Friend's Carbine

AR/M4 Vintage ARs
1

Went shooting with a buddy of mine yesterday. He took along his dads colt AR. I really cant put a timeline on this thing. The barrel is ~12.5" with a 4.5" fh permed. The bcg looks to be a semi auto carrier and the front take down pin is larger than the rear (tried slapping my bcm upper on his lower). It ran 100% though, and he said he hasnt cleaned it in 1k some odd rounds. Heres some pics:

(excuse the cell phone quality pics)

2

No forward bolt assist or relief around the magazine release. What barrel twist rate does it have?

3

couldnt tell you to be honest. the rear sight also looked to be A1-ish.
Could it be something like an xm177? my vintage AR knowledge is lacking.

4

Standard Colt SP-1 Carbine that someone did a chop job on the barrel.

ROT is 1/12".

5

Looks longer than 12.5"

6

With that short a barrel I imagine it has a nice bark to it.