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Old May 6, 2013, 11:01 PM   #1
johnwilliamson062
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All open bolts machine guns?

I have been surfing the web looking at home made designs the last few days. I have seen a number of questionable designs. Maybe foreign made or maybe my eyes aren't accurate at measuring. One thing I have seen is single shots that fire from an open bolt. I was wondering if the prohibition on open bolts is limited to repeating arms, or if these items are illegal.
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Old May 7, 2013, 08:15 AM   #2
Madcap_Magician
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Is there a prohibition on firearms firing from an open bolt? I am not aware of one.

The only real reason to have a firearm fire from an open bolt is to aid cooling for moderate to rapid sustained automatic fire. I can't imagine what the point of a single-shot firing from an open bolt would be.
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Old May 7, 2013, 08:31 AM   #3
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I can't imagine what the point of a single-shot firing from an open bolt would be
simplicity of the firing mechanism.
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Old May 7, 2013, 08:56 AM   #4
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Yes, if it fires from an open bolt it is an easily convertible machine gun in the eyes of the ATF. Don't build or buy one unless you are licensed to build or it's a Pre 84 papered gun.
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Old May 7, 2013, 09:35 AM   #5
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Yes, if it fires from an open bolt it is an easily convertible machine gun in the eyes of the ATF. Don't build or buy one unless you are licensed to build or it's a Pre 84 papered gun.
I wonder how this could be true for a single shot with no provision for a magazine.
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Old May 7, 2013, 09:45 AM   #6
Mike Irwin
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yep, firing from an open bolt is about as simple as it gets.

Most often in such a design the "firing pin" is just a bump on the bolt face -- nothing mechanical about it. In effect, it's a slam-fire gun.

There is something of an art to balancing the weight of the bolt, the power of the cartridge and the power of the main spring to make sure that the system is in balance and that the cartidges don't pop early, or not at all.
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Old May 7, 2013, 09:46 AM   #7
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"I wonder how this could be true for a single shot with no provision for a magazine."

I THINK Savage made such a rifle years and years ago.
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Old May 7, 2013, 10:59 AM   #8
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Generally speaking BATFE frowns on open bolt semi-autos. You will not find any law that outright bans the production of all open-bolt designs. It's simply that no one has figured out a way to make a semi-auto open bolt gun that isn't (in BATFE's eyes) easily converted to full-auto. Theoretically, you could still build an open-bolt gun. In fact, I recall seeing I think SWD made an openbolt 12 gauge shotgun recently. Perfectly legal. Can't be converted....it's a single-shot.

There's probably some way you could make a semi-auto open bolt that BATFE would approve. The mechanism would just be too complex and costly to make.

Last edited by Skans; May 7, 2013 at 11:05 AM.
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Old May 7, 2013, 11:18 AM   #9
Brian Pfleuger
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Originally Posted by Skans
The mechanism would just be too complex and costly to make.
Not to mention all the inherent downsides to accuracy. Folks spend money replacing springs, firing pins and whatnot to eliminate ignition delays measured in fractions of a millisecond. The time it takes for that bolt to close is a comparative eternity.
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Old May 8, 2013, 12:17 PM   #10
johnwilliamson062
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So a single shot can't easily converted to full auto and the ruling does not apply. moving forward with my design.
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Old May 8, 2013, 01:06 PM   #11
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To be a machinegun, a firearm has to fire more than one shot with a single pull of the trigger (or operation of the firing mechanism). A single shot with no magazine can't, by that definition, be a machinegun no matter how it operates.

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Old May 8, 2013, 02:07 PM   #12
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"Not to mention all the inherent downsides to accuracy. Folks spend money replacing springs, firing pins and whatnot to eliminate ignition delays measured in fractions of a millisecond. The time it takes for that bolt to close is a comparative eternity. "

This doesn't include the fact that any open bolt submachine gun I ever handled: the bolt is massive. It is by far the heaviest part of the whole gun. If you take the bolt out, all you have is a tube, short barrel, and the fire control components. So, when you have one of these guns on semi-auto, when you fire it, this massive bolt that weighs several pounds slams forward. I guess I am thinking of guns like the Sten, the Sterling, and things like that. On the Thompson, the whole thing is heavy and massive.
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Old May 8, 2013, 02:31 PM   #13
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Further, open bolt sub guns use advanced primer ignition to keep bolt weight down. When a sub gun is changed to a closed bolt blowback design, either the bolt has to be made more massive or the spring made a lot heavier. Since most closed bolt guns are built on sub gun receivers, increase of bolt mass may not be practical, and the extra spring tension makes initial operation more difficult.

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Old May 8, 2013, 02:41 PM   #14
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It depends what you are looking it for . If you are looking accuracy as said in other posts the open bolt design makes they difficult to shoot accurate on singles shot. That's one reason that the MP/5 is so popular it shoots from a closed bolt and is more accurate than open bolt designs.
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Old May 8, 2013, 03:43 PM   #15
Andy Griffith
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Well, you could have an open bolt pump or lever action too.

Open bolt revolvers are just too complicated to make.
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Old May 10, 2013, 08:09 AM   #16
Mike Irwin
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"Further, open bolt sub guns use advanced primer ignition to keep bolt weight down."

Or, you end up with guns like the Reising and the original Thompsons, which fired from an open bolt, but which had the odd and questionablely useful Blish lock.

I know there have been several other "open bolt, but locked at moment of firing" submachine guns over the years.
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Old May 10, 2013, 08:52 PM   #17
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I had a semi MAC 11 years back that fired with an open bolt. I KNOW that it is now illegal to manufacture these. I would check the laws, or better yet call the ATF and find out. Single shot? How hard is it to braze a magazine well to a tube?
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Old May 10, 2013, 09:40 PM   #18
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I once said that I could make a sub gun out of a barbeque grill and, given some basic tools, I think that is true. In fact the SMG is the easiest of all serious* guns to make, a lot easier than a handgun or a rifle. The hardest part to make from scratch would be the magazine.

*I don't count toys like "zip" guns or those "Philippine guerilla" shotguns.

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