I strongly recommend Mike Pannone's "HK416 Handbook" published by BHI (Black Heart Intl). Bing Mike's name if you want to know more about his extensive background--he's the real deal, and his book contains a ton of useful info in addition to maintenance. While the book is available as an Amazon ebook, don't buy that version since you can't print it out and take it to the range/field with you. BHIdotcom has it available in a spiral-bound edition that is robust and easily transported.
Forgot to mention that Mike's HK416 Handbook describes a "HK416 Reliability Parts Kit" that's available from BHIdotcom (the book's publisher) for $83. The handbook states the Springco buffer spring is not appropriate for the HK416. HOWEVER, Mike and I talked on the phone and he told me that at the time the handbook was written, they hadn't had time to test the Springco unit in the HK416.
They have now tested it and the Springco spring works fine. Hopefully if there's a future edition of the handbook this will be corrected.
Please pay attention to this next part, because it's pretty confusing. Although the HK416 Handbook calls this kit the "HK416 Reliability Parts Kit" and also the "SOPMOD Bolt Upgrade Kit," it's called the "BHI Enhanced M4 SOPMOD Bolt Upgrade Kit" on their website. To cap things off, the currently shipping package is labeled the "Buffer/Extractor Upgrade Kit, M4 Carbine/CAR-15." Follow all that? (LOL)
Not to worry; they're all exactly the same kit. Just make sure the kit you receive matches the photo in the handbook and has the tiny rubber Crane "O" ring included.
I will start this post with the following clarification/disclaimer:
The point of maintenance is not to ensure the cleanliness of a weapon, but to facilitate proper parts inspection and lubrication, to ensure an operational firearm at all times. Soot/carbon build up is rarely the cause for malfunctions, but other foreign debris can be; dirt, mud, small rocks etc.
In short, I usually wipe down all parts with patches covered in some form of lube (I use CLP) to remove most of the soot. I then inspect parts visually for damage/breakage, as well as performing proper function checks on all relevant parts. I then reapply lube to the cleaned parts. All areas of the weapon gets lube, except the inside of the barrel:
Inside upper receiver
Ejection port cover spring and pin
Forward assist (never used, but it's a moving part)
Selector switch and axle
Buffer and spring
Firing pin and spring
For the piston itself, there will be some carbon build up, and I use a bronze or copper brush to remove visible build up. No point in trying to remove the discoloration, as that is natural considering the temperature of the gases in contact with the piston. It will blacken with use.
If I have it available, I will use compressed air to blow off debris from large parts before breaking it down for further cleaning. I would also recommend starting with the optics, in order to avoid getting lubricants etc on the glass.
For cleaning optics, the technique is as follows:
Use compressed air to remove debris (canned air only, compressors can have hoses with oil remnants).
Use a lense brush to remove debris.
Use proper lense paper to polish the glass, start in the middle going out to the edges in a circular motion. Not too hard, in case there is still debris on the glass that can cause scratches.
If there are fat marks (ie from fingers touching the glass), you can help remove it by using either your breath or isopropanol or similar fluids, and wiping it off with lense paper.
Do not use you finger to remove **** from the lenses, unless you are in a critical situation. It only hurts the coating on the glass.
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