DIY Magprime for older blasters

[SirSmoustache]

Hi all!

I’ve recently bought my first “modern” blaster with the Sijun MCX 89-2 and after experiencing magprime, I don’t think I can ever go back. With that said, I have my previous unit the J16 (HK416D) which doesn’t have that and was wondering if it’s possible to jury-rig a magprime system into it?

I’ve looked up videos online but the fact that they’re all posted over 6 years ago worries me a little. Are the methods that was used with those diodes still the go-to way of doing it or are there better ways of doing it now?

Thank you in advance!

P.S: For reference, I mainly used Low Guido’s Chop Shop Mag Prime video

[DChapo]

Magprime really has not changed at all, they practically all use a diode of some sort wired to a switch linking the mag terminals, the diode I think was just so it doesn’t burn out the switch/motor sending the full voltage to the mag.
That’s my understanding of it at least, there are ppl here with a better understanding of electrical components than I.

[SirSmoustache]

Thanks for the confirmation!

Now my homework is to figure out how to fit that switch into the upper receiver and also which wire to attach the diode to. Cable management seems relatively okay so far but not gonna jinx it yet

[DChapo]

Cyma fix a tiny switch to the side right where that fo-bolt carrier plate sits so when you pull the charging handle it gets pressed.

Others, like JingJi, put a switch on top that rides inside the charging handle but gets pressed when pulled.

I’ve seen a kit that replaces the forward assist button with a micro switch and is wired thru the side into the gearbox.

The least stuff around way is just a FCU with auto mag priming, but you loose the function of having to rack it

[DocBob]

@SirSmoustache
@DChapo
The diode is required as a one way valve to prevent the mag prime activation from creating a feedback loop through the rest of the electrical system.

Funny story…
Remember my poor old “Little Pig” 430+FPS Gen8 test mule on the old Forum?

Gelblasters never had any Electronics way back then, no MOSFETS, mag prime, ETU/FCU’s etc, so I always had to build my own mad scientist electronics experiments for my Frankenstein Gelblaster builds!

This was encouraged after purchasing my SKD/STD M4SS and HK416, the very first ever blasters that were fitted with FCU’S

(Yes, @Tiger and I spent too much time at the back table discussing stupid things as well!)

Biggest destruction I ever had, was after building and installing a home made MOSFET and Mag Prime to test out in the Little Pig.

It was sitting at 400FPS at that stage, connected the battery, hit the Mag Prime…
and all hell broke loose!

I was so caught up with the bazillion other technical electronics details, I FORGOT TO ADD A DIODE between the mag terminals and the trigger wire, which meant that the poor old Nylon Gen8 took off flat knacker at 400FPS/FULL RPM, as they never had Semi Auto, and COULD NOT stop the thing other than scrambling to disconnect the battery!

Before I was able to access the battery in the buffer tube… kaboom, crash, bang, screech, wire smoke and then silence and stench of burnt electronics and smoldering grease and oil

It wasn’t until after the smoke and destruction had cleared that it occurred to me that I could have simply ejected the magazine to break the circuit, but never realised that was the issue until I eventually opened up the pile of destruction and noticed that the missing diode was the cause.

YES … it’s possible to add a mag prime.
NO… don’t do it unless you get some proper electrical advice!

Anyrate, shoot me a message @SirSmoustache and I can certainly tell you how to properly set up a mag prime for ya!

[RokSolid]

We never got the j16 here, so I can’t say with much certainty… but I think they are the newer more v2 styled jin ming gearbox and receiver- with blowback?

If it has blowback… you may have a difficult time finding a location for the switch and wiring to live in.
The original gen9 gearbox had a spot for a 3x3x3 microswitch to have mag prime but routing the wiring so it didn’t get snagged and crushed when assembling the upper receiver was a pain.

All the T238 ETUs have auto mag prime, which will detect a mag is connected and prime it, which might be easier to install and set up, but it not for everyone. Depends on how you like to run your blaster. I generally like it, but only have it run fo 0.5second. The newer basic version is fixed at 1 second, which is a bit long for my liking. The more expensive versions can be programmed for 0.5sec increments up to 3 seconds.

[SirSmoustache]

It wasn’t until after the smoke and destruction had cleared that it occurred to me that I could have simply ejected the magazine to break the circuit, but never realised that was the issue until I eventually opened up the pile of destruction and noticed that the missing diode was the cause.

Hey I’ve been there, panic does that to you. Difference is that it was a water valve in my house a few years ago

Anyrate, shoot me a message @SirSmoustache and I can certainly tell you how to properly set up a mag prime for ya!

I’ll take you up on your offer if you don’t mind. I’m still away on a relative’s wedding and probably gonna take a look at it next week. I still need to source a few things like the button switch, spare wires, and the actual diode itself lmao.

[SirSmoustache]

On this note no, the J16 I have were the old pre-2024 ones. Basically an HK416 but with a J9 gearbox and nothing else. I would love to get a 2024 J9 with blowback and magprime since come on, it’s a J9! But my usual importer friend ran out so going to have to wait for a restock on that

The original gen9 gearbox had a spot for a 3x3x3 microswitch to have mag prime but routing the wiring so it didn’t get snagged and crushed when assembling the upper receiver was a pain.

Either I’m blind, mine doesn’t have it, or there are even older Gen 9 gearboxes out there. Here’s a picture of when I disassembled it for reference.

But yeah looking at it again the tricky part is definitely going to be where I’ll be putting the extra wires and diode. Not to mention where I’ll run the wires going to the switch. Worst case scenario I’ll drill a small hole in the gearbox to allow for some space and room but I’ll have to check the fittings again when I’m home.

[DChapo]

In these bits I believe, that’s were other gearboxes have them at least, but they’re usually a bit bigger.

[DocBob]

Yes, that’s the spot!
You beat me to the reply

[DocBob]

This little diagram will give you an idea of why and where the Diode is needed, even if you ignore the Easyfet board and just rethink the wiring running straight from the trigger/motor/battery/prime switch instead.

You can see how the Mag Motor becomes the trigger switch when activated, and once running…it won’t let go!

I can redraw it in the simple wiring diagram that you require to make it easier if you like.

[SirSmoustache]

So thats what it’s for! I’ve seen the same thing on my (sold) BF P90 V4 and have been wondering what the purpose of it. But yeah it is quite small, I’m going to have to find a small enough switch for that online.

Thanks for pointing it out!

[DChapo]

I think that cutout that’s on top of the standard J9 gearboxes are for a similar thing, like how the SLR gearboxes have a switch rather than a button that slots into the charging handle.
The J11 or 12 gearboxes, the one for the HK that looks almost identical to a Gen8, had a switch up there too.

[SirSmoustache]

Hey thanks this is perfect!

I’m using this as a reference for when I’m back next week. Here’s hoping my gearbox doesn’t experience a rapid unscheduled disassembly since my skills with a solder iron is sketchy at best

That aside, so if I’m understanding this right I’ll just have to mess around with the black wire ONLY and turn it from basically this (sorry for the bad drawing, I barely passed physics class in high school, especially when it comes to circuits )

Into something like this right?

Button connects directly from the battery wire and the black diode is connected to the wire going to the motor, creating a Y-shaped solder which I’m going to need to research a bit more on since that doesn’t look safe but what do I know.

I’m also worried on the fact that I’m using an 11.1v battery though. Should I also add a resistor somewhere or is it fine without one?

[SirSmoustache]

I’ll inspect it closer the moment I’m back home and on my bench. Not going to lie I can’t wait to try it out!

Worst case scenario I’m pretty sure I can figure something out to make sure the button/switch stays in place. From the looks of it I might want to consider replacing my nylon charging handle to metal since that part is going to be used much more!

[DocBob]

You have nailed it in one mate
Yes the diode prevents causing a loop from the negative side of the battery bypassing the trigger through the prime switch.

Be sure to orientate the diode so that the white stripe/gate faces to the trigger side of the wiring, and can be connected with the negative side of the trigger switch as the shortest route to run the wiring.

IMPORTANT: Be sure to test the polarity of your wiring before starting any work.
Some blasters run +pos trigger switching, where others run -neg switching.

The only mistake in your diagram is that there’s only one wire that runs through the trigger, either +pos or -neg as mentioned above.

If you imagine that the black wire is the trigger wire, and the Red wire runs directly to the motor/mag terminals, this is how most blasters are wired, and also makes it much clearer as to how the power loop is formed by the prime switch if a diode is not fitted.

It can be much easier to hot glue a small momentary switch into the Forward Assist cavity in the side of the receiver shell rather than trying to set up a fragile charging handle activated switch, but it is up to you which method you prefer, and we can go from there