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A prototype for a bullpup

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#1 Megothitinheadbydart

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Posted 02 April 2006 - 06:48 PM

This design theoretically results in a nerf gun that is reduced in length by the plunger stroke.

Instead of having the plunger tube part of the bolt, which means that the plunger must be at least as long as the piston stroke just in front of the catch, I am using the gun body as the plunger tube. The bolt seals in the body with an O-ring and is worked back and forth by a metal rod the extends over the camber and barrel to a slot above the barrel. The bolt is only about 3 1/2 inches long and is made of a 1/2" PVC endcap and 3 layers of brass.

So far I have made the bold, plunger, chamber/barrel, magazine well, catch/trigger mechanism, and whatever you want to call the main body part thing.
Not bad for one day.
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I still have to make the pump mechanism and the magazine, but I need a mag spring. Right now I am leaning toward a constant force spring from McMaster-Carr, but I'm open to suggestions.
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#2 wick817

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Posted 02 April 2006 - 06:54 PM

I like the concept, i will like to see the finished product. But i did see some spelling errors(check over it before you post next time). Keep working on it, I think it will come out nice.
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#3 GeneralPrimevil

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Posted 02 April 2006 - 07:00 PM

It's nice to see more and more people attempting these, even though you attempted one awhile back which resulted in the magnum pistol. That thing gave me inspiration for a trigger, which I later dropped and replaced with a design more of my own. Yet again, no one but two people know what I am talking about...just wait 'til tomorrow.

Anyway, bullpup, eh? Hmm...not a bad idea.
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#4 Ronster

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Posted 02 April 2006 - 07:03 PM

What is that your using for the mag well?
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-I Nerf, there for I am.

QUOTE(baghead @ Oct 25 2006, 09:55 AM) View Post
Ronster reminded me that I should have warned you all Prepare your Bladder for Imminent Release!!!!

#5 Megothitinheadbydart

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Posted 02 April 2006 - 07:13 PM

The Magazine well is 1/4" ABS plastic stock, 1 1/2" wide, glued together and dremeled to fit the 1" thinwall PVC.
As for the bullpup idea, mabey I wasn't clear, but this nerf gun will be a standard set up, with the magazine in front of the grip, but with the considerable length decrease caused by the new bolt and an extension spring between the bolt and planger you could potentially make a bullpup. That will probably be my next project.
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#6 Ronster

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Posted 02 April 2006 - 07:14 PM

How long is it?
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QUOTE(baghead @ Oct 25 2006, 09:55 AM) View Post
Ronster reminded me that I should have warned you all Prepare your Bladder for Imminent Release!!!!

#7 Megothitinheadbydart

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Posted 02 April 2006 - 07:23 PM

It is 29" long, but the area behind the magizine is much shorter than is comfortable.(around 10") This measn that while the overall nerf gun is longer than many, the clip is very far back compared to most.
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#8 GeneralPrimevil

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Posted 02 April 2006 - 08:02 PM

I really do not know how I got "bullpup" out of that...I may have been reading another webpage at the time or something...


29", for me, is really short. I'm used to homemades longer than 34". My next one has a micro (as in...7" long) barrel with an overall length of somewhere around 41", not magazine fed, but single shot. Then again, there's a silencer on it that is 12"...
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#9 Maverick Master

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Posted 03 April 2006 - 06:01 AM

Thats really nice and a good idea. Also for the mag spring i'm using a spring I took out of my spiral notebook that was out of paper.
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#10 boltsniper

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Posted 03 April 2006 - 10:52 AM

You can buy rebuild kits for AR15 magazines that come with the spring and follower. They run around 7 bucks.

Magazine rebuild kit

If you don`t want to go that route then it is going to be tough to find a spring with enough displacement to function as a magazine spring. At least tough to find one just laying around or at a hardware store. Coil springs, like the one I used in the FAR, work well. Finding them can be tough too. If you can find someone in retail that can get you one from a new or broken fixture. Thats how I got mine.

Good idea you have going on for your gun. I found it hard to design the action so that the section of the receiver where the plunger lies would have no holes, slits, or cuts in it. The operating rod sticks out of hte bolt and through the receiver which meant a long cut down the axis of the receiver. Because of this slot if I didn`t have the plunger tube as part of the bolt all the air would just rush out of the slot. I`m sure there are ways around that. You are right it does add another plunger stroke length to the length of the rifle. I didn`t have a whole lot of issues with that though. The distance from the stock to the pistol grip (length of pull) is dicated by comfort and turned out to be longer than the action needed. The magazine can be placed jsut ahead of the pistol grip allowing suitable distance from the trigger. Magazine placement will dictate chamber placement and beyond that is just barrel which can be whatever length you want. I took all these points into consideration when making hte SCAR. The LOP is as short as I felt comfortable and is still longer than the action needs. I had to perch the spring up about 2 inches from the end of the stock to get fiull compression. I placed the magazine much closer to the grip than on the FAR and the magazine is much shorter. The barrel is shorter at 12". The whole rifle is about 29" with the brake on the muzzle.

I don`t know if you are planning a rifle or not. If you use the system you have going you can drop the stock and make a pretty wicked pistol thing. That would be pretty cool.
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#11 Megothitinheadbydart

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Posted 03 April 2006 - 01:54 PM

Hmmm... The mag rebuild kits seem like a good option, but I think I'll go with acostant force spring from Mcmaster.com like the one on th Far.
To solve the problem of no slots in the reciever where the plunger is, I'm using two metal rods side by side that go up the inside of the body passing right above the chamber and apearing through a slot cut above the barrel.
I was planning on making this a rifle, but it is so short behind the magazine that it may end up with a shortend barrel as a pistol. The next version will use a locking bolt and an extension spring between the bolt and plunger. This should give me the extra two inches less of length that I need to make this thing a bullpup.
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#12 ompa

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Posted 03 April 2006 - 05:52 PM

Very nice! Bars that run up before coming out... That's perfect for solving that design issue. I was also wondering what the best way to solve it was, so thanks a shitload for that. So simple, yet so effective.

EDIT: Actually, looking at your design, you might need a second o-ring on the endcap at the end of your bolt. Otherwise, when you push back the bolt with your bolt handle, the force on the spring might cause the bolt to "twist" in a sense, making it alot harder to push the bolt and plunger back, as well as causing possible problems when the bolt comes back to seal the dart/shell. Of course, that's just what I see, I haven't tested it myself.

~ompa

Edited by ompa, 03 April 2006 - 10:50 PM.

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#13 Megothitinheadbydart

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Posted 04 April 2006 - 04:44 PM

I've decided to go shell-less on this gun, so I redid the bolt as just a piece of 17/32 brass where the old bolt had the extraction mechanism. As for the clip, since a stack of stephans will not compress noticable under it's own wieght, there must be a certain amount of force that will not smash the darts but will still push the top one against the feed lips with a negligable amount of squishing.

Ompa- I just tested the new bolt with the bars and there is no twisting whatsoever. This must be baecause the bars are so close to the inside of the pvc and are in fact rubbing along the top of the chamber, which is made of the same size brass as the back of the bolt where the bars attach.

Also, I've decided on the issue of whether this will be a rifle or pistol. It will be neither but instead it will be a submachine gun type thing, with a folding stock and possibley a shorter barrel.
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