took my brand new DE 44 (6" IMB barrel) to the range yesterday, with a couple of boxes of Magtech 44 Rem Mag 240gr FMJ Flat (product code 44C), wouldn't cycle.  it would shoot nicely, i thought the recoil was kind of mild, sort of like a .45 1911, but had repetitive failures to feed the next round.  it did successfully extract, all the empty cases went into a nice pile behind me slightly to the right.  however, the slide did not come back far enough to strip off the next round from the mag, to chamber.  half the time, the slide would go forward into battery, squeeze trigger, click, nothing.  empty chamber.  the other half of the time, the bolt would get stuck against the the next cartridge, i.e. it came back, but not far enough to catch the rear face of the cartridge.  oh well.  tried several different Magnum Research-brand mags, to no avail.  i did develop some proficiency at racking the slide, though, but it was kind of annoying.  i only got through 1 box of the Magtech in this semi-manual mode (not semi-auto).

i switched over to American Eagle 240gr JHP, wow, what a difference.  the pistol was now very snappy, and cycled perfectly, round-after-round, for a whole box.  that was enough for the day.  i'm a happy camper.

just one data point for other (prospective) DE owners.

maybe i need to wait for the recoil springs to wear in a little before i try to finish up the MagTech.

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ETA: one more data point: HPR 240GR TMJ (also flat-nosed) 44 Mag also failed to cycle my DE.  slide didn't come back far enough to cleanly strip the next round off the top of the mag, bolt would get stuck against the next cartridge.
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ETA2:  Blazer 240gr JHP 44 Mag also insufficient power to cycle.

I had a 357 DE years ago and at the time, the manufacturer stated only one type of ammo. should be used in the firearm. I tried some others but could never get the thing to function with any other brand. Not enough power to cycle.

I'm handloading ammo for my .357 in an effort to get it to cycle reliably.