I know it sounds crazy, but I really like the round.  I'm toying with the idea of building a pistol in .357SIG.  I've got a 10.5" in 9mm, and would love to see what it can do being a reloader.  I'm presuming I would need to use a dedicated Glock style lower & magazines.(What is the highest capacity 40 S&W/.357SIG mag GLOCK makes?)    

Is there any kind of a mag block available to use a standard 5.56 lower? (No clue as to what magazines you would use, I just like the Colt style like my 9mm build.  Not sure about blow-back or gas operation either.

And if I'm wishing in one hand, how hard would it be to make a mag block that used P226 or P320 magazines?    I'm not bashing GLOCK, but I pretty much only shoot Sigs, so using the same magazine is a bonus!  

Thanks in advance!


@Mad-Machinist

Well currently there is nothing that uses Sig mags, there are only a couple of companies that make pistol mag lowers that take mags other than Glock mags, Wilson and one other company but I don't for the life of me remember their name... I don't think Wilson does Sig mags but it would never hurt to inquire.. most lowers are going to be Colt, Glock or grease gun mags. Someone could come up with a mag block if Sig mags fit in a standard lower (I don't have any Sig mags to verify) or someone could draw up a lower that would use Sig mags but they would have to see a market for it to make it cost wise. If you do want to move ahead and use a Glock lower Ron at RMW XTREME has built several DI 357 Sig guns and they are a hoot to shoot. I would stay away from a blowback 357 Sig as it requires heavy springs and buffers to keep it from opening too soon. The DI uses standard spring and standard carrier and a 223 bolt opened for the sig rim or I think a 6.8 bolt will also work but may need tweaks. If you go with grease gun mags they hold 35 rounds if I remember correctly.

?? Nothing in his post says he is hell bent on using Sig magazines. He can use Glock 357sig/40s&w mags.
No idea of largest capacity Glock mag for 357/40.
Olympic used to make a mag well block and mags for a 40s&w conversion..?

One of the things op did state was he shoots mostly Sigs and Sig mags would be a bonus.... Good luck getting anything from Oly, last I heard they either closed or were going to close? I also think they had stopped making mag adapters long ago???
The other company that has started making lowers that take other pistol mags is Nordic Components with their 9mm PCC Rifle  they currently have Glock and M&P magwells available.
The easiest way to do what the op wants is with a Glock small frame lower, several companies make them with and without last round bolt hold open, not sure if that matters to op but there are several to chose from. I have an NFA Glock small frame lower I use with my DI 9mm, 40 S&W and 45 GAP uppers and all of them run great with factory Glock 9mm/40 mags, ETS 9mm mags (I haven't tried their 40 mags yet) and Magpul 9mm mags that I have tried so far and I am very happy with the NFA Glock lower and if I were going to do a 357 Sig I would use it on that lower as well.

I'll certainly testify that Rudy is the only game in town for PCCs - especially in more exotic calibers than 9mm.  The man supports and backs his products to the hilt.  

Thanks all!  Correct, while Sig mags would be icing, it's not mandatory, as I was just hoping not to have to buy a bunch of GLOCK mags, when I already have the Sig mags.  I guess the only benefit to GLOCK mags, is they are cheaper than Sig mags!  I've already bought a plenty of Metalform's for the 9mm, so it can be done.

Ok, so I found this thread, and it looks like 22 round 40 mags work, which is fine, as I prefer the 20's on my 9mm.  Right now it appears that Macon is working on their blow back offering and Ron has done DI.  

Hmmm, looks like there are some choices.  As an FYI, I reached out to QC10, and at one point they were working on a lower to accept MP5 magazines, and had talked about a lower that uses SIG mags, but I think it's on the back burner right now.  They discontinued .357SIG barrels due to abuse on the gun and are looking to address the situation.  Although they don't mention it specifically, CMMG has their own hybrid system they use in the 45 and mentioned they are looking at other caliber options in the future, so hopefully we can have some more options.

I'll update more as I find out along this journey!

Thanks, and if you have or find anything, please share!

My .357 SIG AR upper that Rudy at Macon Armory built for me, ( the lower is a complete Glock one from Lone Wolf, )





I use twenty-two round .40 cal Glock mags with +5 Aredondo extensions, along with all of my thirteen and fifteen round .357 SIG mags.