A bit of an FYI for those running HK416/MR556 guns.
One of the biggest complaints about the HK system, or any piston AR for that matter is the lack of parts compatible with DI guns and of course, the expense of proprietary replacements.
Due to an online discussion I was involved in about what was the best gun for long term disasters, DI or piston, where one of the most common shortcomings of Piston systems that was mentioned was lack of spare parts, I decided to find out exactly what was and was not compatible.
As it were, a "full" HK rifle will have less parts compatibility unless some entire sub-groups are swapped out, such as LPKs, the entire lower, etc.
I decided to forgo the headache of not being able to use any mag avalible and just bought an MR556 upper and modded it to fit my needs (reprofiled the barrel, cut to 14.5, perm attached a bi-lock, had the whole thing QPQ nitrided, etc). I also went with a Noveske lower and a Vltor A5 stock system (LMT sopmod for the stock body).
This means that my entire lower is fully compatible with any standard DI gun.
But what about the BCG?
Considering that the carrier itself is considered a major component and is expected to last the life of the firearm, replacement and compatibility is not an issue. The bolt, ejector, extractor, firing pin, etc. are an issue and a possible liability. Especially considering that when spare HK bolts can be found, they are stupidly expensive. Like 5 times the cost of a DI bolt.
Thusly, I decided to do a full swap of a brand new Noveske bolt assembly into the HK carrier and check critical dimensions. Turns out that it fits right in en-mass, and firing pin protrusion is proper and it head-spaced as it should.
But, as someone on another site pointed out, fit doesn't necessarily translate into function. So I headed to a square range today to do two tests. The first one was to finally shoot this bitch for groups to see how the barrel work affected accuracy (took the profile from 1" under the handguards down to .7"), especially considering that piston guns have a bit of a reputation for less accuracy due to the extra mass moving over, and impacting on the barrel.
After a thouroughly frustrating ****ing day trying to get good groups (the scope wouldn't zero for shit at first. Now I know why they guy gave it away for free) I finally got a 5 shot group (with ****in' flyer of course) that I could live with and said "screw it, i'm done for the day".
I then did some happy blasting to with video to show that the DI bolt was working fine, and then re-attached the Aimpoint/po-boy and re-zeroed. That's when I nearly shit myself, but in a good way.
Point being is that for all 170 rounds, the DI bolt assembly worked fine.
I shot the entired session with the DI bolt in place and took two videos, one showing install shortly after setting up the target, etc., and another after the last accuracy group that I shot where I was only shooting to demonstrate that the gun being fired was in fact with the AR bolt, as I took the BCG apart after shooting to prove that it is in fact a DI bolt, cam pin, and firing pin(and the HK firing pin spring was retained).
I also took several before and after pictures of the bolt head itself to see how 170 rounds might affect it. My primary concern was for the bottom lugs of the bolt. Since the bolt fits into the HK carrier freely, even though it still has the gas rings, I was expecting that upon the bolt coming in contact with the top round in the magazine, it would be cammed downward as it stripped the top round from the magazine and possibly come into contact with the bottom lugs on the barrel extension, thereby being out of alignment and causing excessive wear and marring of the bolt lug edges.
Turns out I had nothing to worry about and the brand new Noveske bolt barely shows any signs of wear, much less mis-alignement strikes/wear.
The biggest difference I noticed was brass throw. The DI ejector spring is in fact weaker then the HK ejector spring, so brass was ejecting further forward as the bolt was able to travel rearward further before the brass actually struck the deflector. It still ran flawlessly though, and while deflected to a different location then normal, the ejection was still consistent.
Before pics:
After pics:
And here are the videos I shot. First one showing install, second one showing shooting and then removal for proof.
http://smg.photobucket.com/albums/v1...oltinstall.mp4
http://smg.photobucket.com/albums/v1...otandprove.mp4
This leaves the only incompatible parts being obviously the barrel/gas block, the piston assembly vs. the gas tube, the carrier, and the firing pin retainin pin which is proprietary in the MR556, but can be easily fabricated.
Being that the barrel, op-rod and piston are also considered major assemblies that should last the life of the gun (especially with the QPQ nitriding), my only possible concern would be the op-rod spring, and the piston gas rings.
I'll do another test shoot later to see how the gun works without rings in place just for shits and giggles.
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Originally Posted by Arctic1
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