GBB pistol rate of fire quickly slows down
 

I used to have a plastic TM hi capa 5.1. I never had any problems with it. I could fire 10-12 shots in rapid succession and would still get a decent rate of fire or speed.

When it broke down I was strongly recommended to buy a KJW full metal hi capa 5.1. I have to admit that I am not very impressed. Whenever I shoot very fast with it (like 5-6 BBs one after the other) the speed of the BBs gets very low or the gun has a hard time cycling, the sound changes and it shows and obvious lack of power.

Would this be explained by the extra weight of the metal slide compared to what I was used to with a TM or it's another problem?

Partly, it is also because the TM is more gas efficient than the KJ.

I speculate that KJW valves are higher-flowing than TM valves to support the increased mass of the metal slide. Higher flow = more gas consumption = more rapid mag cooldown. Couple all this with the inferior design of KJW mags, and you have a less than ideal situation.

I'd say that if you used CO2 magazines, you wouldn't have any issues like this one!

I use mostly TM mags if it helps.