well again for the first one a 240(and thats gonna be the lighter of the two you suggested you could have a full auto M4 with a tax stamp and it would be much more versatile -you could even make a bipod attachment similar to that of a pintle mount so you could mount it up there. -with an auto/burst M4, its the lower that is registered, so you could have different uppers, you could have one upper for dismounted use ie hand held at the range ect, and then a longer heavier upper for mounted almost like a snipers rifle -they also make M4s in 308, the civilian term is AR10 -military term is M110 but it is still only semiauto, not sure if they make a full auto or burst M110/AR10. i think your 1919 looks similar to a M2(.50 cal) but again my concern is its bulk and ease of use, as well as availability of parts -and the same goes for the 240, but those should be easier to find. i have never used a 1919 or worked with one, so i can not comment on their reliability and service ability, but i know they are old school and relics, not really in use anymore except by collectors ect. but having been a heavy weapons team leader as well as been 240 gunner i know the 240 pretty well -i know it hates blanks, and will require modifications for the feed tray if you want to use blanks(dont tell the army). and learning how to clean one, well lets just say that in all my years in the military i have yet to find one 240 that has been cleaned properly and all that i have recleaned and or used had some serious corrosion issues going on. great weapon and i love it, but its bulky/heavy hard to shoot from the shoulder, but it can be done -i have done so but its not gonna be near as accurate. and again personally a semi auto version is only gonna look cool... and thats a lot of money to just look cool