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You can get one setup with 2 calibers 9/45 which is pretty neat. They just don’t look like a iconic SMG. Like a SW76, sten or UZI.
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Don't get me wrong, I think they are neat guns. But, at a price point of 14,000, there are better options out there.
Right not the Uzi market is super soft. I think someone just mentioned that they got a RR Uzi for 12,000. With an Uzi, you have a superb 9mm gun with simple 22 and 45 conversions. Here's a huge packasage for 14,800 http://www.sturmgewehr.com/forums/index.php?/topic/13564-fsvector-fs-uzi-reg-3cal-huge-pkg-14800/. Here's a great deal on a Vector Uzi with a suppressor for 11,900 http://www.sturmgewehr.com/forums/index.php?/topic/13455-wts-vector-arms-uzi-transferable-excellent-11990/.
Next, don't get hung up on caliber conversions. One of my machineguns is an Uzi. Although I have conversions for 45 and 22, I rarely use them. The gun us designed for 9mm and runs best in that caliber. 22 is frankly boring and 45 is expensive and brings nothing to the table.
Finally you hit the nail on the head about "iconic" machineguns. The Stemple will never have the selling power of an Uzi or a Sten. People like to buy historically relevant guns or guns they saw in movies. As such, the iconic guns are much easier to sell.