(this is largely crossposted from a P&S Buyer Beware thread, but it's the most concise "the story so far" I've written on the subject)
Do not buy a Zenith MP5 sight unseen. My lust for an MP5 has turned into a money pit hell of my own making, as I've paid over $1,900 for the gun plus a few hundred dollars in optics/accessories, and out of the two months I've owned it, it's only been in my hands for a cumulative total of about a week and a half, and has been in a working condition for about 3 of those days. It's a comedy of errors, bad luck, and worse QC that eerily echoes a similar experience I had as a teenager desperately trying to get an airsoft version of the game gun working.
I bought a Zenith Z-5RS from Kelly Enterprises in late May after a year of waiting for Brownells to fulfill a backorder that was consistently being pushed out another month every time I called. Chuck was a very helpful (if chatty) seller who informed me that April 2019 imports of the Z-5RS were markedly subpar, often with heavy amounts of rust inside the receiver as they had been sitting on a dock for months (read: since 2018) before Zenith received final go-ahead on importation. In addition to finish/corrosion issues, there were numerous reports of poor magwell fit. He assured me that he had inspected all the 2019 production guns he received with the May shipment and that they exhibited none of the issues that were present in the April batch, so I ordered.
Since then, I've endured issue after issue with this gun, and when I would ask HKPro or other MP5 groups, I was often either mocked or flat-out ignored because I didn't own a Dakota Tactical or SP5K. The first and most obvious issue was that the recoil lug on the top of the receiver was welded on off-center. While this is still technically "in-spec" according to Zenith's head armorer, it's an annoyance that cannot be fixed. My B&T mount and Aimpoint PRO do sit a bit left of the front sight post as a result.
The real problems began shortly after. My first range trip had me ready to go - I've wanted an MP5 all my life, and I finally had one. 150 rounds later, I had encountered something in the neighborhood of 50-100(!!!) individual stoppages - I stopped counting in the 20s, but the gun jammed often every other shot, either with a failure to feed, failure to eject, or the casing would fall in front of/behind the hammer, requiring a field strip to correct. I discovered shortly after that the BCG not dropping free of the receiver under its own weight was not in-spec, and that such severe binding was causing the BCG to pinch towards the rear, slowing the carrier down enough that ejection was improbable at best. The gun went to Zenith to have the receiver re-formed and brought back into spec, as well as have the parkerizing touched up on the 3-lug and paint touch ups as needed.
Zenith fixed the issue with the receiver, retouched the 3-lug, replaced the extractor, touched up the paint, and ran a variety of different ammo types through it to confirm function before returning it to me. Upon receipt, I too tested the gun with about 200 rounds of various 124gr/147gr FMJ ammunition and found no issues, but did note that the touch up paint they had applied seemed uneven and blotchy on one side of the gun. I reached back out to Zenith and they offered to do a full blast/repark/repaint in HK Black Duracoat on their dime. Since I had an SPA/SOLGW Armorer's Course that weekend, I took them up on it and sent the gun back out. While it was in their possession, I also asked for them to inspect and adjust the "triple tree" if necessary based on reports that the cocking tube was making contact with the FSP on newer imports, preventing the barrel from free-floating per HK spec. Zenith confirmed they would do so and added it to the work order.
The gun came back late last night, and the finish looks fantastic from top to bottom. The cocking tube is perfectly centered (though I'm unsure if it was off before), and while the trigger group and brace now fit much tighter than before and require a punch and mallet to tap on/off, they do fit. I was ready to clock out early today and hit the range to confirm function, but when I pulled the bolt carrier group to lubricate it, I discovered yet another problem - the BCG tail is bent on the side cut for clearance of the receiver block. While I am unsure if this will cause functional issues, it still seems like something obvious enough to the naked eye that it should have been caught the first time the gun went in for repairs, especially given the issues present with the receiver being out of spec and binding the BCG on that trip. There are at least half a dozen other people with the same issue on their Zenith MP5s on one of the FB MP5 groups.
I've sent another email and call out to Zenith's warranty/armory department and I'm currently waiting to hear whether or not this is an issue worth remedying, and how/if it can be troubleshooted. However, regardless of whether or not this needs to go back a third time, regardless of whether or not Zenith pays for round trip shipping again, my patience and trust in the company has been completely exhausted. Zenith's CS team is polite and patient, but it seems like they do not check the guns for issues that any person with passing knowledge of the MP5 platform could easily spot (not to mention their entire warranty staff appears to leave the office at 2PM EST daily).
I have already warned them that if this gun has to go back a third time, there will absolutely not be a fourth - just a demand for a refund, which I will either use to buy a competitor's product (Dakota Tactical or Parabellum Combat) or simply dump into an 11.5" AR or night vision. As much as I love the MP5, the feeling does not appear mutual, and Zenith's inability to spot issues with their own product on multiple trips is not at all confidence-inspiring, especially at a $1,700-$1,900 price point.