Hi all. This a Practical Comparative Review of the Tanaka, G&G, and Bar10 conversion Kar98k. By "practical" I mean mainly in-game usage in the hands of one user/owner for all three rifles (me). So this is all first hand experience. I will not go into a detailed review of each rifle separately, but rather focus on what sets each one apart, advantages and pitfalls.
Background:
I've been very very fortunate to have these three rifles on hand to compare head to head. I built the Bar10 conversion or "Bar98K" in 2009. I was able to buy the G&G kar98k this year. Then i was lucky enough to get a pre-ban Tanaka Kar98k for less than half the price of the G&G. Today was the first time i brought them all to a game.
The Bar98k
My Bar98k was built using a JG Bar10pro and a Dboys Kar98k with wooden stock. It has been upgraded with Edgi precision parts and shoots 550fps with 0.2g bbs. Upgrades include: tool steel sears, steel spring guide, SP01 Edgi spring (rated for 500-550fps), piston, 6.01id custom bull barrel, stock hop rubber with anti-blowby rings. Stock parts are the cylinder, cylinder head, receiver, hop chamber. Optimum weight bbs: 0.4g. Total cost of build and upgrades: USD393
Advantages:
*Most consistent fps (range of only +/- 5fps)
*Best shot-to-shot precision/accuracy (probably due to the tight barrel and the amazing design of the hop chamber)
*30 round magazine
*no consumable gas required
*lightest of the three
Disadvantages:
*You will need time, money and skill to build one for yourself
*Will require a LOT of work to make receiver look spot on
*Using the Bar/VSR, the rear sight will ALWAYS sit too high due to the cylinder design. You will have to either compensate your aim (aim low), raise the front sight blade, or use a scope.
*Will have to cut a large magazine well out of the bottom of the stock
*Hardest to cycle bolt..especially if upgraded.
*magazines don't fit in k98 pouches
The G&G G980
G&G was the first to release a more affordable and skirmishable Kar98k. I was able to get their 2nd release, with the new steel hammer components. The 8g CO2 cartridges were readily available packaged as soda chargers. The package already comes with 2 mags, sling, and a box of good quality .30g bbs. I bought mine new for about USD460. The problem is that i got a lemon. My magazines were only giving me 120-200 fps. My friend who bought from the same store had the same problems. I emailed G&G about it and they said that it might be too hot where i live (Manila, Philippines), making the pressure too high for the hammer to depress the firing pin. But according to the gun tech i brought it to, it shouldn't be the case. First of all, it's not that hot over here (24-30degC). And second, all other CO2 airsoft guns work fine over here. G&G advised me to return the mags for them to have a look, but due to logistics, i just decided to have the tech fix the mags. He got them to fire at around 520 fps. I was able to buy extra bb clips directly from G&G. All stock. 9 round bb clips. Best weight ammo: 0.3-0.36g bbs . Total cost: USD583
Advantages:
*Widely available, skirmishable Kar98k
*Solidly built
*Surprisingly, shot better than the Tanaka kar98k
*FPS variation range +/- 20fps (not counting last few shots)
*no-tools hop adjustment
*bb clips and CO2 cartridges fit in k98 pouches
*easier to reload compared to Tanaka kar98k, even when wearing gloves
Disadvantages:
*Possibility of still getting a lemon even after paying a high premium
*heaviest of the three
*stock is too square
*hop up range of adjustment too small. Can't get enough spin on 0.4gbbs which are the ideal weight for this fps range.
*Sometimes the cartridge gets stuck. Can get it out without the use of a drill but you can't do this in the field
The Tanaka Kar98k
I consider this a very very lucky purchase. Lucky because the previous owner really wanted it gone. The going rate for a brand new one (IF you can find it) is USD700-1000+. The one i got is preban (without the fps restrictor) including 2 magazines, a sling, BG hop unit, KM TN 6.04 barrel all for only USD261.
Advantages:
*Best fit and finish, hands down.
*Just felt great in the hand...i don't know how to explain it. It felt Right.
*It's THE airsoft Kar98k, what can i say...
*Shoots amazing considering it's age (preban)
Disadvantages:
*Hard to find, most expensive of the three (usually 
)
*Surprisingly, had the largest fps variation +/-25 fps
*Have to use a flat head screwdriver to adjust hop. Mine felt waaay to loose. Is that normal?
*Shots were a bit scattered compared to the G&G and the Bar98k
*front sight rotates...is that normal? Can i just glue it in place?
*magazines don't fit in the pouches
*extra mags hard to find, expensive
*parts getting hard to find
Comparison shots
from top to bottom: Tanaka, G&G, Bar98
from top to bottom: Tanaka, G&G, Bar98
Receivers top to bottom: Bar98, G&G, Tanaka. Notice the Blued finish of the tanaka? Beautiful. Notice the AK rear sight on the Bar98? At the time of this build, i didn't have tools to use the rear sight from the Dboys
Magazines: L-R: Tanaka, G&G mag and clips, Bar98
Tanaka & G&G mags are NOT interchangeable.
Final word:
There are a lot of second hand info regarding comparisons of these rifles. "Springers are the best" "Tanaka will out perform anything 'cause it's the best rifle EVAH" "G&G sucks"...So here are my final thoughts, without any bias or elitism...
Best for precision sniping: Upgraded Bar98k
Most accessible, most available and surprisingly a good performer (if you get one working fine OTB): G&G G980
Best 1:1 Replica: Tanaka
Nice one. 
The G&G G980... i just decided to have the tech fix the mags. He got them to fire at around 520 fps.
Any details of what was done? My problem is the reverse with way over 500fps as stock so I would be curious about any adjustments made to the mags.
He had to enlarge the valve openings to the main gas vent. He was so experienced that he just asked me what fps i wanted it to fire at...enlarged the holes...tested it once and went "There you go". Permanent fix though, no getting it back to firing at a lower FPS. G&G did send me new firing pins to try out for free, but i never got to use them. You could try a nozzle with a smaller bore?
Ah yes, enlarging holes makes sense. I'll detail my efforts to reduce fps elsewhere...
I was pretty surprised that the G&G surpassed the performance of the Tanaka. I was very frustrated with my G&G purchase because it didn't work well OTB, despite it's high price. And every time i took it to the field, there a different problem always came up. Including the day i brought all three. I couldn't use one of the mags that day because I couldn't put the bolt handle down (firing pin was sticking too far out). In fact i've been trying to sell it for some time now.
But come game time, it performed exceptionally well. Plus, reloading was much easier, even when wearing leather gloves. Everything fit in the k98 pouches, too! Replacing the CO2 cartridge was easy as well. As long as you get a cartridge that doesn't get stuck. I've noticed that this has more to do with tolerance variations between cartridges more than an actual defect with the G&G mags. I don't think the cartridges expand and get stuck. They get stuck at the neck and not the body. Could possibly have something to do with over-tightening. All you have to do to get em out if stuck is to remove the bottom plate, and loosen the two screws underneath. No drilling required.
I guess after tackling all the niggles (which you shouldn't have to at this price point), the G&G can be a great performer.
So I guess I'm keeping mine. 
Small update: I ordered the G&G scope for the G980. It's a clone of the Tanaka "ZF41" and fits both the Tanaka and the G&G well.
