The Sig 250 I shot was a Polymer frame. They might be in Alloy or
Steel now, the one I worked with was Polymer.

http://cheaperthandirt.com/blog/?p=19072

"When SIG introduced the P250 series of polymer framed, modular
pistols in 2009, the marketing hype surrounding those guns made it
easy for potential customers to forget that SIG had already been
making polymer guns in the same calibers for a decade. It seemed like
the SIG SP2022 was destined to be one of the best-kept secrets of
pistol world...."