10 or 17 rounds

[LAB55]

I'm not too familiar with semi-autos. Besides holding 7 more rounds what is the difference? Is the grip larger?

[NickSS]

I donot quite know what you are asking?  Revolver grips and semi auto grips come in all sizes.  Some are really small and some are really large.  Generally semi autos come with either single stack magazines or double stack ones.  Generally (though not always true) pistols with single stack mags have thinner grips than double stack ones.  Most pistols with single stack mags hold between 6 and 10 rounds of ammo.  Double stack ones hold between 10 and up.  Like for my Glock 17 I have standard mags that hold 10 and 17 rounds and I also have one mag that holds 32 rounds and they all fit in the same pistol.  With a revolver you are limited to the number of chambers in you cylinder (usually 5 or 6 rounds though some hold 7 to 9 rounds)

[spruce]

Are you referring to the fact that sometimes identical models are listed side by side as one holding 10 rounds and the other holding 17 rounds?

If so, it's simply to make them legal for sale in some states because they have restrictions on magazine capacity. 

In these instances you can use either magazine interchangeably in either gun (where legal).