Besides the rear sight and rail system, what are the differences between the 6920 and the 6920 SOCOM?

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Primarily the barrel is the only relevant difference.

A needlessly heavier barrel on a semi-auto rifle.

This

I have both barrels in 14.5". Quite frankly I cannot tell the difference, but when you mount rails, lights, optics, lasers, and have to carry a rifle for a long period of time, the weight becomes a big deal. I carried an M-4 for an extended period time, and after awhile with all your gear on, you can't wait to take it off.  Boy, I sure miss my AUG

I'm glad we are allowed choices, I'd go heavy barrel every time when given this choice.
I respect there are differences, I'll save weight somewhere, not in the barrel.
Oh, the beauty of options... Yet, I respect your decisions also.

Until you have to carry it for any amount of time.  I carry an m4a1 with that barrel profile at work.  
It's needless in MOST user's applications, and I'd even say all applications on a semi-auto AR.
But I agree, whatever works for you.

Ref. Colt model LE6920 SOCOM roll markings "M4A1 CARBINE" (Ref. archive LE6920 SOCOM thread link - click here)




My Colt model LE6920 markings "M4 CARBINE"



Between the two barrel length are the same 16" except the LE6920 have a government profile barrel under the handguards and the SOCOM have the medium profile SOCOM.

Other differences are roll markings as shown in the pics, SOCOM have a new ambidextrous selectors with a short right side lever, standard LE6920 have the standard reversible selectors, the LE6920 have H buffers and Colt Defense-Rogers SuperStoc (and rear push button sling swivel) compared to H2 buffers and standard M4 buttstock for the SOCOM model.

The LE6920 are also supplied with two Colt 20rd USGI mags while the SOCOM are with two 30rd USGI mags.

I don't know how accurate it is, but the Budsgunshop description also says it has a 2 stage tactical trigger, in addition to the stuff already mentioned.


Also, I was looking at one and it actually has a LAW ENFORCEMENT CARBINE  rollmark on the left side.  Unlike the current crop of rifles I have been seeing.

Don't know why they'd advertise it with a 2 stage trigger, it doesn't have one. But it does have an ambidextrous safety.