I have generally given up hope of keeping my WE M4 in good condition anymore. This usually results in it running dirty yet (usually) functional, if I go shoot with it. Perhaps not the best practice, but I’ve been shooting (and pouring money into) mine for many years past its prime – I probably should have given up on this one a few generations before WE “open bolt”/open chamber GBBR and conversion kits even hit the market. But what the heck, I might as well keep it running.
I recently have been encountering issues with most of my 9 working magazines refusing to lock back (I have somewhere in the range of 14, but the old gen internals just leak too often to be worth maintaining). Years ago, I did most of the common mods you’ll find on the internet and then some, in regards to getting the finicky WE M4 “open bolt” bolt stop to actually, well, stop the bolt when the mag runs out of ammo. A combination of many of those seems to have fixed my issue, but it has since returned… all things considered, I’m a little surprised that I’m just experiencing this now (much later than expected).
After inspecting the usual suspects and confirming those to still be in functional condition, I still couldn’t get it to work right with all my mags. A few ideas popped into mind – mag part that pushes up on bolt stop, magazine outer shells, or lower receiver.