Laws.
You can't make it open bolt. Making it closed bolt changes the fundemental operation of the firearm and how reliable or durable it was would be based on that entirely new action.
So it would only superficially look like a grease gun.
Open bolt firearms, especially full auto with even fewer required moving parts than semi, can be both crude and reliable. The bolt in many such guns does the role of several things in closed bolt actions. The trigger merely holds the bolt back against spring pressure. The bolt when released strips the round, chambers it, and fired it with a fixed firing pin. With recoil pushing the bolt back against spring pressure and the process repeating until the bolt was caught by the trigger mechanism being released. So its just a tube with a piece of metal bouncing back and forth in it. The most complicated part of the entire firearm is the magazine to insure reliable feeding.
Change it to closed bolt and the refinement required to be as reliable is much higher.
Second you can't make subguns thier realistic sizes that the market would actually buy and the ballistics of a pistol caliber are ideal for.
This is due to SBR laws that shrink the market to a much more limited NFA crowd, and also add a $200 tax to something which is a lot if you are making something to meet a low price point.
Most of the market looking at low price point substitutes and willing to deal with a lot less refinement to reach it does not overlap with the market willing to pay at least $200 extra, wait months, and go through the hassle of the NFA.
As for the barrel length: Pistol calibers using fast pistol powders excel in shorter barrels.
The point of diminishing returns where additional length is worth the gains varies by caliber and load, but I recall in .45ACP the Thompson barrel length was actually created with that in mind, and was 10.5".
Subguns even in semi auto would be quite handy and a good ballistic, weight, and size, compromise with such length barrels. With modern improvements like telescoping or collapsing stocks you could have something compact and easily maneuvered when deployed and little bigger than a handgun when stored.
But toss in NFA restrictions mandating a 16"+ barrel for most of the market and a rifle in a rifle caliber of the same size makes more sense.
If it has to be rifle sized with a rifle length barrel that could benefit from more powder and longer burn times it might as well be a rifle cartridge.
Another area pistol caliber carbines offer consideration is with suppression, where they can perform almost normal ballistically and be highly suppressed compared to rifles that rely on supersonic velocities for the majority of thier energy. But that again involves the NFA, and reduces the market size.
Third crude subguns were made for volumes of fire. Without fully automatic capability they lose thier primary role. If you can't have volumes of fire then precision becomes more important and more precision requires more refinement and that typically involves a more stable action and higher price point.
So for civilians I would say the laws play a huge role.
You can't make it open bolt. Making it closed bolt changes the fundemental operation of the firearm and how reliable or durable it was would be based on that entirely new action.
So it would only superficially look like a grease gun.
Open bolt firearms, especially full auto with even fewer required moving parts than semi, can be both crude and reliable. The bolt in many such guns does the role of several things in closed bolt actions. The trigger merely holds the bolt back against spring pressure. The bolt when released strips the round, chambers it, and fired it with a fixed firing pin. With recoil pushing the bolt back against spring pressure and the process repeating until the bolt was caught by the trigger mechanism being released. So its just a tube with a piece of metal bouncing back and forth in it. The most complicated part of the entire firearm is the magazine to insure reliable feeding.
Change it to closed bolt and the refinement required to be as reliable is much higher.
Second you can't make subguns thier realistic sizes that the market would actually buy and the ballistics of a pistol caliber are ideal for.
This is due to SBR laws that shrink the market to a much more limited NFA crowd, and also add a $200 tax to something which is a lot if you are making something to meet a low price point.
Most of the market looking at low price point substitutes and willing to deal with a lot less refinement to reach it does not overlap with the market willing to pay at least $200 extra, wait months, and go through the hassle of the NFA.
As for the barrel length: Pistol calibers using fast pistol powders excel in shorter barrels.
The point of diminishing returns where additional length is worth the gains varies by caliber and load, but I recall in .45ACP the Thompson barrel length was actually created with that in mind, and was 10.5".
Subguns even in semi auto would be quite handy and a good ballistic, weight, and size, compromise with such length barrels. With modern improvements like telescoping or collapsing stocks you could have something compact and easily maneuvered when deployed and little bigger than a handgun when stored.
But toss in NFA restrictions mandating a 16"+ barrel for most of the market and a rifle in a rifle caliber of the same size makes more sense.
If it has to be rifle sized with a rifle length barrel that could benefit from more powder and longer burn times it might as well be a rifle cartridge.
Another area pistol caliber carbines offer consideration is with suppression, where they can perform almost normal ballistically and be highly suppressed compared to rifles that rely on supersonic velocities for the majority of thier energy. But that again involves the NFA, and reduces the market size.
Third crude subguns were made for volumes of fire. Without fully automatic capability they lose thier primary role. If you can't have volumes of fire then precision becomes more important and more precision requires more refinement and that typically involves a more stable action and higher price point.
So for civilians I would say the laws play a huge role.
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