[Ross]

The nozzles break because they are junk. As a troubleshooting step, you can rule out the overloading of magazines by charging the gun first and leaving the chamber open before inserting the mag. If you do this every time you reload, and the nozzle still breaks, then you know it's not from overloading.

In some cases the npas can cause more problems than its worth. When you have the npas installed, less gas is used to propel the bb forward, and more of it is expelled outward, freezing internals and adding more pressure to the recoil operation.

Which means more chance of cracked nozzles.

If your current installed nozzle is cracked, then it is most likely the reason you are shooting 370 fps instead of 410. Also huge factors is temperature. If you are chronoing your gun outside and it's cold, your fps will be reduced. A cracked nozzle would also be the reason you are shooting 270 fps with the npas wide open.

The bolt not locking back could be a problem with the bolt catch or the bolt carrier itself. I find the shimming method to work well.