HFC C96/M712 Mauser Shooting Very Low Range and FPS

[para1944]

From what I've read this gun shoots very hot. 

However, when I tested it out today, I only saw a few infrequent shots get up to a reasonable FPS. 

Most BB's could not make it past 20 feet before hitting the ground.  BB's traveled at a very leisurely pace with a very obvious downward arc started at the barrel.  Some BB's had even less range--I could probably throw BB's faster and farther than that.

Adjusting the hop up didn't do much:  too much and I got a bunch of jams caused by double feeding.  Too little and the BBs' range decreased even more. 

I've disassembled the gun, and could not find anything out of the ordinary.  I applied lubricant and it is still having the same problem.  I noticed that the "button" at the top of the mag the gun's "firing pin" is supposed to hit was impossible to press down so I degassed the magazine from the filling port at the bottom, which made the button at the top very easy to press down. 

The gas I was using is green gas (propane from adapter, with silicone lubricant).

Does anyone know what happened and what else I can do?

[2nd Bat]

These typically are very hard shooting guns but obviously something is off. The good news is it's an inexpensive gun and for all intents and purposes did not see action in WW2 anyway!

[stuka]

Well it did see use in the CBI and I am sure there were a few people with them in the ETO and MTO as well. It wasn't common but it was still there

[shiftysgarand]

I don't own one of these, so I can't be sure, but I think the hammer spring might be toast, or it might just be too weak to release the valve with green gas. Try some 134a gas, and if that doesn't work, I would suggest (if you can find one) a new hammer spring.