So I keep hearing how short stroke and long stroke piston rifles are heavier than direct impingement ones, but how much heavier are they? How much weight do short stroke and long stroke pistons tend to add to a rifle? Like how much does the individual piston system weigh?

The piston weighs a little. The gas block weighs a little more. The barrel is usually heavier because the piston has more effect on accuracy of lighter barrels.

The piston guns are a little heavier.  I think it is a relatively small increase and that the real difference for a noticeably heavier rifle depends more on the barrel and the foreend used than anything else.  For example, earlier HK416s are heavy but the rail and the gas system, which get a lot of the blame, are simply not all of the problem (a 416 rail by itself is actually relatively light aluminum considering the integrated picatinny rails), thus the push in later models for sought after lighter barrels.  By way of example, I have a newer DI build with a Geissele rail and FN 14.5 barrel which is much lighter than a DI KAC SR16 E3 CQB build with URX3.1 rail -  I think the weight in the KAC is in the barrel and the rail system.

It definitely depends on the system. I have 2 ARs I built that are EXACTLY the same, but one has a SA piston system and the other has a titanium carrier w/ an MOA gas block. The difference in weight is about 6 oz.  They're both however, sub 4 lbs without any irons or RDS.

I have a PWS long stroke system that is my heaviest setup. My BRN180 upper is light in comparison however. My MCX setup is heavy as well, but BRN180 has an extremely small gas block for piston, but retains heaviness in the carrier and the hybrid profile barrel.

A mid length Superlative gas block weighs 2.50 oz. and the op rod 1.25 oz.  The monolithic bolt carrier probably weighs a tad more than a standard "full auto" DI bolt carrier.  For sure the gas block from my Adams Arms system with the 1913 rail on top weighs more than 3 ounces.  If you are comparing overall weight to a system using a classic A2 gas block/front sight/bayonet lug then the difference, depending on your front sight choice, is minimal.  If your alternative is a low profile titanium gas block you can save a couple ounces.  My Sig 516 isn't heavy because it is a piston gun, it's heavy because it is built like a tank with a quad rail.

If it doesnt weigh that much, why do most non US militaries make their piston based rifles weigh around 3.5kg(7.7 pounds) without a magazine?

Heavy barrel profiles are the new hotness. Take a look at the Legacy MCX setup compared to the current Virtus. The Legacy system weighed in close to the same as a standard AR15 setup at around 6 pounds with a 16" barrel. The current Virtus weighs in close to 8 pounds, in comparison. The biggest difference in the two is the barrel profile/weight.

Sub 4lbs!?! Holy crap. So you have a functioning piston AR that weights 3lbs 10oz? Do you have a parts list?

And despite the heavy profile barrel (which makes the Virtus extremely accurate) it is still an extremely well balanced rifle which does not feel as heavy as it is.  The Sig MCX Virtus barrels are really incredible shooters.

Here is some real world actual data

Two of my LMT rifles.  Identical build, stock, grip, optic, optic mount, barrel profile, one is a piston the other is a DI



The DI weighs 8 lbs-11.25 oz , the Piston weighs 9 lbs-2 oz, the Piston is 6.75 oz heavier

Now the DI is a MARS lower so there may be a very slight increase in weight with the MARS but it can’t be much.