BEST Fittings – Dedicated SIG Sauer MPX / MCX High Pressure Air (HPA) Upgrade. NEW 1100 psi Output Regulator.

(3 customer reviews)


SKU: 88S3-HP03

Dedicated SIG Sauer MPX / MCX High Pressure Air (HPA) Upgrade. Complete with HPA Adaptor, 13Ci 3000PSI Bottle, Butt Stock and Quick Coupler Socket for filling the tank.

NEW 1100 psi Output Regulator.

Please read the full description below for important additional information.

Questions about this item? Contact Us


BEST Fittings have responded to customer demand and produced this bespoke kit to convert your SIG SAUER MCX or MPX to run on High Pressure Air (HPA).

NEW 1100 psi Output Regulator.

With all of the components included in one low cost package, all you need is a dive cylinder or stirrup pump to fill the tank. No additional parts will be required to complete your conversion.

Weighing in at only 650g there is hardly any difference when compared to a full 88g cartridge and the standard butt-stock. (565g)

All of the parts of the kit are also available separately should you already have purchased one of our adaptors.

Tank Only.

Adaptor Only.

Butt Stock Only.

Filling Connector.

During trials, we discovered that fast fire capability was not so much about pressure, but more importantly it’s about air flow. We trialled our own standard Air Magnum to Co2 or HPA adaptor with actuating pin, and found that even increasing regulator pressure to 1100psi, the fast fire capability was very unreliable.

We then re-designed the HPA Adaptor with a fast flow actuator, and fast fire capability with a standard 750 – 850 psi regulator output was achieved. A great result in our book!

However, following up on customer feedback, and further product research, we have concluded that although fast fire capability is possible, a drop in performance is also noted when running at the lower 750 – 850 psi regulator output, so we now supply our kit with a NEW 1100 psi Output Regulator.

As a service to customers that have already purchased one of our kits with a standard 750 -850 psi regulator, we are happy to modify your existing bottle and return it to you. If you would like to take advantage of this service, simply send an email to: [email protected] and we will explain the procedure for returning the bottle.


  • We recommend that one or two drops of silicone oil be placed into one of the holes in our bottle adaptor between recharges to maintain good internal lubrication of the action. DO NOT OVER LUBRICATE!
  • The weapons were designed for use with Co2, and the operating pressure of Co2 varies greatly:Co2 @ 22ºC is approx 855PSI
    Co2 @ 30ºC is approx 1050PSIThe regulators are fitted with a Burst Disk on both the Low and High pressure side of the regulator. In the highly unlikely event of the regulator failing, the low pressure (1.8KPSI) burst disk would rupture and vent the bottle contents through the Burst Disk Port. It is commonplace in industry to use Regulated High Pressure Air bottles in Co2 applications, as the gas is far more stable under changes in temperature.
  • You may invalidate the manufacturer warranty in fitting this upgrade, and do so at your own risk.

SKU: 88S3-HP03

3 reviews for BEST Fittings – Dedicated SIG Sauer MPX / MCX High Pressure Air (HPA) Upgrade. NEW 1100 psi Output Regulator.

  1. gym25 (verified owner)

    Converted my Sig MPX using this kit,It works very well and only adds a bit to the weapons overall length.The supplied stock cover fits well and the valve is clear and easy to see.
    The valve convertor fits like a glove.The bottle is top quality.
    I haven’t noticed a drop in round velocity and the unit came in record time and was extremely well packaged..
    Best Fittings are an absolute pleasure to deal with and I would use them again without hesitation and would recommend them
    Sam and his staff are the gold model with regards to customer service.
    I have absolutely no connection with this company just a satisfied customer

  2. niatrinh (verified owner)

    The conversion kit is very well crafted and works extremely well.

    I found the unit adds around 2 inches to the overall length of the rifle which meant i had to move the scope back a little to accommodate, not really an issue.

    Initially, the kit was charged with a stirrup pump but after a couple of refills, realisation dawned on us that this isn’t the way to refill the bottle, its waaaaay too much hard work. So i have since purchased an air compressor which fills the bottle in a few minutes. Now the gun can be used indefinitely without needing to purchase expensive CO2 bottles again or break one’s back pumping it.

    The previous reviewer suggested they haven’t noticed a drop in velocity but unfortunately i believe i have. We did a test fire using an 88g CO2 bottle, 15m distance and a used soup can and you can pretty much pepper this thing with holes from all directions! However, when used in conjuction with the HPA kit, we found that most – if not all – pellets were not able to penetrate the soup can. On an average day like today when its around 22C outside, CO2 is supposed to be around 850PSI where as the kit is supposed to be 1100PSI so i cannot figure out how or why CO2 shots would blast holes in the can while the kit can’t. The only logical conclusion is that there is less power coming from the HPA kit.

    I have called and left a message with the technical team to discuss and try to resolve the issue (if indeed there are any). For the time being though, i can only give this 4 out of 5 stars since the experience hasn’t been 100% satisfactory. Hopefully we can resolve and i can edit to a 5 star.

  3. Stuart Christie (verified owner)

    Bought for my MPX as CO2 causing freeze up’s during rapid fire.
    I have read that power can be down on HPA but quite honestly and not having a chronograph I can’t tell.
    This isn’t a hunting gun it’s a target and can killer. Even forgetting cost savings HPA is a much better solution for this gun and it’s rapid fire capability.

    No complaints over service, quality or price from Best Fittings.

    Thank you folks!

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