This:
Or This
I’m guessing it’s personal preference but I must ask, why the open trigger guard, doesn’t that just add small parts to the build.
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Open trigger guard gives access if you’re wearing gloves, particularly winter gloves.
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My understanding is the open guard is due to the forging process, whereas billet allows a fully machinable trigger guard area.
I used to forge M16 uppers and lowers, and a friend up the street from me did/does some edm work on the M16/m4’s receivers, but no milling, so that portion is an unknown to me.
Not sure why the forged area is open, but it’s been 30 years since I worked on them. Possibly a cost issue on starting a cut and getting a good finish in that thin wall area with a forging.
Edit…guess I just bought a forged receiver that looks like billet, but Aero is noting the effort they put into that area…my other Aero X-15 receiver has the typical sign of being forged, and now I’m even happier with my DD upper/ Aero m4 lower build, due to the fact the barrel and both receivers are forged. I’m just a sucker for forged stuff,ever since forged pistons went into a race bike years ago, and hung together while my cast pistons sometimes ended up in marble sized chunks in my k & n’s 
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Yup, totally agree. In fact you can purchase “larger” guards exactly for that purpose.
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You have to remember that the M-16 was designed with the “Last” war in mind which was Korea and ergo WWII. There were great pains involved with producing a “Winter Trigger (Kit)” for the M1 Garand as well as the M-14… Both are available on the surplus market. This was in response to GI’s opening the “trigger guard” on the Garand during WWII and Korea so they didn’t have to take off their gloves. The only problem was after a round or two the trigger housing fell out of the gun or was displaced enough to not make it work.
The M-16’s response was to have a pin captured trigger guard that would allow the use of gloved hands. Not really worth a dog but there it was built into the assembly.
Me personally, I am over the moon on the Aero Precision M4E1 lower for a whole host of reasons not to mention the solid trigger guard. Glove technology has come leaps and bounds since lambs wool and canvas.
Cheers,
Craig6
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Full disclosure, R’s is built on the Noreen billet, the first picture. She seems to like it.
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Don’t think too many troops were wearing gloves when the M16 hit Vietnam…
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