DD Mk18 buffer questions

AR/M4 AR Technical Discussion
1

I’m curious about building/assembling a Mk18 pistol. I wanna build a pistol lower with the SBA3 brace and mate it to a 10.3” DD stripped Mk18 upper. Older threads include a lot of references to overgassed DD 10.3” barrels with port sizes of 0.080” or more, but Brownell’s has measured the gas port size on these uppers with pin gauges at 0.070”, and they claim this measurement is confirmed by DD.

My questions concern the buffer system. Since the SBA3 I’ll be getting comes with a carbine RE, would I be good to go with a Springco white spring and an H2 buffer? Would a blue spring be more appropriate? The gun will only be fired unsuppressed, as I live in a state where NFA items aren’t allowed. Most shooting will be done with moderate .223 handloads, but I want reliability with 5.56 ammo as well.

My searches only seemed to turn up info on the older, overgassed uppers. From what I saw, a lot of people were running syrac adjustable gas blocks and A5s. I’m still pretty new at this, but I would imagine a different setup is needed for .070” and .080” gas ports. The A5 route would be additional $$$, and I don’t want to change gas blocks unless I absolutely have to.

2

DD Mk18 barrels manufactured after a certain date have proper .070 gas ports.

I want to say it was summer of last year, but would have to check to be sure.

If your upper is new production, it would be properly gassed.

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3

You seem to be on track. I’m such a fan of the A5 system, I say buy that. You don’t need an adjustable gas block if your not running suppressed. Not running a can minimizes all your problems.

4

Forget an adjustable gas block. First measure your gas port size once you get the barrel.

I got a replacement barrel on my DD and it came in at .073, put a sprinco Blue, and a H2, see how that works, still over gassed get a LMT Enhanced Full auto carrier (L7Q3A) and that should make you golden.

5

If I went with an A5 tube I’d use a green springco spring, right? Which A5 buffer weight would be best?

6

You could dial it down with one of Clint’s gas tubes or ports.

7

Those look sweet, but I’m going to avoid messing with the gas block and/or tube unless I have problems that I can’t solve with the buffer.

8

A5H2 & Sprinco green spring. When the gas flow is right, this combination is smooooth.

9

I’m pretty sure the answer is yes* but will a pistol lower with an A5/intermediate RE and the SBA3 still be a pistol? I ask because I don’t know if the SBA3’s ATF approval is contingent upon the use of a carbine RE, and I don’t want to “modify” the SBA3 system and accidentally create an SBR.

*I put an asterisk because I know that the ATF’s position is subject to change, but the SBA3 + carbine tube is kosher for the time being