Question for someone truly knowledgable about Colt lower receivers. Are lowers with the "Sporter Lightweight" designation made of a lighter weight alloy than other Sporter (HBARS, Target, etc.) receivers. Or is this just marketings way of designating the carbine Colts with 16" standard barrels as having lighter overall weights. I ask because someone had made the comment that "Lightweight" lowers were less expensive and less desireable than their counterparts. My observation has been that the most "Lightweight" models seem to have all the features (raised fence, small pivot pin) that are more desirable. Less the sear block, of course. Hey, I wish Colt had never dropped the "AR-15" designation in favor of the weaker and more PC "Sporter" monicker, but that aside,
does it really matter? Heck, isn't there a post-ban Colt model called a "CAR-15 A3 Tactical"? What's up with that? It's not even a carbine!

"Lightweight" has nothing to do with the lower receiver.  It means that a thinner, lighter weight barrel was used.  It's lighter, but it's not as rigid and it heats up alot faster.

The real difference between different companies' lowers is the method used in their manufacturing.  Cast lowers are made by molten metal being poured into a mold, allowed to set, and then milled.  Forged lowers are made by heating the metal to a point well below its melting point, hammering it into the rough shape of the receiver, and then milled into the final piece.

Forged receivers are always more desireable than cast.  The metal is denser and more uniform.  The result is a stronger receiver.  It's also more expensive.

Cast receivers ARE lighter than forged receivers, but this is because the metal is not as dense as a forged one.  It's lighter, but not as strong.  

Colt went with the "Sporter" moniker in the late eighties because of some high-profile shootings with AR's.  The only difference in the original Sporters was that the bayonnet lug was ground off.

The weight difference will be so slight you will not be able to measure the difference between cast vs. forged.

I do not have the numbers for aluminum, but for steel the difference is 0.2% in density.  So if the cast receiver was 16 ounces of steel, the forged receiver would be 16.032 ounces.  A small enough difference that variance within the specs. of the machining would account for more weight change than cast vs. forged.  Again, I do not have the specific numbers for aluminum but I would expect the results to be similar to steel.

Kent