Quoted:
1)When did they start forging? I'm assumming the cast was the first offered.
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The forged started sometime around 99/00.
2)What was the S/N range for the cast
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Forged lowers have an 'F' prefix on the serial, making them a seperate series and there is no cutoff date. The still make both types.
3)Are there any real detraments to a cast unit?
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If you hit the receiver really hard with a hammer beyond the metals ability to bend, the metal of the forged part will stretch resulting in a large dent while the metal of a cast part will crack. A hit that hard to either lower will likely take it out of service. The forged lowers are cosmetically superior IMO. Especially the lettering and logos.
4) What does it mean in an ad when the seller says the block has been removed?
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Most (all?) makers of AR lowers reduce production costs by only milling enough space in the inside rear of the lower to let the safety and the rear lug from the upper fit into the lower meaning that there isn't enough room to insert a registered drop in auto sear or a registered lightning link. "removing", actually milling out the area in the rear, is done so you can use one of these very expensive devices if you wanted to.