4-1/4"

KKM | Precision made custom pistol barrels