I recently acquired a preban Colt Sporter Rifle with a serial # in the 20,000 range. It has the bayo/hider. The rifle was retired from a state police agency and when I got it there was not a single scratch or wear on the lower. It was configured more or less along the lines of the current model of Colt AR15 A3 Tactical Carbine (I have one of those, too, however, it is LEO/Gov restricted).

What I didn't notice when I purchased the Sporter for my personal use was the mag release side of the lower is smooth, without the raised rounded "well" around the mag release button and other areas that I can only assume were for structural reinforcement (?)

Then it dawned on me that the lower was identical to that on the M16 I use (it's a surplus A1 model). That Colt M16 has had thousands of rounds through it over the years and is still an effective weapon.

So, what difference (other than aesthetics) does this slab-sided lower make in the performance/durability or whatever of an AR15?

Is there any functional difference or difference in internal parts compared to the newer designed lowers?

Also, I assume this is one of the rifles made during the transition from the AR-15 model name to the more PC "Sporter" name. With a 20,000 serial can you determine the approximate date of manufacture?

Thanks for your comments.

The "fence' was designed into "newer" lowers to reduce the possibility of something bumping the mag release button and dropping your mag unexpectedly. It's been around for a long time. The only other differences that I can think of off the top of my head that would differ from the older models would be the reinforcing rib on the lower receiver where buttstock is, the larger holes for the fire control parts on the newer(blue label) Colt lowers and the larger front take down pin holes on the Colts. If it doesn't have the "fence' then it is very likely a pre ban Colt. One more thing as far as the "newer" more PC name sporter, what do you think the SP in the serial number means? You have an old SP1 model Colt AR15. Does it have triangular handguards? Does it have a light weight barrel with a 1/12 twist? Is the handguard ring at the front of the receiver flat or beveled? Is there a trap door in the buttstock? along with a few other features the preceding are early SP1 features. I can't tell the date of manufacture but maybe someone else can point you in the right direction.

The lip or fence around the magazine release is a feature of the M16A1 circa 1968.  One of the changes from the XM16E1, the most common rifle during the Vietnam War.  If it's "slab sided" it's not an "A1."

Colt's, for reasons never explained, kept using the XM16E1 machine pattern on the AR15 until fairly recently.

I carried a slab-sided XM177E1 for a year in mountain jungle without losing a magazine.

-- Chuck

So, from what you are saying (thank you very much for your thoughtful responses) is that it is primarily a design update vs a fix for a design flaw that would render the older receiver "weaker" without what appears to be reinforcement...

Congrats on the find Dingo.

Here is the serial number list I have bookmarked. It seems your rifle was made in '72-73. Presumibly your ser. no. has an 'SP' prefix.

The Sporter name has been used by Colt since almost the beginning of production. Just to differenciate between civvy semi-auto and military select fire.

www.biggerhammer.net/ar15/ar15serial.html