I then attached suppressor an observed eject to about the 2:30 to 1:30 (gun always ejects a little more forward after it warms up which is when it ejects to about the 1:30 to 2:00 position)
Most important observation regarding ejection is that it was consistent with a healthy throw and ejected in a predictable manner. If brass seems to simply be "falling" out of the side of the gun with an unpredictable throw, this generally means the ejector spring is shot.
A quick check with the pin gauges shows the standard 416 vented gas block (with sling loops) for a 14.5" barrel to be 1.5mm.
So, a 10.5" barrel with a .073 (1.93mm port) and a 1.5mm (.0591") vent will work fine for unsuppressed and suppressed.
Keep in mind that the HK system has two variables that effect function in the gas block and they may or may not work in conjunction with eachother depending.
If your gun isn't getting enough gas due to small gas port and too short of dwell time, the vent won't mean that much. This is why HK initially had no vent for the 10.4 barrels, thinking that instead of opening the port, they'd just close the vent and call it good. Until people ran them suppressed and the guns started beating themselves to death.
Experience taught us that the gas port must be opened up to adjust for the lack of dwell time and there must be some form of vent to allow bleed off of pressure from the system once the system actually gets enough gas to run. To accomplish this, HK has a fixed (non agr) block with a smaller gas vent and they've opened up the barrel port (HK now uses a 1.6mm = .063" gas port) to ensure the system has the gas it needs. Screw too much more with one of those without adjusting the other and you lose the "self adjusting" aspect of the gas system and its ability to run both suppressed and unsuppressed.
With your smaller vent on your gas block, you're now allowing some venting but now don't have enough pressure (small port/short dwell time) to run the system before the vent opens and releases the pressure. Your .03" vent is .76mm which makes just slightly over half the size of your gas port which is running approx. .055" (1.4mm)
That should tell you something about the efficiency and delicate balance of the system when the port is kept small. That being said, I chose to go with a 416 full vent block and modded the gas port until I got the results I wanted. Same results as HK, different ratios. I prefer a more robust gas system that allows a wider operating range so I went with the larger gas port to match the larger gas vent.
In the long run this means I'll experience slightly more gas port erosion, but in my years of working .mil guns, I never ONCE coded out an M4/M16 barrel for gas port erosion. Only for physical damage (bullet/frag impact, getting run over by vehicles, etc.), pitting/damage to the chamber or bore, bent barrels, or failed gauge (barrel straightness, bore/muzzle erosion, or headspace).
The biggest drawback to what I did is that the gas venting out of the front of the gas block has blasted a shiny spot on the rear of my suppressor. Not exactly a show stopper and I really don't care so long as the gun works.
I have one question: Did you have any issue with chatter when boring the gas port? I've talk to someone about doing this, he did not want to do it due to his belief there would be too much chatter; he didn't want to take the chance of screwing up my 416 barrel.
Thanks Again for the info.
"Just because you're paranoid doesn’t mean they're not after you!"
On the other hand, I experienced serious bit chatter when I first started turning down the barrel profile. Although I don't know the exact material type, I suspect that there's a fairly high nickel content in HK barrels and they seem to work harden. So in my experience, lathe work needs to be done relatively quickly and steadily because it seems to harden as you're machining and cause bit chatter if you go too slow. Carbide did work better and made cleaner cuts.
I really have not shot mine enough to show a lot of wear but there is wear on the front edge of the deflector and 0 wear on the ejection port. This is a cut down to 10.4" MR556 barrel with a non vented 416 gas block which is all there was at the time.His gun is beating itself to death.
What is your opinion on whether or not I need a vented gas block. I do not have a suppressor and don't want one.
What is the gas port size on a MR556 barrel. I showed my upper to a class 3 dealer and he said it was wearing in nicely. Thanks.
Last edited by Tuvia; 09-25-2017 at 07:00 PM.
If you do change your ejector spring to a milspec version and see your ejection heading to the 1:00 to 1:30, you might want to think about first changing your action spring and if that doesn't do it, perhaps consider the 10.5 vented block, but also realize that for that to properly work your port will need to be opened up to 1.6mm.
Also understand that ejection pattern as a diagnosis tool is limited and works best when the rifle is working close to milspec performance metrics (bolt speed, USGI parts, etc.) If you continue to use the HK ejector spring, understand that ejection pattern diagnosis no longer really applies because it throws brass slower and therefore makes your gun seem overgassed even if it isn't.
Your gas port (unmodified) is .055 (1.4mm) Don't worry about port erosion.