I'm considering building a 5.5" carbine in .357 Sig.  My understanding is I can just use a 9mm barrel and a .40 SW bolt.  Is that accurate?  Anything I should be aware of?  What sort of buffer weight would I need?  And would I be able to switch to 9mm just by changing bolts (and possible buffer springs)?

I believe your going to need to make sure it chambered is cut to fit the .357 sig so the case will fit the barrel or else there is nothing supporting the case when it is fired.

Nope, just nope.  You have to dedicate an upper to each caliber (you can't switch caliber/case between 357 Sig and 9mm on the same barrel).

You could maybe have a 9mm barrel chambered to 357, but there might be feeding issues you would have to deal with.

The barrel has to be chambered for .357 Sig.

The bolt face is the same as a 40 S&W.  I suspect that a 40 bolt will work, but there may be some tuning issues (buffer/spring).

You will find that will have to find a magazine solution first.  (Most likely a glock mag lower if you want to switch calibers).

To make a 9mm 357 SIg switch caliber you need:
Minimum - 1 glock lower with 9mm and 40 S&W mags (these are different). 2 different uppers, 2 different bolts, likely 2 different buffers/springs for best reliability.

I guess you could use a regular ar15 lower if you go with Olympics system of mags/uppers, but that's pretty pricey with the mag costs.

OK did not know that re: the barrel.  The more I think about it the less appealing it is since the high capacity glock magazines for .40 have a full 11 rounds less than the 9mm ones, and it'd be much harder if not impossible to get the .357 sig to shoot subsonic, which is part of the appeal of the 5.5" build.  So if I really wanna do a .357 sig build, I'll probably just do a separate upper with a longer barrel chambered in .357 to take advantage of the increased velocity.

QC10 is prepping .357 bolts and barrels right now. They are taking bbl length preferences to get ready for production.

That will be your easiest and most direct route to a 357Sig AR.

Contact heavybuffers.com for a buffer and spring suggestion. I'm guessing he will say the 45 buffer and extra power spring is your best bet.

You are going to have a high velocity, flat shooting gun. Should be fun.

https://instagram.com/p/03KOHTNJx8/

QC10 posted this yesterday

Very interesting...  Muchas gracias.  The glock mag lower I'm gonna build takes .40 and .357 sig mags......  More money into the black hole.

In shooting the 357sig I found the recoil to be much sharper than the 40sw.  On my first try I used a 5.5oz buffer and normal spring and noticed that the spent rounds had lost their bottleneck.  I ordered a 8.5oz heavy buffer and wolf extra spring.  The gun shoots much better and the cases come out normal.

I considered it.  At heart I'm a 357 magnum guy so I went that route.  I was trolling the internet when looking at a 357sig carbine and found a couple of useful reads. Here.  A great deal of good information, some jacktacular information as expected on a forum.  Those guys know their ballistics and its a good read.  

Why I wanted a 357 magnum upper?   Just a want really.  No real purpose that a rifle chambered in a rifle cartridge would not accomplish.  

Ammo commonality has its merits, but not many merits.

Our country is built on want.  Knock your socks off, and post pictures for us when it's done.  

I'm building a 9mm now, pretending that it's to save on ammo costs.  I only have to shoot ten cases before I break even.  After that, it's a free pistol.  

Interesting, thanks Russ.  Got one of your 9mm barrels a couple weeks ago, excited to put it to use soon.