ibo7

Seconded they are very good guns, but it is next to impossible to get replacement parts for maruzen guns.

the Maruzen NBB P99 is as quiet as the Mk23 and actually a nicer pistol to use too. However they are only made in small ish batches so you might have a little while before one comes up 

A stock MK23 and socom supressor yes. If you start adding 3d printed stuff and fart flaps the MK23 is probably a touch quieter. But as I say I use the longer supressor on mine. It's quite nifty with proper baffles and cones inside. But we tested it at the receiving end and you couldn't tell someone was shooting at you. 

Even if you did, would they take the hit 🤬

Psst... it's airsoft, prob wouldn't hit what you aimed at anyway. will still be fun for plinking tho.

I think that's problem with all Makarovs as the sights are tiny and it's a relatively small pistol.

https://www.actionhobbies.co.uk/products/ics-pm2-non-blowback-pistol-black/
 
https://www.firesupport.uk/ics-pm2-non-blowback-pistol-nbb-black.html
 
 

It's worse. Even heavier than the mk23 and double action only

The ICS PM2 makarov is as quiet as the MK23. The only downside is that it doesn't have the after market support. The MK23 can be made to out perform everything. The PM2 scores very much on style and is a lot smaller. It takes a CO2 bulb but will chrono no problem. It has a very heavy trigger if you want to rapid fire it so it's better to cock the hammer with your thumb. If it had the same after market support as the MK23 it would probably be the go-to sniper backup but that doesn't look like it's going to happen. Still if you want something smaller than the MK23 and just as quiet, but don't need it to perform circumcisions at 70 meters it's an option. I like mine. I did buy the larger suppressor for it. And the magazines are little sticks that are very easy to store. 

So exactly like a mk23 then 😂

Those can be fun but my god is the trigger horrible

The quietest pistol you'll ever have.

Oh, is this for garden use rather than skirmishing?  The ASG Ruger is quiet, can be had for under £50, but comes in at 1.3J.  You can verrrry carefully file down the hammer to reduce that if you want to skirmish it.
 
If this is purely for slinging BBs at short ranges, you could also consider an AEP. Any of the CYMA AEPs will make more noise from the motor than the pop from the tiny cylinder.

The Makarov isn't that quiet. It has quite a loud pop from the CO2. Quietest is the TM MK 23 with silencer attached. Used to run around Copehill at night and my giggling as I shot people was louder than the pistol. 
If you fancy branching into small form factor rifles instead of pistols, I reckon you could make a TM VSR 1 pretty quiet.

Have you considered revolvers like the Dan Wesson 715 and th orange of Smith  & Wessons from Umarex.
Western Arms used to do some 1911 NBB which can still be found second hand occasionally, they are really Seecamp Double Action Converted 1911's.  There is also th excellent Tokyo MArui Range of NBB 1911 amongst others.

The range of nbb pistols is definitely a fraction of the size of the gbb market, I guess most people prefer blow back despite the disadvantages (and I'm certainly one of them). I don't think there are any nbb 1911s available but there is a Makarov
 
https://www.actionhobbies.co.uk/products/kwc-co2-makarov-airsoft-pistol-nbb/
 
I've no idea what they're like though.  I've never owned one as they're way too overpowered for skirmishing at over 1.5j but if you only want it for plinking that isn't an issue 

The split slide versions of Hi Capa's might be an option if you're set on 1911 ish designs. Less moving mass, so inherently quieter.

All non blow back pistols are quieter than GBB ones due to less moving parts. The mk23 just gets mentioned the most as they're the most easily available. Also being able to take a suppressor allows them to be even quieter.
 
Is there a specific model you're interested in?

The quietest aeg that you can make is most likely something like an Aug or previously mentioned an F2000. The gearbox is very enclosed and the areas around it can be padded out with foam and etc for further sound proofing. 
 
Although that's only if all variables between all the different aegs are constant. 
 
Noises come from every single part of the gearbox. The motor, gears, piston, piston head and cylinder head slapping and other bits. 
 
The fact is if you take 10 aegs out of the box. All the same model and from the same manufacturer you will find each one sounds different 
 
 
 

This. Also it's worth remembering that we're using things which are relatively mass produced and put together without any precision or tuning so 2 identical guns could easily produce different noise levels depending on whether they were put together at the start or 10 minutes before the end of shift

I got around that issue by short stroking the sector gear by two teeth, giving the whole thing a damn good shimming and fitting a high speed Warhead motor.
 
 
 
But I have more money than sense so your mileage may vary!

You are not going to get a good answer, no one has done any sort of noise comparison of a wide range of stock AEGs (or any that I can find). Even a custom build gun is going to make a minimum of difference to that actually decibel level from a gun. Mods will change things like pitch a frequency so the sound may not travel as far.

Suppressors do work very well on hpa/gas non-blowbacks where theres minimal action noise.

G&g ump with the suppressor; one of the few suppressors that actually work!

Out of the box the Cyma M14 is a particularly slow cycling, noisy gun. Most new ish M4s are pretty quiet if you run them on an 11.1v battery and make sure you adjust the motor height properly.