I've answered this question at: Sig 45 slide won't release...
It is unnecessary to duplicate a post.
I just purchased a Sig Sauer 1911 C3. When I picked it up at my FFL dealer, I was checking the gun to make sure everything was OK...I pulled the slide back and locked it..When I tried to release the slide, it wouldn't move...I handed the gun to the dealer and he had no luck either...When he dropped the magazine, it closed just fine...Put the mag back in and it wouldn't close..We then dropped the mag, loaded a round, inserted the mag and it released OK...Just wondering if this might be some type of safety feature??...I was wondering if anyone else has had an experience like this...I'm going to contact Sig and see what they have to say.
If your answer was, "Sig magazine springs and followers are just that stiff when they are new." you were probably right.
Both of mine were like that when new and they are STILL significantly tighter than all my other mags.
You'll never have to worry about your slide not locking back that's for sure.
I posted the question in two different forums thinking that someone reading the 1911 forum might not be reading in the Sig forum or, vice versa....Again, thanks for the help.