Could a 30 cal carbine round function from a open bolt system in a full auto OR is the pressure too high? I figure the mass of the bolt to be about the same as a bolt for a 9mm subgun. As the 9mm fires a 115 gr bullet & the 30 cal Carbine fires a 110 gr. Put on your thinking caps. ???

You talking Blowback system, Boy ? This one came close.

http://en.wikipedia.org/wiki/Cristobal_%28carbine%29

Grizz

Firing from an open bolt position and firing in a blow back system are not the same.
Firearms that do fire from the open bolt position do also use the blow back system. Many other firearms that use the blow back operating system fire from a closed bolt.

The blow back operation is simply that the bolt or action is not locked on firing. Some do use a form of delayed locking (or opening) with blow back.

Open bolt systems use a fixed firing pin like the M3 grease gun. blow backs are like the MP40.

Didn't they make an experimental Thompson in .30 carb.? Thought I saw a photo of one some place. if so it would have been open bolt, blowback??
C.G.B.

The BAR fires from a open bolt and is not a blow back system.

The M2 fires from close bolt.

So to your original question Doug, you are asking for a 30M1 Carbine that will fire from the open bolt position and also be a blow back operation just relying on the bolt spring and mass producing enough forward inertia to keep the breech sealed long enough for pressures to drop to a safe level before the extraction and ejection cycle is initiated.

I'm not sure if there is any formula for determining the suitability of any given cartridge for this style of operation. I think it comes down to a combination of sidewall pressure holding the case in the chamber, and the momentum reaction force on the bolt to the bullet and powder gas exiting the barrel, while keeping the breech block and spring mass at a sensible size that enables manual cycling. I guess you could make any cartridge fire in a blow back action if you made the spring and the block heavy enough and strong enough. Problem is you could probably not cock the firearm by hand and the whole action would be ridiculously heavy.

Open bolt and blow back seem to 'mostly' go hand in hand with low intensity cartridges with the open bolt designed for cooling purposes on rapid fire Military sub weapons. I would have thought if this method could have successfully worked for the 30M1 Carbine cartridge then it would have been simple just to have followed the Sten, Tommy and most of the other squirt subbies in manufacture

Eagle thats exactly the track of thought I was seeking. I appreciate your input.

Have you ever noticed that it's easier to rack the slide on a conventional "short-recoil, Locked breech" 9mm handgun than it is to do the same on a straight blow-back 380Auto?

The 9mm will function reliably with less mass and less spring
BECAUSE of the locked breech short recoil system's ability to allow pressure to drop before the action opens.

Sure you can make a straight blow-back 9mm, but they tend to have a high reciprocating mass and/or substantial spring.

Open breech sub-machine guns tend to bring another factor into play... "Advanced Primer Ignition" where the primer is actually struck before the cartridge is fully seated and the blowback actually has to work against the forward inertia of the (still in
motion) breech block.

In auto-cannons this is sometimes refered to as the Orlikon principle, because of who used it first in medium sized auto-cannon.
The primary advantage of using this system is it makes the total weight of the weapon lighter. it is what made aircraft mounted auto-cannon practical

For the whole system to work the weapon is uaually designed so that the bolt actually protrudes into the chamber

In small arms it is usually desired that the primer is initiated
as the projectile engages the rifling and the cartridge reaches pressure before the bolt it entirely arrested from it's forward motion.

So you can see primer initiation and peak pressure have to be carefully timed events.