M4 w/ Two Upper Receivers Project... Advice?

[dizzale21]

Inspired by some of the builds in the Media section, I've decided my airsoft project for the new year will be an M4 with two upper receivers - one with a carbine/CQB length barrel and another with a DMR length barrel.

My question is this: does anyone gave any suggestions for a good base M4 platform to start with? Ideally, it would allow me to swap fps with ease, since I'd like the carbine upper to shoot around 375-400fps and the DMR upper to shoot around 450. I guess it would have to be rear-wired to a crane stock (or perhaps I'll run a lipo in the buffer tube), since a battery in the front end would be difficult given I'll be swapping the uppers.

I know ICS has the split-gearbox thing going on, but I also hear that they have a lot of proprietary parts, which I'd like to avoid. Suggestions, anyone? Classic Army? KWA?

Furthermore, does anyone have any suggestions on parts (stocks, RIS units, barrels, optics) in general that I should be looking for?

I'll have cookies for anyone with advice.

[Qlong]

Most of the AR builds with extra uppers are with GBBRs, being easier to swap uppers by the push of 2 pins.

If you are doing an AEG build with multiple uppers, the battery has to fit into the stock and your stock/battery options are more limited. To be able to switch between two variations of FPS in an AEG you are pretty much stuck with a PTW, ICS, or a V2 gearbox with the ability to quick swap springs.

GBBRs on the other hand will require you to have 2 bolts carriers with the ability to be adjusted to your specified FPS or you can have just one if you don't mind adjusting the FPS when swapping uppers.

[TokyoSeven]

Swapping uppers is one thing.

Having an AEG with swappable uppers and an ICS style mechbox is another. Its entirely possible but I would imagine very expensive due to the fact that ICS guns require ICS bodies I believe or at least some modification to fit the mechbox if you are not using an ICS body.

Alternatively you could attempt to source a prowin mechbox if you have a ridiculous amount of funds available to pour into this project and have oodles of money and time to mod the receiver and extra uppers to fit the mechbox as well.

And finally you may wish to look into building this project off an ARES armalite platform. While its still expensive, ARES also has a quick change spring feature as well.

[Qlong]

He does not necessarily have to go with ARES, recently Chinese manufacturers released gearboxes with quick interchangeable spring systems for around $30.

http://www.khmountain.com/airsoft/pr...oducts_id=2465

[bareass]

any word on te quality of that mechbox?

[m102404]

Going from a CQBR to DMR might be a bit problematic....accounting for the "extreme" difference in length between the two setups (i.e. that's stereotyping...but for whatever reason guys seem to think they need some house ladder length rifle setup to have a DMR).

However, here's a setup that has worked well several times. It doesn't exactly fit what your requirements but it might give you some ideas and a starting point. This is for "normal" AEG M4's....not specific to recoil systems, GBBRs, etc... Also, the following was usually done to have a switch gun setup for indoors CQB and outdoor field games.

- metal bodies that slide apart front to back (i.e. CA/KA/DBoys style) are easiest to work with. Hurricane/Guarder style bodies that have tabs and capture the mechbox in the upper receiver are not suitable since you basically have to disassemble the whole rifle to switch it around (then you might as well just keep a second rifle).

- outer barrels that can take an extension or be elongated with a supressor are worth considering since then all you need to do is swap the hopup/inner barrel as a complete unit and alter the outer barrel accordingly

- with a CQBR length rifle setup with a M4 cylinder, good compression, bearing spring guide, bearing piston head (some say two bearings aren't necessary), short tightbore (say 247mm) and a good hopup unit/rubber....often you'll hit 345-360fps with a M100 spring. Once the hopup is set that's often just under the 350fps indoor. If it's still just a touch high...try a stock 6.08mm 245mm inner, that'll bleed off a bit of FPS and for a lot of CQB you don't need a tightbore.

- take the above setup and switch to a SIG/M4 length tightbore barrel (i.e. 363mm), good hopup unit/rubber and you often see the FPS jump to 385-390fps. While it's not at the magic 399.999999fps for most 400fps MAX limit field games, it's fine in practice.

So...really, what have you done?
- you've setup a nice M4A1 length rifle and have it working efficiently for that length
- then you've "compromised" the setup of it by short changing the inner barrel (and you can fiddle with the hopup a bit) to essentially be less efficient...in order to shoot a different FPS
- I have not done what you're aiming to do specifically...but I strongly suspect that the same approach can be applied.

The techie upsides of a setup like this are:
- two setups doing different things
- same grips/slings/accessories
- same mechbox setup (i.e. same motor balanced to the gear ratio and spring strength)
- same battery
- same ROF (although in CQB you may tend to shoot more semi than full auto)
- minimal storage space...especially if you get a barrel that can take extensions and/or use a suppressor.

So far as what specific parts to use....my experience has taught me not to buy clone sh*t. Stick with parts from Modify/Prometheus/G&P/(and I like guarder clear rubbers and their infiinite torque up motors...but other guys poo-poo them)/etc...for internals. Stick with KA/G&P/Madbull parts for externals. Madbull and Modify have some great bang for buck parts. I will not list off all the bits piece by piece....do your own shopping.

[dizzale21]

Thank you all for your replies! I appreciate your input.

I'm not interested in building a GBBR, and I'm leaning towards not using an ICS based build for the reasons TokyoSeven listed. A PTW is a little out of my price range... but the ARES quick spring swap thing does intrigue me. I know that STAR (which, correct me if I'm wrong, is now ARES) makes a G36c which a unique open-backed mechbox that allows access to the spring simply by folding the stock... am I correct in assuming the ARES m4 functions similarly (I suppose once you separate the receivers)? Does anyone have any suggestions for any reputable retailers that sell ARES M4s?

Above all, though, I'm really intrigued by m102404's idea of building the DMR upper and using "less than perfect" parts to get a lower FPS for the CQBR upper. It's an idea that didn't occur to me, and has a ton of advantages!

My question: I know in your example, m102404, that you used a CQBR and mid-length carbine as your different "uppers"... but could this work with a mid-length carbine and a DMR? To put it differently, could I build the mechbox and DMR upper for 450fps, and then "downgrade" the fps using a specific hop-up and inner barrel assembly for the mid-length carbine that will bring it down to 375-450 fps?

Forgive the newbiness of my questions, as I've only been in the sport for less than a year and the mechanics of fps and gun building is still beyond me (this is why I wanted to start this project; to learn more about how guns work... previously all my gun work was done by local gunsmiths).

Thanks again in advance for your help!

[Kos-Mos]

Best option for AEG would be ICS.

A base gun would cost you about 300-350$ with a metal receiver.
A second upper mechbox and upper receiver is about 120$ for both.

Then you only have to get any front end you want, you have a metal AEG with swap capability and the split mechbox is a lot more sturdy than any other V2 style mechbox.

ICS uses proprietary receivers, mechbox and anti-reversal latch. Most of the uppers are threaded in SAE threads (G&P) and the stock prong require a little modification to accept any type of stock (or it can take any stock that fits over the stock buffer tube). The hop-up unit is also special, but I strongly recommend you to keep it. It is way better than any M4 hop-up out there, including the Ultimate Hop-up.

[Wantabe_Warrior]

Will DBoys/CA/KA uppers fit SRC gen3 pro HK416 lowers? I was already thinking about a double upper rifle system, it's for performance and looks.

[TokyoSeven]

King Arms uppers are design to work with King Arms lowers due to the functioning bolt catch gimmick they have.

[Kos-Mos]

The other way around.

Uppers fit properly, lowers don't unless you remove the bolt catch.