Besides not being full auto what are the difference between the HK 416 and the MR556?

What does everyone think of the HK quad rails, Remington defense, an the Giesllie rail?

I am looking to buy one in the future and want to know what to look for.
                                                                                                                                       Thank you for your time,
                                                                                                                                          Brian

There are different generations of HK416, so it gets complicated depending on what's being compared to the MR556 (of which there are 2).

Most of the differences are pretty obvious when comparing product photos on HK's website.
http://hk-usa.com/hk-models/mr556a1/
http://www.heckler-koch.com/en/products/military/assault-rifles/hk416-a5/hk416-a5-11/overview.html

The current MR556A1 ships with a keymod-ish (HKey MRS) fore end. The older 556 used a quad rail identical to what had shipped on many 416 models (up to the A3). The current 416 A5 has a FDE quad rail with an integrated folding front sight.

The barrels and gas blocks are different. MR556 barrels are profiled differently (heavier) and not chrome lined. MR556 gas blocks are not vented and none have sling loops. HK416 A3 10" gas blocks are vented and have sling loops (all until the A5 had the loops). The HK416 A5 has an adjustable gas block. The HK416N (for Norway) was the first to feature an adjustable gas block.

You can see some differences in the buttstocks and castle nuts.The HK416 A5 has ambi lower receiver controls, while the MR556A1 does not (neither did HK416 up to A3).

You'll find original HK416 uppers (A3 and earlier) for sale if you look. More common are MR556 uppers modified to resemble them. Often people will swap a genuine 416 gas block onto their MR556 barrel. Sometimes they use the 556 gas block and drill a vent hole when shortening to 10". This is common when combined with a Geissele rail, because the sling loops are not compatible (ground off when mounting on real 416). If the barrel isn't chrome lined, then it came from a 556. The uppers are laser engraved with the model, but could be re-marked.

The benefit of the Geissele and Remington rails are they are lighter, longer, and more modular than the original HK quad rail. Word got around years ago that Delta/CAG was using the Geissele rail on their 416 10" and DEVGRU/ST6 was using the newer Remington Defense rail on theirs, so this makes them very popular with milspec cloners.

If the MR556 barrels aren't chrome lined, then what is done to protect the rifling from rust? Some type of nitriding process?

I bought a MR566 with quad rail about two months ago, and sent it off to Jayson at Investment Grade Firearms to have the following done:

1) shorten and re-profile barrel to 14.5"
2) pin AAC 51T
3) nitride both barrel and gas block.

Why did I buy a MR556? Well, I wanted something different, had the cash, and I was on a HK kick, so I figured why not? I have used HK416 10.4" rifles in the past, and own a LWRC M6A2 10.5" SBR. What I don't like is that HK did not chrome line or nitride barrel, and the upper in it's original form is very heavy. For the price of the upper you figured they would have added irons and chrome lined the barrel. Oh well, a fool and his money.

I still a waiting for Jayson to finish up the upper, and hopefully will get it soon

MR556's have a tab on the barrel extension and a corresponding slot on the carrier to prevent use of a 416 FA carrier.  Easy enough to remove on the extension.

Barrel, as noted, is thicker with no chrome lining. It is the same material though, as the 416 barrel, and is CHF.  It will need to have a good, solid copper removal clean from anywhere between 1k to 6k rounds depending, or copper buildup will start causing keyholeing. Not sure if this is caused by the tapered bore or the material it's made from.

No nitriding, although the barrel steel is remarkably rust resistant without.

The MR556 gas blocks are the exact same gas blocks as the 416 with the exception that the sling loops have been removed after forging.  The poster who stated above that the MR556 doesn't have a vented gas block is incorrect.

The difference between vented and non vented gas blocks is between the 14.5 416's and the 10.5 416's, and even then that's only with the older model 10.5 416's that don't have the vent whereas the newer ones do have a vent now.

Edited to add:  People do not drill vents in gas blocks when shortening MR556 barrels, they actually have the pre-existing vent welded up or have it welded and re-drilled at a smaller size to allow some venting but not as much as the original vent.  Some also have the vent threaded and plug screws with different size vent holes drilled through, installed to control the vent to run suppressed vs. unsuppressed and "tune" the gas system.

To further cluster up the gas block stuff, the mr556's most lacking feature is the bayonet lug! sheesh.

In other words, differences only wanna-be Operators would obsess over.

LOL.