If the 5 round mag is simply a blocked 10 rounder, you would be essentially loading it half full and that would not put much of any strain on a decent spring. That is, of course, assuming yours has a decent spring.
I can load 10 in my standard mag, press the top round down a bit and close the bolt on an empty chamber. To use, I simply pull and release the bolt handle and fire.
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"Robo is right" Fuzzball