Do they exist? Not the cheap pieces of junk made by Promag, but something that's quality and reliable?

It's child's play to find 33 round magazines for Glock pistols. It's even easy to find good quality 30 round magazines for Beretta pistols. But so far I haven't seen anything for SIGs. Do they simply not exist?

Depends on the Sig your looking for. P226? P320? P220?

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Might also depend on the year of the gun itself.

Glock and Beretta both made machine pistol versions of their standard handguns (M92FS/M93r, Glock 19/Glock 18c) which is why factory extended magazines are available for them. I don't think SiG ever made a full-auto pistol.

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P226. Because you can't beat a classic.

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Sig itself makes a 20 round magazine for the 226 for about $50 per. The stupid Promags are 32 at $24 per(But I wouldn't trust 'em, I haven't had good luck with Promags.). Found a 30 round for the 226/228/229 at buymymags.com at $25 per.

Edit: The brand of mags at buymymags is unnamed.

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When we were still shooting regularly, we had a lot of luck with the 18 round Mec-Gar magazines. I can only assume their 20 round magazines would be the same case.

I've also heard that other than the location of the retaining hole, the dimensions of the Beretta and SIG magazines are identical. So maybe it'd be possible to get a 30 round Beretta magazine and simply cut a new notch hole if I'm careful.

Good to know about the buymymags thing, though.

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I would be extremely careful about modifying magazines of one firearm to work in another. You might also find that the geometry of the feed lips are different, or that the feed ramp on one is lower or higher than another, and you wasted X amount of money on mags that are now useless.