A few questions.
1. how long have you owned the gun?
2. have you ever been able to load 357 mag since you have had it?
3. are you sure it is a gp 100?
4. will any of the cylinders accept a 357 mag cartridge?
5. have you looked in the cylinders for any type of blockage?
Sorry for all the questions. a lot of people on here don't know much about guns, and these are the things that would help diagnose the problem.
Ok, look in the cylinder. If you have been firing 38 special in the gun, then there may be a dirt-lead build up at the point where the 38 brass ends. this buildup can prevent a 357 mag from going all the way into the chamber. a good cleaning will solve this problem.
make sure the gun is a 357 mag. some rugers are chambered for 38 special and not 357 mag. while they look a little like a gp 100, they won't accept the longer 357 mag.
the caliber should be stamped into the barrel.
No offense, but you would not be the first person on here to not know anything about a gun they recently purchased.
