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.308 semi-auto carbine

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.308 semi-auto carbine

Unread postAuthor: Bando_Red » Sat Oct 26, 2013 8:17 pm

So the past day I haven't been able to get this idea out of my head. An ultralight, ultra-compact semi-auto in a mid-sized caliber. I've outlined the general goals of the project but I'm still figuring out the build details, the remainder of which will all take shape based on trigger mechanism and establishing a seal between the magazine and barrel when discharging air without manual working of the action. So far my best idea is to combine aspects of GBB airsoft guns and big bore hunting air rifles by using a tapered barrel (which is how Quackenbush achieves a bullet seal with the higher-powered projectiles) and utilizing the blowback principles used in GBB pistols to allow new rounds to be chambered, only most likely with a charging handle rather than the hammer and slide. I'm not sure about a trigger yet. I'm curious about electronic triggers so I'll do some more research into them.

the ballistics and ammo from this are borrowed directly from the ".308 Exile" Hunting Airgun by the revered Mr. Quackenbush. He's done some really impressive work that I'm certainly not capable of replicating, and as such I am going to attempt to purchase a barrel from him. I do intend on casting my own conical bullets as he has done. With the right carrier I don't believe they will be any more finicky to feed than BBs, Just deciding whether to use a tubular magazine or vertical stack, or whether it will be detachable in the case of a stick mag

I think my brain is finally exhausted. Gonna rest and think on it.
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Re: .308 semi-auto carbine

Unread postAuthor: jackssmirkingrevenge » Sat Oct 26, 2013 9:31 pm

Bando_Red wrote:An ultralight, ultra-compact semi-auto in a mid-sized caliber.


I think this is a flawed premise to start off with. Compressed gasses very poor in terms of energy density, so it is unlikely that you would achieve the required performance in a setup that is both light and compact.

Let's look at the Caselman air machinegun, roughly the same calibre as you are thinking of:

http://www.doomlabs.com/projects/caselm ... 0Flup.html

Image

It has the performance you probably have in mind, but as with most powerful airguns, it still needs a big bulky air source and a long barrel.

You need high pressure to keep things compact, in the 3000 psi region paintball bottles can take.

This implies strong parts, do you have a machine shop to hand? This isn't something that will be built on a budget either.

Some commercially available airguns to research are the FX Monsoon and Revolution which use muzzle blast to recycle the action, and the recent Evanix series (for example) which use a hammer driven by an electric motor in the same vein as the piston on an airsoft electric gun.

Just some thoughts to chew over ;)
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Re: .308 semi-auto carbine

Unread postAuthor: Bando_Red » Sat Oct 26, 2013 11:54 pm

It was more of a long-term plan. I guess I wouldn't say ULTRA light or compact. Like I said I'll be buying the barrel, and most likely the tank as well.

The space thing I'm not worried about. The overall package will be roughly p90-shaped, so its pretty space efficient. I'm thinking a 16-20 inch barrel and a bullpup feed, so the breech will be near the butt of the gun with the magazine running directly under the length of the barrel it if I go with a tubular magazine like a lever-action or directly under the breech if I use a traditional bullpup style magazine. Not sure which would be best yet. In any case, thats only a few inches apart taken up lengthwise, leaving plenty of room below for a tank, grip, and trigger, just a matter of positioning and shaping them correctly.

Weight is another concern. while I'm obviously not going to compromise safety at pressure points, I'll be using plastic and aluminum whenever possible. I don't anticipate the reciever weighing much more than a pound assembled at most, possibly almost half that depending on how much plastic I use. Most of the weight will be the tank, with the barrel and the various valves and other solid brass taking up more.

I don't have a machine shop, which is why although this a long-term project I plan on "outsourcing" so to speak the major components like the barrel and tank. I can put together a firing mechanism ,reciever, and furniture once I have the plans together. My main concern right now is achieving a reliable seal in conjunction with reliable feeding and cycling. Once I figure that out, It will help me determine tank placement and magazine type. I'm also a little worried about the timing on a semi-auto action and what kind of valves to use, but I'm fairly certain there are examples of that in the showcase. Still on the fence about whether to go mechanical or electric trigger but I'll worry about that later.

Once I have some preliminary sketches I'll post them as well as the "proposal" as they say in the military.
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Re: .308 semi-auto carbine

Unread postAuthor: wyz2285 » Sun Oct 27, 2013 6:35 am

Evanix series (for example) which use a hammer driven by an electric motor in the same vein as the piston on an airsoft electric gun.

Done that 8) even though not very pleasant to use. The noise of the motor and mixed with muzzle blast and the need for battery...
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Re: .308 semi-auto carbine

Unread postAuthor: jackssmirkingrevenge » Sun Oct 27, 2013 7:05 am

wyz2285 wrote:Done that 8) even though not very pleasant to use. The noise of the motor and mixed with muzzle blast and the need for battery...


The idea has been in my headfor a very long time. I wonder if the people at Evanix read though this forum...

My main concern right now is achieving a reliable seal in conjunction with reliable feeding and cycling. Once I figure that out, It will help me determine tank placement and magazine type. I'm also a little worried about the timing on a semi-auto action and what kind of valves to use, but I'm fairly certain there are examples of that in the showcase. Still on the fence about whether to go mechanical or electric trigger but I'll worry about that later.


I would say trying to replicate an airsoft gun would be a bad idea, they are not very efficient. The FX series linked to above is a good example of a very efficient mechanism as it uses the muzzle blast to drive the reloading, air that would have otherwise been wasted. Using electricity to drive it means that no extra air is wasted, but you have to add the weight of batteries to the setup.
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