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What is the ruler test?
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Compressing the spring, by pushing down on the follower with a ruler. Lets you feel how strong the spring is and how smooth the follower travels.
Fwiw, I used to be as big a D&H fan as there was. But got some bad ones in 2012 or 2013; some that failed the ruler test (sticky follower travel) and some that wouldn't seat, much like the early E-Landers. Both of those typically speak of dimensional problems. Probably 10-12 out of 70-80 had problems, but it made me give up on them and sold them to a dealer (yes, with full disclosure of their issues in my guns).
Only all-aluminum magazines I buy or own now are Okay/NHMTG. Been using them for 34-35 years, and have literally never had a failure with one other than from blatant physical damage. Still use some pmag & lancer, but for gi-style magazines, I'm monogamous to that one brand now.