I built an sbr ar15 in 300aac. When I use it with a pmag or a usgi mag I have no issues. When I use my lancer 20 round clear mag I get a bolt over cartridge jam on the 3rd to last round every magazine. If I clear the jam, load the jammed round back into the mag, and either cycle the action or lock the bolt back and release it with the bolt release it all works fine. Does that sound like a weak spring to you? or symptoms of some other issue? The mag is only a couple years old and has not been used more than a few dozen times. It worked fine on my blackout using the same ammo before it was a side charging sbr. not my picture, but this is what it is doing.