Update on functionality (12/01/2013):
My findings so far seem to indicate that so long as you’re getting a WE M&P that has been proven to work the way it should, you’ll have a pretty damn sweet gun for the money. However…
(1) WE M&P mags seem to tend to leak and/or overfill. Theoretically, overfilling should be impossible, but I’ve gone through two mags that have leaked as a result of being filled past the normal capacity of gas. I mean past the normal capacity in the sense that both mags continued to take gas from my propane tank w/ propane adaptor well beyond the normal time it would take to fill a magazine of this size, followed by the valves failing to open (puff of gas only) when inserted in my M&P and I attempted to fire. Solution – actually need to count the seconds that I’m filling my mags with gas now. I exchanged one magazine as under a QC issue, but one of the brand-new spare magazines I purchased the same day does this. I will monitor this issue and keep you guys posted. Update: QC issue was indeed on that one mag thus far.
(2) WE M&P magazines have a very thin O-ring sealing the bottom plate in the magazine. Concerning issue, as this design will be prone to leaking. I will be wrapping this seal in all of my magazines with Teflon plumber’s tape ASAP. Here’s to hoping that my mags don’t leak there. If they do, I’m not sure how confident I will be in tightening up that screw hidden by the magazine baseplate. WE metal has already stripped on me numerous times before when snugging up various bolts in other guns; if the WE metal strips under gas pressure in this magazine design, there will be a very-fast-travelling chunk of metal flying out of my magazine. Definitely not a good day if that bit of magazine frag hits people or things. Update: WE M&P mags have been holding their gas charges without any leaks with shooting, refilling, and storing magazines gassed up as since January.
(3) WE M&P tends to fire a 2-burst on a single trigger pull. Some of the guns I tested while QC’ing the WE M&P’s at work tended to fire a 2-burst once or twice within the 25-or-less shots fired in a magazine’s worth of gas, in the cold temperature of the unheated warehouse. Note that I haven’t been able to replicate this issue exactly, as it seems to be an occurrence on only some of the M&P’s. For my personal M&P, I just exchanged lowers as a QC issue. After spraying some silicone oil into the completely-dry hammer mech, the issue appears to have gone away on my personal WE M&P. The Head Tech at work will be looking further into this issue. I will attempt to do the same (but on my own time) – hopefully it is just a lubrication issue rather than premature wear or shitty tolerances. The issue has been reported to WE by my boss, and I’m hoping that WE will fix this in later production models of the M&P. Update 17/01/2013: issue is caused by partial trigger pull. Firm press all the way to the rear of trigger seems to be much more consistent in terms of firing semi-auto as it should. Update: fix posted here fixes this issue.
(4) WE M&P disconnector tends not to work 100%. On almost all of the WE M&P I’ve personally tested, if you pull the trigger on a slide that is locked back (empty), you can hear a slight click. Upon insertion of a gassed-up magazine, when the slide is returned to battery, the gun will fire. Upon no insertion of a magazine and the slide is returned to battery, there increased force towards the returning of the slide forward and there is another click when the gun is in battery. Seems to me like the disconnector is not working exactly as it should be, and that the sear is slightly slipping when the trigger is pulled with the slide back. Update: fix posted here also seems to fix this issue.
(5) Update 17/01/2013: hop up adjustment wheel is too tight to turn to adjust hop up. Problem is that the hop up adjustment arm is the wrong shape and size than it needs to be on the end that fits into the dial. When everything is snugged up in the hop up unit, the arm more or less gets locked into place and won’t budge (even with significant force). The DIY remedy is explained here: http://www.youtube.com/watch?v=FeMrGGyOzGs. Thanks go to “Driftinsti” for finding that and sharing it with me.
From my testing, the key here is to carefully check and test fire your gun before you buy it. Do note that I’m unsure if these issues are the same with other WE M&P’s sold at other stores – but you have been warned.
Part 2 – Update
Part 5 – Work-In-Progress Hop Up Fix (not the most current info I have)
Part 7 – WE M&P Hop Up Fix Part 2 (this is the most up-to-date fix I know of)