Having both the HK416D and the M&P15-22, I much prefer the S&W product as it’s more like the original in function.* The HK has the look of the real thing, but beyond that, it’s just a cosmetic shell.* Some comments:
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The S&W breaks down like a normal AR in that you have front and rear pushpins that separate the lowers.* In addition, the whole bolt and recoil spring assembly pull out the rear similar to the normal AR bolt and carrier, allowing normal access to the chamber end of the barrel and the full bolt for cleaning.* You can also remove the charging handle, as on a real AR.* On the HK, you push out the rear pin, which is very tight to break open the upper and lower.* However, the bolt and spring are in a metal shell that is in the cosmetic upper receiver and are not meant to be removed.* The only access to the bolt face or chamber is through the ejection port in the gap from the retracted bolt.* For cleaning the barrel on the HK, you either have to come in from the muzzle, or use a bore snake or flexible cleaning rod with a very short brush if you want to clean from the chamber end.
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The S&W has a fully functioning bolt hold open/release on the side that works just like a normal AR.* Press the bottom to lock the bolt back, press the top to release the bolt.* On the HK, the BHO is a non-functional lump molded on the side of the receiver.* The only way to lock back the bolt is with an empty mag, and the only way to release it is to pull back on the charging handle and let go.
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The muzzle threading on the S&W is AR standard, so you can change out the flash hider for other things with the same thread.* On the HK, the threading is unique.* The HK barrel is actually pencil thin; what you see on the outside is a shroud, and the FH actually is used to align the barrel inside the shroud.* You need an adapter if you want to attach standard AR muzzle accessories to the HK. Also, the S&W has a normal diameter barrel instead of the pencil barrel on the HK, although with the FH locking the barrel to the shoud, the accuracy with the HK is pretty good.
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The S&W dummy buffer tube and receiver are close enough in size to normal that AR stocks and grips can be used.* My 15-22 came from the factory with a Magpul MOE stock and grip (the stock fits fine, but there is a gap between the upper rear of the grip and the underside of the lower receiver).* I’ve also heard that the trigger parts are close enough in the S&W that some AR parts can be used to improve the trigger.* The HK trigger is quite different from a normal AR trigger.* Don’t know if the grip and stock on the HK are standard size.
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The mags for the S&W are cheaper in my experience than the ones for the HK.* The S&W comes with a 25 round mag, while the HK comes with a 20 rd mag, although I think the HK/Umarex mags look more like metal AR mags than the S&W version.* Also, I think the S&W is cheaper.* I paid less for my 15-22 with Magpul MBUS, MOE stock, grip, and ladder rail covers included, than I did for my 416.
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The S&W is lighter, although it’s worth mentioning that it has a polymer upper, lower, and quad rail.* I'll also mention that the S&W lacks an ejection port cover, whereas the HK does have a normal AR type cover. The HK also has a more normal "heft", which could be good or bad depending on what you want.
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I own the HK416D, the M&P15-22, and a SIG522 SWAT, and if I had to get rid of one of them, I have to say the 416 would probably be the first to go.* I think the look is the best of the bunch, but the functions like the inability to remove the bolt and non-functional BHO are big turnoffs for me.
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My two cents.
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