Would these internals work?

[Meathead316]

Hi, im thinkin of moddina famas for a project im makin mini gun) i thought explainin it would take ages and be too confusin, so here in an animation showin my idea, do you think it will work? i think the only drawback is the hopper.

 

 

[Sale]

I'd bet that gravity alone won't reliably feed BB's when you're firing full auto. How about using a highcap magazine?

 

-Sale

[Hissing_Sid]

I'd bet that gravity alone won't reliably feed BB's when you're firing full auto. How about using a highcap magazine?

 

-Sale

<{POST_SNAPBACK}>

Seems to work OK on the TOP AEGs.

 

Having said that, something like an M60 is supposed to kind of "plonk away".

 

A minigun SHOULD produce an absolute hail of BBs. In order to achieve this, maybe a 9.6v battery is in order, in which case maybe gravity won't be enough?

 

Basically, if the guy is using a standard FAMAS and 8.4v battery then gravity-fed should be ok but if he is planning on upping the ROF then some kind of powered mag might be in order.

In that case, as you say, a hicap mag mechanism might be adapted to feed BBs constantly.

[Meathead316]

hhmm, i thought feeding would eb an issue, give me a coupel fo hours while i adjust my plan

[Buttons]

paintball solution: air assist down the feed tube.

[HardCoil]

The TOP aegs have a complicated rotating thingie that showels the BBs into a long spring and loads them. It's not just gravity feed.

 

Try turning a BB dispenser upside down to find out how well gravity feeding works.

[Hissing_Sid]

The TOP aegs have a complicated rotating thingie that showels the BBs into a long spring and loads them. It's not just gravity feed.

 

Try turning a BB dispenser upside down to find out how well gravity feeding works.

<{POST_SNAPBACK}>

Very true.

 

I suppose it might work a bit better with an AEG vibrating away underneath it.

 

The easiest way might be to have some kind of mechanical shaker/stirrer wired into the contacts for the motor.

 

On the other hand, bastardising a Hicap so it feeds BBs constantly into the hop-up might be the way to go.

 

Main thing is the height. Unless you tipped the hicap onto its side and then created an angled feed-tube to funnel the BBs down into the hop then it'd stick up in the air the height of a hicap AND the height of the hopper above that.

 

If it was me I'd build it with a hopper made out of a box such as this one and then, when I discovered it didn't feed very well I'd get a small electric motor with an eccentric weight on the shaft (such as the motor in the "Operation" board game) and bolt it to the back end of the hopper box.

 

Hopefully that would shake the hopper around enough to keep the BBs flowing.

[Meathead316]

a shaking hopper, i think we may be onto a winner. i was also thinking of using a hose only wide enough to fit one bb in a little bit of room to move but not big enough for a jam, and then making a spiural out of it, then u load the bbs at one end they spiral down and then straight into the hopper in single file, hopefully no jams. maybe i can adapt that odea and use a spring to push the bbs through the hose, or maybe a mixture of ideas, a "hose shaker"

 

other than the feed mech, do you think my plan for the rest will work?

[HardCoil]

Many non-marui AEGs use a spring as a flexible BB hose for feeding into a hop up at weird angles. That works very well.

[Meathead316]

sounds like a plan, all i gotta do it save up £170 for a famas just so i can take it apart

[davecymru]

I've come across exactly the problems that are mentioned here trying to upgrade one of the cheepo CYMA p90's.

 

Unlike TM P90's this CYMA one is gravity fed and the inside of the magazine is just one huge slide with a pretty pattern of ribs by the dispenser hole to minimise blockages.

 

Also this gun uses a Copy of the TM boys gearbox and uses a cheepo Chinese motor that means that it fires (in theory) very quickly at a rate of 400 bb's per sec! But, and this is a MAJOR but, it jams every few shots and is only really usable in short bursts. After upgrading, the gravity feed becomes even more of an issue.

 

I've taken a TM mp5 hi-cap apart and was thinking about grafting the nice clockwork feed into the P90 mag. This would be VERY similar to what you are suggesting, but i think that it's going to be quite tricky.

 

Also on a related note, this might be relevant to your plans to create a homemade real fast firing gun. In this cheepo gun, the gearbox move so fast and the power isn't that good that the BB's are still only halfway down the barrel when it tried to load the next. This is causing a vaccum that SEVERELY slows down the BB in the barrel!! The two solutions i have seen are:

a short barrel. This seems to be the preferred option in the majority of the cheepo guns i've been playing with. People usually say it's due to cost saving, but in fact it's due to performance!

or

and this is the P90 solution. Drill a hole through the barrel one third of the way down! I saw this, thought "ohh quick power gain" and tried blocking it up with tape and the FPS fell instantly! remove tape and all good.

 

So the feed and speed of your planned beast may be even more important than you thought!

 

Keep us informed how you get on

[Hissing_Sid]

Also this gun uses a Copy of the TM boys gearbox and uses a cheepo Chinese motor that means that it fires (in theory) very quickly at a rate of 400 bb's per sec!

Shurely shome mishtake?

 

40 BBs/sec, maybe?.

 

Also on a related note, this might be relevant to your plans to create a homemade real fast firing gun.  In this cheepo gun, the gearbox move so fast and the power isn't that good that the BB's are still only halfway down the barrel when it tried to load the next.  This is causing a vaccum that SEVERELY slows down the BB in the barrel!!  The two solutions i have seen are:

a short barrel.  This seems to be the preferred option in the majority of the cheepo  guns i've been playing with.  People usually say it's due to cost saving, but in fact it's due to performance!

<{POST_SNAPBACK}>

Allow me to apply some maths to this...

 

A normal AEG fires at around 800 RPM. This equates to 13 shots a second.

Let's assume that our supergun actually achieves 40 shots a second.

 

It's firing with muzzle energy of 1J. That means it's shooting BBs at 328ft/sec

Or, to look at it another way, the BB is travelling 1ft in 1/328th of a second.

Let's assume the barrel is 24" long.

The BB will exit the barrel in 1/164th of a second. That's 0.006 of a second.

 

We've decided the gun is doing 40 shots a second.

That's 1 shot every 1/40th of a second, or every 0.025 seconds.

 

So, A BB fired at 1J will exit the barrel in 0.006s and we're only firing a BB every 0.025s.

 

Conclusion: We are firing nowhere near fast enough for one BB to still be in the barrel while another is being loaded.

In fact, we'd need to fire 4 times faster (0.025/0.0060), or 160 BBs/sec before this became an issue.

[davecymru]

My apologies, it does a quoted 400 bb's per min NOT per sec. Now 400 bbs per sec would be impressive!

 

Hi Hissing Sid, thanks for that, but your maths don't or "don't seem to" apply in exactly the same way to the weapon currently on my workbench.

 

Let me explain myself, ( as i have no intention of upsetting/annoying/having a row with anyone, that's why i got married )

 

 

In the case of this cheepo D90 (CYMA seem to call it that rather than P90, probably for licensing reasons):

 

*It's only firing at about 140/170fps (max)

*The barrel is about 12" long with a hold drilled through it at 5"

*if i cover that hole, the fps drops through the floor to round 50fps (note: the inside bore of the barrel is still clear of obstructions.

* I have also tried replacing the barrel with a new solid one (no holes) and got the same really bad fps.

* It is worth noting that the tip of the piston chamber actually goes slightly inside the BB chamber (only about 1 mm), rather that resting tightly up against it (as it does on my ics mp5) and i account the majority of the 'suction' to that.

* I don't believe you took the acceleration factor into account in your calculations. i.e. the bb does not go from 0 to 380fps instantly and is therefore still increasing in speed as it leaves the barrel. But, i think that a more accurate calculation may be difficult without taking into account, bb acceleration rates dependant on bb weight, friction of firing chamber, friction of barrel, point of bb release from chamber compared to piston position, speed of piston, size of piston, acceleration of piston within outer chamber, size of gas release hole from outer chamber. blah blah...

Please,once again, no offence is meant by this, and i would greatly like to see your revised calculations with this factored in as they may aid in my experimentation.

* Also, along with that point, there is also the lag before the cam on the main gear starts to pull back the outer chamber for the next shot.

 

I was only trying to get across performance and operation of this individual gearbox, hop and barrel assembly based on personal extensive testing, tinkering and modifications over the past few weeks with it in my workshop and seeing in practice what it's doing. I'm confident in backing up opinions on this matter due to this and thought that my experiences may be of some use.

[Hissing_Sid]

Do you know how long the cylinder of your D90 is?

 

I suspect it might be a bit shorter (or narrower) than a Marui one.

 

I'm guessing that it'd be shorter. This would result in a shorter stroke and thus improve the ROF and produce a gun with lower power due to a softer spring (which, again, would help with ROF).

This set up, combined, gives you your 150fps and 400 RPM along a 5" barrel.

What's happening is that due to the lower velocity and (assumedly) smaller cylinder, the cylinder is being wound for the next shot before the first BB has left the barrel so you're experiencing some barrel-suck.

 

My calculations don't account for BB acceleration but, as far as I can see, they don't need to.

 

A first BB is fired and it leaves the barrel with a muzzle-velocity of 320ft/sec. The second BB, fired 0.025 seconds later, then follows it.

By the time 0.025 seconds has passed the first BB is 8ft away from the barrel, having left it travelling at 320ft/sec.

 

I do have calculations which could work out how a BB might accelerate along a barrel but, frankly, it's not worth doing.

 

To digress for a minute, it's like regarding calculations for "barrel suck". If you work out the volume of your cylinder and the volume of your barrel it turns out that you'd need a barrel almost a metre long before you get barrel suck on a normal AEG (Assuming minimal bypass, of course).

The barrel on an M4 is only 12" long so worrying about barrel suck just isn't worth the effort.

If you're losing that much air out of the system as the gun is fired then you've got more to worry about that barrel suck.

 

As far as I can see it's the same with this idea, but from a different POV.

 

Basically, you're firing a BB every 0.025s and, at muzzle velocity the BB should take 0.006s to travel through the barrel. Sure, it's had to accelerate from 0 to muzzle velocity but it has 4x the theoretical time it needs to do it so it's not even "borderline" IMO.

 

To look at it from another POV, calculations show that a cylinder might have 3 barrel volumes of air in it.

So, from the moment the piston moves forward it expels enough air to propel the BB out of the barrel, then enough air to do it again, hen enough air to do it again.

It must do this before it completes its stroke and begins winding for a 2nd shot.

What are the chances that the BB is still in the barrel at this point?

 

If it is then, again, you've got bigger problems than your ROF. You've either got a seriously mismatched barrel/cylinder or you've got a massive air leak somewhere.

 

Please don't get the impression that I'm picking an argument BTW, replying to your posts has been the most interesting and thought-provoking thing I've done all day.

 

And I'm not usually so full of facts and figures. I just thought it'd be interesting to try to verify what you were talking about.

[davecymru]

humm, that's raised a few interesting points that would directly affect this concept.

 

So, applying these ideas to the original topic.

 

If you were to modify a famas, then the whole gearbox,barrel couplings would have to be pretty accuratley machined/moulded in order to mate up at exactly the right point in time in order to avoid the problems we've described above. Ignoring the magazine issues at the moment as i'm convinced you could mate a clockwork hi-cap internals into just about any container you wanted (there's a topic for another thread!)

 

Here's an idea.

 

If you're going to experiment with this concept and run with it, how about changing the doner weapon to one of the cheepo ones (£30ish) while you refine things and then changing out the gearbox for a good quality one when you're confident that you're not going to mash it up?

The gearboxes in these cyma guns are a standalone module much like the more expensive ones (but made of cheap plastic) rather that relying on mouldings on the weapon case to function like the mini series.

 

Also, in case you're trying to source your barrel(s). As i mentioned elsewhere, B&Q do a cracking aluminium tube with a "just" over 6mm interior for a few pounds per meter that i've used with good effect in the past.

[Pendra]

I made a calculation sheet. From this you can see lots of things. The colored cells contain formulas, don't write into them or you may mess up the calculations.

Calculations

[davecymru]

Impressive! I'll take some accurate measurements tonight and plug them in and see what i get!

[Meathead316]

i got some of that pipe from b and q a few weeks ago, is awesome stuff! i think i will get the famas, make a copy of the bit of the body where the mag goes (some how), so that i can take a mag, cut the top off and put the topinto the gun as normal so it feeds 1 at a time, then make some sort of hi cap feeding system to go into the mag bit.