Also bought a JP GMR 15, same store i assume.
Buffer detent was already out.
If you buy the MBX buffer it comes with a yellow carbine spring, don't know if it's a Sprinco.
You also need to buy the MBX tuning kit.
Shot it yesterday with a blue spring and short buffer.
Ammo used was Fiocchi 115gr factory ammo.
Trigger reset was reliable, also activated the lrbho.
Need to try more setups with this ammo.
If you want it to short stroke (i don't) you can also use euro's in the buffer tube.
Met vriendelijke groet,