Probably been discussed before, but I'm too lazy to look.

Anyway, I've got a Glock31 (.357, I'm sure everyone here knows that, but I still have to go to the glock site to cross reference the caliber) and I know I can swap the barrel for a .40.  What I don't know is whether I have to get .40 mags for it or if I can use the same ones I have.  I compared the followers to a friends and saw they were slightly different.

I was going to wait a while, but I made the unfortunate discovery that the pistol is not partial to Selliet & Bellot.  It was misfiring ALOT and frequently doing FTF's.  I am told this is due to a hard primer.  I just need a cheaper alternative for blasting away.

If it is just going to be a range gun then I would see how the .357 mags work.  If they work fine then just use them.  If they don't work then pick up a couple .40 mags.  I believe there is a slight difference in the angle of the follower.

I have shot a few thousand rds of S&B without any problems in both 9mm and 40S&W. You might want to check that the firing pin channel is clean, dry and oil free. I find often times crud or oil is a cause of some failure to fire.  Hopefully that will help narrow the problem to the hard primers.