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Discussion Starter · #1 ·
Yesterday I was taking some measurements out of my MR and I realized that all changes, that HK made, could fit into regular M4 upper. And it adds up, because originaly HKM4 was supposed to be overhaul service to existing rifles. So why HK decided to go with higher, non-standard (per AR15 world) rail?

One answer comes to my mind:

Railed AR15 uppers are less rigid than original carry handle design. Did HK want to reinforce upper by additional material on top of it? Was there other reason?



BTW Does HK barrel nut use some threads that M4 nut?
 

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I would think that the added strength of the full top rail and the anti-rotation tab were part of it.

With most AR piston systems, they have to have a specially made rail that is even thinner at the section that is actually over the gas block. Because of the size of a piston gas block vs. a typical DI gas block, this can create a rather weak rail section.

On other AR piston systems, they leave the gas block entirely forward of the handguards, and while not as thinned out as the handguards that sleeve over the gas block, they do have to be modified and made thinner as well. Also, any two piece, non-free floated handguard for the AR series rifle will require major modification to the forward part as well as the handguard cap just behind the gas block. As noted in the early HK M4 test guns, they modded the **** out of the handguards and handguard cap to get everything to fit.

To be able to have a full length, freefloated rail with no weak spots that could also accomodate the extra space that a piston system takes up that is also able to be easily removed for maintinence of the piston system (seeing as how the piston cannot be removed from the front of the gas block like on many other AR piston conversions) it makes sense to modify the rail and upper to match if you have the ability to and are not simply making a "drop in" piston system.
 

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Yesterday I was taking some measurements out of my MR and I realized that all changes, that HK made, could fit into regular M4 upper. And it adds up, because originaly HKM4 was supposed to be overhaul service to existing rifles. So why HK decided to go with higher, non-standard (per AR15 world) rail?

One answer comes to my mind:

Railed AR15 uppers are less rigid than original carry handle design. Did HK want to reinforce upper by additional material on top of it? Was there other reason?



BTW Does HK barrel nut use some threads that M4 nut?
The reason was to make sufficient room for the piston rod and metal insert through which it travels in the upper.
G3Kurz
 

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Discussion Starter · #4 ·
The reason was to make sufficient room for the piston rod and metal insert through which it travels in the upper.
G3Kurz
That is what I always assumed. But there is whole 10mm of space over piston system and edge of steel rod bushing. That is a lot. Half of this height to it would do (if not even less), if this would be only reason. Also piston rod operates in exactly same place as M4 gas tube goes. So I do not buy "place for rod and bushing" as only reason anymore ;)

Handguard top rail could sit little lower as well (but I think no more than 3mm, must measure it), but maybe making handguard stronger is an answer. I hate to agree with Grumpy, but he made valid point here.
 

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I dont really see how you could have a continous rail on the HK416, if you wanted the rail on the upper reciever lower. There is not much room inside the handguard around the gasblock to achieve that:






So unless you want an offset, non-continous rail or a different rail design, it won't work.
 

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I wonder if it also has to do with them recycling the older G36 series proven piston. It's pretty big compared to other modern piston designs, but again, it's proven.

Most AR gas tubes are bent to come down from a lower axis. Although, the Noveske switch block uses a straight tube but needs a shorter rail or a long rail with top rail section cut out for switch access.
 
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