I have a stock MR556 I submitted SBR form 1 on and Nefarious Arms 10.4 socom profile barrel I purchased 2 years ago. What gas block combo will be best suppressed and unpressed? I know system will alway be overgassed some. Suppressor will be surefire socom RC2. What about using stock gas block?

In my experience, one can use the stock gas block with full power ammo suppressed with a M4 action spring and it may/may not work unsuppressed as it will border on being too undergassed (due to the short barrel and large gas block vent). Mine worked fine both ways and I preferred it suppressed anyway.

If kept HK buffer and spring would non vented be preferred?

No, even HK fixed their mistake and began venting the 10.4's soon after release. So, the vented 10.4s have been around for a couple decades after the unvented 10.4s were out for a couple years. The difference is a smaller vent as compared to the MR556. All the answers you seek are on HKpro.

Yes, some people will tell you unvented 10.4s function fine, and I'm sure they do, but there's literally no reason to have them run so harshly.

The vent on the stock MR556 gas block is too big.  You’re going to need a 10” vented gas block as I believe Marvin drills the ports on his barrels to stock HK spec.

Get one before it’s gone!

HK German 10.4 vented gas block

Just grabbed one earlier today from Atlantic. Thanks. I’m going to be using my surefire SOCOMRC2 mini and at some point I’m going to be like an RC 3 so hopefully it would cycle with both.

This really depends.

NA originall manufactured their 10.4 barrels with both gas port sizes, depending on customer request. Depending on when you purchased it, it bay be one or the other.

Original 10.4 barrels meant for non-vented GBs were ported at 1.4mm and the barrels made for vented GBs were ported at 1.6mm.

To truly understand which gas block you need, requires a proper measurement of your NA barrels gas port.

If you have a 1.4mm gas port, your options are:

1: to get a non-vented GB (which i would heavily recommend  you not run suppressed due to excessive bolt speeds, damage to your dissconnector, excessive carrier tilt, and damage to the rear of the ejection port caused by brass impact.

2: get a vented GB and experiment with different strength springs and buffer weights until you find the combinantion that will reliable lock the bolt back on the last round.

3:  have the barrel gas port opened up to 1.6mm and get a vented 10.4 gas block.  

If you keep the 1.4mm port, choose to use a vented GB, and experiment with different spring and buffer strengths/weights, ensure the last round bolt hold openpeoperly works  when dirty and dry or you'll be dancing on the line of full function reliability.

In my early days of learning gas port/gas block combos, i would find a combination that seemed to work when the gun was clean that would then later fail to lock back after the gun got dirty and dry.

If you have a 1.6mm port, you really should run a vented GB and stock spring and buffer combination as that's how the gun was set up from the factory.

If you choose the experimentation route, i'd recommend using the different strength springs from sprinco, and getting one each of the different buffer weights (carbine through H3) so you're nit constantly driving out the buffer roll pin to change between steel and tungsten weight combos.

As of this writing i have a 1.4mm ported barrel that i have a vented GB on and the full array of springs and buffers to test, i just haven't gotten to the range yet.

10.4 socom profile I bought in June of 2022. Any idea which gas port that would be?

Most likely a 1.6mm, but it's best to measure with a pin gauge set.

Or, you can simpy install your new vented gas block and try it. If it has weak gas, either try the spring/buffer musical chair party, or open the gas port to 1.6m.