New AR Build - Technical Questions (Buffer Weight/Port Size, et al)


Sep 6, 2014
Los Angeles, CA
Hey All,

I come here with a few questions that I'm hoping the resident Wiz-AR-ds (see what I did there?) can help me out with. I've gotten my feet wet building ARs, and now I can't stop, and I've picked up a lot of practical know-how I didn't have before along the way. That said, I don't know everything, and there's a few very technical elements that elude me. I'm writing down the rough blueprint of the build to get your thoughts, but basically my questions are to do with gas port size, buffer weight, and barrel length, and optimizing them for the build I'll outline below. Here's what I'm thinking:

  • Basic Mil-Spec Forged Upper
  • 11.5"-12.5" FN Barrel
  • YHM Phantom or AAC Blackout Flash Hider
  • MOE SL Handguard (if Upper comes with FSB/GB Combo)
  • 12" ALG EMR V2 (if Upper comes with low-pro GB and is 12-12.5"
  • MBUS Rear (+front if Upper comes with lower-pro GB)
  • AIM Surplus NiB Bolt Carrier Assembly
  • BCM Gunfighter Charging Handle Mark 4 (Medium)
Upper Recap: I'm pretty sure I'm going to get an 11.5 or 12" barrel, but more knowledge is always welcome. I know it's worth jumping from 10.5" to 11.5" for the increased dwell time and the greater reliability that that entails, but I'm not sold on the jump from 11.5 to 12.5" so I may go with whatever's cheapest on PSA between the 11.5" or 12" when I decide to build it. If somebody can weigh in on whether or not that extra 1" to get to 12.5" is going to make a noticeable difference, that'd be awesome.

I'm also curious as to what size gas port I should drill. I would like to eventually put a silencer on the gun, but I'd like to be able to run both suppressed and unsuppressed. Is it truly as simple as getting an adjustable gas block? Adjustable gas notwithstanding, what's the optimal gas port size for 11.5", 12", and 12.5" barrels, respectively?

As far as the muzzle device goes, I was going to go with whatever reduces flash the most, but I think by the time I am in a position (and State) to buy a suppressor, the offerings on the market may have changed so I'd rather not be beholden to AAC's proprietary 51-tooth mounting system, hence the YHM. I was also debating a Noveske KX5, but would like more flash suppression.

Finally (as far as the Upper goes), I'm going to stick with the AIM Surplus NiB BCG unless somebody can explain why, outside of the brand recognition and reputation, it might be worth jumping up to a $200+ BCG rather than this NiB offering from AIM Surplus.

  • 80% Aluminum Lower Receiver or JMT Polymer Lower Receiver
  • Magpul MOE+ Grip
  • ALG QMS or ALG ACT, eventually to be replaced by Geissele Combat Trigger or Wilson Combat Trigger
  • Standard Buffer Tube
  • H2 Buffer (I'm truly unsure about what buffer to get for max reliability. This is one of the elements I really need help with)
  • Mil-Spec Parts Kit + Anti-Walk Trigger Pins + Enhanced Takedown Pins

    So for those of you who've been kind enough to read this far, I have a few main questions.
    1. Assuming I pick a 12" barrel, what should my gas port size be?
    2. Assuming a 12" barrel, what buffer weight should I use?
    3. What springs should I use?
    4. If I get an adjustable gas block, what should I use when firing suppresed?
    5. Will my buffer weight need to change accordingly?
Thanks for your patience if you've bothered to read this all. Any suggestions/comments/advice are welcome.

Thank you very much!