Inlines Using Modern Primers Reclassified as Firearms by BATF

Roger Laird said:

It is also interesting on the consumer end that once you have installed the drop in cylinder you have manufactured a modern firearm and are subject to those charges on the federal level

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There is nothing illegal about "manufacturing" a modern firearm for personal use. If you go to sell it, though, with the drop-in cartridge cylinder installed, it's subject to the same restrictions as in selling a modern revolver (such as not selling to an individual across a state line). The simple solution is to re-install the percussion cylinder. Installing a cartridge cylinder does not change the nature of the gun permanently.

Note the date on the page the ATF agent gave him; 1997. It may well have been rescinded since then, and if not every gunshop in the US that has sold an inline is in potential deep doo-doo.