Can anyone confirm if the Springfield XD 4" 9mm will function with a .40/.357 10 round mag and that it will hold 15 rounds of 9mm with no mods?  Does it fit flush to the mag well?  Is it reliable?  

I have read in one place that this is correct and would be a much better way to go in the 4" model than the Beretta Hi-Caps that you can buy from Springfield.  I bought 5 of them and they don't fit flush into the mag well; at least 1/4" gap.

Is it legal to use a the wrong caliber post ban 10 round mag in a gun in order to allow for increased capacity (i.e. .40/.357 10 round mag to hold 15 9mm rounds)?

Quoted: Is it legal to use a the wrong caliber post ban 10 round mag in a gun in order to allow for increased capacity (i.e. .40/.357 10 round mag to hold 15 9mm rounds)?

Conflicting reports.  Some say that it is illegal and some say that it isn't.  Both have proof of this.  I would say ask local LE or contact the ATF before you try anything.  Its better to wait a little and find out, than lose all your firearms because of a stupid mistake.

I guess a letter from the head of the ATF's technical unit in charge of making such determination's is not good enough for some?  He was asked specifically if it would be ok to put 15 rounds in a 10 round post ban .40 mag and his answer was absolutely alright to do so.  Check in the Springfield section of this forum and you can find a link to a copy of that letter.

members.aol.com/hs2000fanatic/letter.pdf

go to www.xdtalk.com and check the FAQ.

Modifying mags is perfectly legal, the only restriction is that it must still work in the weapon it was originally intended for after you modify it.

Since modifying magazines and using them in other guns is legal, then using them (unmodified) in other guns is even more legal, i e less likely to get you into trouble.