Gas barrel length: how?

The longer the barrel, the higher energy creep. With gas guns, you'd have to have a 20 meters barrel to get into under-volume issues

Ok, what is energy creep? I have heard that phrase befor.
With paintball, I know an 18” barrel gives slower velocity than a 12” barrel.

Energy creep or joule creep, basically the heavier the BB you use, the higher the energy output. Assuming you have the correct barrel length and barrel to cylinder ratio.

In your example of paintball, that particular system has too long barrel, resulting in energy loss.

In typical gas airsoft guns, even the stock configuration is "over volumed", resulting in you gaining energy the longer your inner barrel is. Pretty much you can install the longest barrel in the market currently (I see 800mm as the longest so far) and still gaining energy with whatever BB weight you use.

As for your main question, it is still outside my realm of understanding so pardon me for I couldn't give inputs about it

Paintball is a little different. The gas systems are designed to be highly efficient in a certain zone of pressure and volume. Usually the stock barrel on most paintball guns is 12 or so. (Might be different now, been a long fat min since I played.)

In airsoft, while HPA setups are "common", they are designed and put out to use far more gas then they need to to compensate for a variety of circumstances in builds.

GBBRs are different from HPA and AEGs as they use compressed gas (CO2, green gas, mapp/pro, propane) in there liquid forms to load into magazines. They don't really have any kind of regulation in them other then tuned springs.

For Marui system (negative pressure) GBBs - which is practically every GBB outside of GHK - the nozzle valve will almost always be closing the nozzle exit only after the BB leaves the barrel (IE the gas released will always route into the barrel first). Unless your inner barrel is some really unholy long kind, you will practically never reach to the point where you run into negative Joule creep.

That is all really cool, and supports an idea I have for a good sniper/sidearm pair. A rifle build, that uses gas blowback pistol mags, but is bolt action, while carrying a sidearm that uses the Same mags. Not only would it he extra barrel length increase velocity, but all gas would propel the bb in the rifle, rather than work the blow back, and the pistol could be stock velocity for close combat.
Think de lisle carbine if realism matters.

My friend has a similar idea about it, something about a sniper using gas pistol mag.

But it won't be a bolt action in its truest sense. Instead, his idea is to give TM Mk23 a bolt action look / bodykit, and the bolt cocking action would makes it cock the hammer. The idea is by cocking the "bolt", we will have a single action, giving us a light trigger pull. And the side arm would be a normal standard TM Mk23.

Kind of a silly idea if I am being honest, since for me this idea is somewhat destroying the whole point, that is having a bolt action primary and a side arm with rapid follow-up shot capability while all using the same mag.

But nevertheless, I am quite intrigued by this.

But why not get an HPA VSR or a KJ M700 if that's the case? The KJ M700 is literally the thing you are trying to do regarding to a non-blowback BASR.

That way, we will then have 2 different types of magazine, something my friend is trying to avoid. As for KJW M700, he held it before and it is too heavy for his taste anyway. TM Mk23 is the best match for him so far, with only downside is the heavy trigger. That's where his idea came from

I mean, if you're doing sniping in a serious sense you're not gonna run sniper ammo in a pistol (if just to save a few bucks of ammo and for faster projectile speeds in closer quarter combat) in the first place. Using identical magazines just seems like adding more trouble than it's worth.

The hard part about making a bolt action (non-blowback) gun work with GBB mags is gas flow cutoff.

In gas blowback guns, the gas flow is controlled by the valve locker disengaging when the bolt / slide has travelled back far enough to cycle, while bolt action guns rely on the gas pressure and valve springs counteracting the hammer strike. It's a press vs a hit.

Not to mention this idea implies using sniper ammo for the pistol, which would make the pistol shoot waaaaay to slow if keeping without CQB joule limits

Yeah, I am mostly familiar with paintball valves.
I see this as similar to a stacked tube pump action pb gun, but backpressure at the valve may not be enough with a green gas mag, or the spring and striker weight would need to be balanced so that the momentum of striker opens the valve, but the back pressure can close it against the striker spring pressure.
As to bb weight; 2 possibilities; 1) run mid weight bb, a compromise in both guns, or carry extended mags for the pistol to shoot your way into or out of a situation. Perhaps one short mag in pistol with lighter bb when holstered, and the heavy bb short mags would only go in pistol in a less than ideal situation.
But my research is coming up with more and more I don’t know, so I appreciate all your wisdom.

Well, you got the point right on using the same BB weight for different scenarios is not ideal. He uses 0.3g though, maybe it is still fine in close combat scenarios since he's camo'ed and all, and he rarely gets to enclosed spaces where we need speedy reactions