problem firing in classic army m15a4
 

many sites say the m15a4 can take a m120 spring out of the box, so i got one and when it fires it makes like a grinding as being fired. I have an 8.4 volt 3800 mah battery which is probably alright. so can you guys please enlighten me on the problem?

Grinding is not good you may have lost a shim.

the grinding noise means that one gear is moving laterally, and teeth are chewing on stuff.
You'd better not operate it and have it fixed.

sounds good thanks