Guitar cases are good for standard length rifles, though it is best to gut them and add foam to fit, and also add cut outs for mags. I've seen guys get creative and fit two rifles in a guitar case. This would also work fine for SBR's, of course. Acoustic guitar cases work best, IMO.
Edit: someone is now making these cases to sell. Here's a pretty nice example.
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The guitar case seems good but I've always had the thought that a theif is far more likely to break a window for a guitar case than a tennis bag.