m16a3 firing problems first few shoot few meters then rest are fine

[Tomm]

skirmished my g&p m16 today last week changed my spring, basically when i got to fire they basically roll out the end then after the first ten bbs say it fires fine but then change mag and does the same again so had to mess about clearing the first couple of bbs before i can fire properly bit of a pain when your taking fire, just wondered if any one else had this problem could this be a air nozzle or hop? even though before i changed my spring i did replace the hop rubber and was firing spot on only since replaced the spring its been playing up.

thanks in advance