I have a Kel-Tec sub 2000. It will eat up 115 gr. bullets flawlessly, but 124 gr.(nato) bullets give me occasional, failure to EJECT (not extract) The bolt comes back and crushes the non ejected empty shell. Is this a recoil spring problem??? Do I need a heavier recoil spring for the 124 gr. ammo??? Any help would be appreciated.