I no longer have a Universal but checked my USGI for magazine movement. There is a front to rear movement of approximate 3/8" which is the caused by the magazine compressing the magazine release spring.
Does your magazine have any up/down movement? Does it lock in place?
Put your magazine in place and watch the magazine release spring as you move the magazine...you will see what I'm talking about. There is a small hole just behind the magazine release.
Universal carbines were made to accept USGI and similar magazines.
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Papa G, check the latching 'bumps' on the back of the magazine. The original ten-round mag for my IMI M1 Carbine (commercial with GI parts) has the bumps so worn down, the mag falls out when I shoot it. No problem with the three new aftermarket 30-rounders I bought later.