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Mini MP5 Upgrade Experiments

Mini airsoft guns are the cute and light versions of their more expensive counterparts.

They can be handy for fiddling with, and I was interested in seeing just how much tampering I could do to a mini MP5 and whether the performance could be enhanced usefully at all. I wound up fitting extra batteries, an external spring-loaded magazine, and modified the spring.

What Was Done

  • The unreliable hopper feeding mechanism has been replaced with a 38-round spring-loaded magazine (the unpainted brass tube in the picture above).
  • A mini normally uses 4 AA cells. Two additional AAs are concealed in this gun for a 50% increase in RoF and better battery life.
  • The main piston spring is replaced with a slightly weaker spring for an increase in RoF. A heavier spring could also be introduced for an increase in FPS (muzzle velocity).
  • Stress on battery power is reduced (and life of batteries extended slightly) by a buffer capacitor on main power supply.
  • Useless safety switch is disabled.

The gun used for modification was a HFC Mini MP5. The HFC minis are actually quite good quality to begin with (so far as minis go).

Here are some sound clips:

Stock Mini with 4xAA Cells
Mini with battery upgrade (6xAA Cells)
Mini with battery upgrade and Max RoF Spring

Upgrades and Mods, One by One


To the rear of the piston and gears, you can see the 4700uF 16V electrolytic capacitor. It buffers the power supply (the batteries) so that the current draw on them is smoothed out somewhat. In practice, I’ve not measured what difference it makes (it wouldn’t be much). But there was room for it and the theory is sound, so I put it in. 🙂


The fake magazine and the cocking tube have been re-commisioned into battery holders. Custom brass tube assemblies hold the batteries in place in addition to the 4 AAs in the handle. This increases the voltage (and on-demand current capacity) of the power supply from 6V to 9V.


It’s not a visible change, but the stock spring was replaced with a slightly weaker spring to increase the RoF (rate of fire) of the mini (in addition to the RoF increase from the battery upgrade). It could just as easily have been replaced with a stronger spring.


The hopper resevoir is somewhat reduced by the battery and wires in the cocking tube. But the hopper system has poor reliability at best when the hopper is full. I replaced the whole hoppper system with a non-removeable magazine feed. The magazine comes out of the side of the MP5 and runs along the side of the gun. It is spring-loaded and holds approximately 38 BBs.


This was easily accomplished by removing the return spring for the safety lever and gluing it in the FIRE position with plastic cement. Disabling a safety feature isn’t normally something that’s first on my list of things to do, but the safety catch on this thing really bugged me.

Tools and Materials

If you plan on trying your hand at some of this, it’s recommended you have the following available:

– Dremel tool with variety of tips
– Brass tubing of various sizes
– Tube cutter
– Soldering iron and electronics supplies
– Epoxy Putty (great for bonding and molding)

Assembly Shots

Sorry there is no step-by-step, but these pictures should show you some of the finer points and give you your own ideas.

The trickiest was the magazine (close second is the battery holders). I had to put two 90-degree bends perpendicular to one another in the plastic joiner between the magazine and the breech. This was so the magazine can lay flat against the gun. It was either that, or have the magazine stick straight up (Bren gun style) or out the side (Sten gun style).

Stock HFC Mini MP5, open.

Additional Battery #1 – inside fake magazine

Additional Battery #2 – inside cocking tube

Making a max RoF Spring (i.e. a lighter spring) by cutting off a few coils, then sanding the sharp edge.

Making the New, Spring-loaded Magazine

There’s a lot more than meets the eye, here. Mostly inside the top tube in the leftmost pic. It contains the spring and everything.

The tube (bottom of the pic on the left, and visible on the gun in the pic on the right) is filled with BBs, then the tube with the spring is fitted on. Think of it like this: instead of putting the BBs into the magazine, it’s more like you’re putting the magazine around the BBs.

Also, in a Mini the BBs feed from the top – from a funnel-like gravity feed – into the breech.