I bought the Colt / Umarex M4 OPS and was pretty disappointed with the zinc alloy innards. Sold it and purchased a CMMG Sierra .22lr upper for my S&W M&P 15T (it will fit any mil spec AR lower) and couldn't be happier.
MUCH better quality with an all steel bolt and a real, full profile steel barrel, not the puny little pencil thin Umarex barrel in an aluminum sleeve.
My CMMG upper has been both reliable and accurate.
I honestly can't see how Colt/Umarex, S&W and others are selling their AR15-22s when these far superior uppers are available.
CMMG also sell a complete AR15-22 rifle (the Quebec) for about the same money as the cheaply built Colt, and you can put a center fire upper on the CMMG Quebec.
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