hi all, i have a question for experianced ar builders. ive just completed my first ar build. its an m&p 15 stripped lower with rra lpk and an yhm complete upper, with a rra six position stock. the problem is that the weapon will not fully cycle through. the bolt only comes half way back and stops before extracting the shell. i run good ammo and the weapon is well lubed. the one thing i notice is that the buffer spring is very stiff. being a former 0311 ive had plenty of trigger time on the ar platform and i think this may be the problem, the hammer spring is pretty stiff as well. i would appriciate all opionions and info offered thanks.
Is this during firing, or only hand cycling? Do you have the correct “carbine length” buffer installed or could you by chance have installed a “rifle length” buffer?
thanks for the reply dos. yes this is happening while fireing rather theirs a mag inserted or not. when i cycle the action manuely it functions properlly. the buffer spring came with the rra stock. i actually when to the local gun shop and tried out the action on a few of the ars and mine is definatly a lot stiffer.
What is the overall length of the spring you are using?
the overall length of the spring is about 14"
14" is way long for a carbine. Has to be a rifle spring.
A carbine spring should be approx 10.5-11.5" in length.
thanks for the info guys
I believe thay max. length for a carbine is 11.5 or 11.75 inches. So you most likely have the rifle spring in the weapon.
well i cut the spring down to 10 3/4" the weapon is now running perfectly. thanks a lot for all of your help guys.
jeff
Good diagnosis guys! 
A question regarding springs: Are rifle springs and carbine springs of similar stiffness, or would OP benefit from buying a spring specifically meant for a carbine?
springs are probably the same. you switch out buffers to adjust “stiffness”.