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Discussion Starter #1
I realize there are several people who have chopped down their mr556's but have any of you had to have Gas port work to keep the gun 100% reliable? I've been told that when some some of the scar guys did that, they were having function issues.

It's going to be run very hard.
 

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Discussion Starter #3
Had you done it or read anyone has that did not have issues? I realize the scar is different but an 1.5" did make a difference with that platform.
 

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I have little first hand experience with the mechanics of the SCAR platform, so I can't help you out with that.

I can tell you that the position of the gas port is in the same position on all 416/MR 5.56 variants. And possibly with the exception of the 10.4" (if that) the gas ports should be the same size as well. Given that the 14.5" 416 functions reliably, why should an MR556 with essentially the same mechanical specifications not run? It would be more likely to have to modify the gas block if anything.

Keep in mind that the 416 system does not have the same issues with peak pressure and dwell time as its gas operated counterparts. Again, not knowing the SCAR platform intimately, I can not comment to the alleged issues you allude to.
 

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I test fired my MR556 after cutting the barrel to 14.5 and the ejection pattern shifted slightly to the rear(about 4 o'clock), but remained well within acceptable standards.
 

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I don't believe there are any differences in the gas ports between 416 barrel lengths.

The actual gas block is the part specific for barrel length...and I don't think the 14.5" gas-block is choked-off anymore than the 16" gas block.

I think there are a few members here that have had IGF cut their barrels to 14.5" without issue. You could ask them, or contact Jayson at IGF.
 

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Had you done it or read anyone has that did not have issues?
HK uses same gas block (with same size dump hole) for 14.5" and 16.5" barrels. This is hint.

Gas block for 10.4" is different - it does not have dump hole.
 

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Discussion Starter #8
HK uses same gas block (with same size dump hole) for 14.5" and 16.5" barrels. This is hint.

Gas block for 10.4" is different - it does not have dump hole.
This is exactly what I wanted to know. Thanks Montrala!


Does anyone know if reprofiling the barrel will hurt the accuracy? Everything that I've read here says no or not by much that would matter to me for this rifles purpose.
 

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Have not noticed any reduction in accuracy yet but have not had the time to properly test it out at 100+ yards(wait too long at range and dont get out of work on time during weekdays). Same accuracy out to 25yds as with the 16" barrel least using irons. Personally imo if you are cutting the barrel from 16" to 14.5" or less, long range accuracy shouldnt be on your mind.
 

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Discussion Starter #11
I'd like to keep the 1-ish moa since I know there are other AR variations in similar setups that do it.
 

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I'd like to keep the 1-ish moa since I know there are other AR variations in similar setups that do it.
Actually cutting it down can improve "short range" accuracy (less barrel harmonics), while will degrade "long range" accuracy (less speed makes bullet loose stability faster).
 

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Actually cutting it down can improve "short range" accuracy (less barrel harmonics), while will degrade "long range" accuracy (less speed makes bullet loose stability faster).
I've seen some sharpshooter rifles that can attest to that. Cutting a .30cal bolt gun down to 16" barrel increases accuracy out to roughly 500 yards. Once you start reaching out into the 800-1000yard range the 20" barrels come back into power. BlackOps Precision makes their rifles almost exclusively with 16" barrels.

EDIT: A lot of the U.S. military experienced lower qualification scores and difficulty qualifying when they transitioned from M16 to M4 length rifles. However, the cause and affect of this was actually the shorter sight radius as opposed to any degradation of accuracy from the barrel itself.

The SCAR does not vent the gas from the gas block/piston, but instead contains it, and then sends it back down the barrel after the bullet exits. HK uses a vented block (apparently with the exception of the 10.5" version), in which the gas pressure in the block releases dependant upon the pistons movement, more than the bullet. There can be such a thing as too short, and a minimum amount of time needed for the gas block to pressurize and operate the piston. However, by design, it shouldn't be as sensitive as the SCAR. For reliability, I can't see HK fine tuning it that far. If anything, the system gets an excess amount of gas, as a precautionary measure. Especially since it will simply vent out any excess, once the piston moves far enough to open the vent hole.

As a little background: I believe I remember reading that Stoner's original direct impingement design needed at least 4-4.5" of barrel after the gas port to pressurize down the gas tube and into the BCG. The amount of time required would still vary by design, but the piston directly in the gas block won't have that delay associated with pressurizing the entire length of the gas tube.
 

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Does anyone know if reprofiling the barrel will hurt the accuracy? Everything that I've read here says no or not by much that would matter to me for this rifles purpose.
I was wondering the same thing but decided to not reprofile my barrel on the 10.4" conversion from IGF.
 
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