I picked up an almost new looking full size USP 45 this weekend. Stainless, date code AB, with 2 10-round mags. Great gun but it does not hold open after the last round. It does this with both mags. Is this likely a mag-spring problem or something else?
I guarantee I gripped my 45 the same everytime and only the extended mags didn't hold open the slide. The gun is used but the ext mags don't have much markings on them as compared to the flate base mags with some markings but the flat mags hold open the slide everytime.
I just had a similar problem with my USP and it's jet funnel mags. After a good cleaning and some testing I realized it was a spring problem. So, I stretched the spring a little bit and put the Mag back together. After the stretch the Mag is now locking the slide back. We'll see how long it works for and in the meantime I'm looking for new springs.
I don't take offense at the limp-wrist suggestion, I am sure it happens. It also seems to be offered as the cause for most malfs -- ftf, fte, hold-open. However I shoot a lot and I don't think I suddenly went limp because I had a new (to me) gun. I have also tried limp-wrist shooting, right up to the point of being unsafe, and have never found that it consistently produced functioning problems.
I cought some vid. on Fireingline of someone trying to limp wrist and he could not produce any failers. Just for jollies I tried a few rounds with the limpest grip I could safely hold my USP .45 with and I could not get the gun to fail, including lock back. Frankly, I now believe the "limp wrist problem" is a falicy.
It is very easy; if you are right handed, to find your thumb just slightly resting on the ledge of the slide release. Afterall, that thing is HUGE. I had mine shaved down a few years ago and no issues since. Just a thought-good luck.
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