m16a1 vs m4

[timelinex]

In reference to this thread, where I can't ask the question since it is closed:

A few things were said that I did not understand and wanted to know the answer for curiosity's sake:

Quote

That colt replaced the original lower post 1986 with a new M4 lower. 

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The original lower was sent to Colt for repair and came back an M4.

1. What makes this an M4 lower? I thought the only difference between m4 lowers and m16 lowers is literally just the marking. The marking on this lower is m16a1 still. Is there some difference between m4 and m16a1 lowers I do not know about? And if there is, did they then use the m4 lower buy stamped it "incorrectly" with m16a1 to keep it consistent with the lower that they were replacing?

2. Even more to the point....How could they have "replaced" an NFA lower and kept it legal? I thought that is why taking care of the MG part of guns is so crucial (like in an M60). You CANNOT replace the part that is the MG. It has to be "fixed" in any way possible, but if replacing , it is now considered a new MG and not allowed to be owned by civilians. What am I missing? Is that a newer rule that was added sometime post 1986 or something?

[taylorwso]

back in the day the original manf could replace receivers, then the fatf said no.

the m4 have more meat in the buffer extension are, and something else is different (i forgot).

 

 

Edited October 12, 2021 by taylorwso

[SGT Fish]

Yep, manufacturers used to be able to replace a transferable MG receiver for warranty work until the AFT changed their mind (as usual). 

M4/M16A2 style lowers are a different casting that (as mentioned) has more meat by the buffer tube, and may have other features that the original lacked, such as a fence around the mag release, or different size pin holes. You can Google all the variations of Colt lower receivers. It's a good way to verify how old a lower actually is. 

[timelinex]

Awesome, thanks for the info guys!

[Ryo]

 

5 hours ago, SGT Fish said:

Yep, manufacturers used to be able to replace a transferable MG receiver for warranty work until the AFT changed their mind (as usual). 

M4/M16A2 style lowers are a different casting that (as mentioned) has more meat by the buffer tube, and may have other features that the original lacked, such as a fence around the mag release, or different size pin holes. You can Google all the variations of Colt lower receivers. It's a good way to verify how old a lower actually is. 

Thank you Gemtax.  Asking AFT for clarification for everything isn't a good thing.