Stock fps, what makes the difference?
 

The CA G36K has its stock FPS ranging from 290 to 340 according to different websites.

With my new CA G36K, I did 6 coke can tests with .20 BBs, and pierced both sides easily. I got bottom center pierced once, while other tests only cracked it without penetrating. So I assume it shoots at least 340fps.

It's great, I won't need to fiddle with the mechbox.

What I'm wondering is why there is such a difference between the "possible" stock fps for the CA G36 (or any other AEG). Aren't stock parts all the same?

Depends on many things. Type/Brand/Charge of battery, Weight/quality of BBs being used, and probably other reasons I'm too lazy to think of.

It's also possible that the reviews you're reading were written for early and later versions of the gun. The TM G36C, for instance, began its life with an EG700 motor, but is now currently built with the EG1000 motor.

CA is VERY inconsistent in the quality and finish of their guns, some shoot 330 for a year, some shoot 310 for half a mag and the tappet plate breaks.
If you want better consistency I've found the best thing to upgrade is piston/cylinder heads and the spring guide

Battery has ABSOLUTELY NOTHING to do with velocity. Now that that's out of the way, it's all about allowable tolerances.

When companies order a part from someone, the more precision in tolerances you ask for will cost you more money. People need to be paid to sort through parts and weed out those that are outside of "spec". If you don't care about tolerances, you will get the best price per part.

When companies build something for a retail market, do they care about getting the cheapest part, or getting the best part? After all, if you want the "best" part, there is a whole marketplace full of people willing to sell them to you for profit, so the manufacturer doesn't need to be concerned with that. It's simple economics.

This holds true for anything that is manufactured from parts, including AEGs.

Last time I chronied mine it was shooting 315 fps. Around five rounds shot I had a variance of +/- 2-3 fps, so a range of about 312 to 318 fps. This was with a fully charged Elite 1500mah, 8.4v mini battery. Only upgrades to the gun is a stainless steal 6.04mm high precision barrel. This was with Guarder .20g BBs.

Friend of mine sports the CA G36 (full length version), totally stock his fires 315 FPS +/- 2 fps.

-Dr.