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The GP100 is primarily a six shot .357 magnum/.38 special revolver. The Redhawk is primarily a six shot .44 magnum/.44 special revolver.
Every now and then, Ruger muddies the waters a bit just to keep things interesting. They recently introduced a 5 shot .44 special GP100, as well as an 8 shot .357 Redhawk.
You can also get a Redhawk in .45 Colt that also shoots .45 acp with moon clips.
The msrp prices Ruger lists for their revolvers are kinda scary but real retail prices for them are more reasonable. Also, keep in mind that there are lots of used Ruger revolvers out there that are in excellent condition, and that usually are a good bit cheaper than brand new ones.