Pot metal is heavier....but not many wants those either.....
The issue is that its not just weight, 6061 and 7075 are also what I'd consider as engineering material, they have the strength to take some beating. A lot of the WE and the some Japan OEM uses heavier zinc alloy to get the weight and feel of the gun, but just as many people have found out, they are weak and will often break just like plastic slide.
There are shops that makes kit just for the heavier kits, brass, bronze kits aren't THAT unusual in some WA 1911(TM system supposedly not so great running heavier moving parts), but knowing the material it is likely not that robust, but the main market for those are collectors.
They can like you say, just make it out of steel, while the tool path might be the same, the more expensive tool bits and longer machining time might mean much more expensive product because not as many people might be willing to pay for a steel kit(especially when not many are willing to buy more expensive kit like Nova or PGC as it is).
Edited by RacingManiac, 17 June 2009 - 02:40 AM.
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TM Pistol: IPSC Limcat and SVI, Kimber, Glock 17, 18c and 26
KSC Pistol: USP Match
WA: Kimber Compact, SVI 3.9
WA GBBR: Iron Airsoft Larue PSD + MUR 16" Upper
GHK GBBR: Tactical AKM