Hope I'm not preaching to the converted - apologies if so!
If like mine your AGM MP40 upper receiver is a little loose fitting to the lower receiver/body ie there is some movement between the 2, I've found that once the 2 halves of the gun are separated for battery changes etc, simply push-fitting a 10mm (or there abouts-not critical) rubber "O" ring onto the exposed end of the Hop-up takes up any excess movement once the gun is reassembled -it creates an excellent air seal between the gearbox and hop-up too, which otherwise is where the movement manifests itself and can affect shooting performance-which can only be a good thing? It also eliminates annoying squeaks in the stock to boot! 
A little care must be taken to make sure the "O" ring stays round the mouth of the hop-up unit whilst reassembling the 2 halves or it could end up falling off or worse still inside the gearbox if you're really unlucky! 
Packets of "O" rings can be found cheaply at DIY stores in the plumbing section - mine were from B&Q.
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
Nice tip.
Now please tell me how to properly secure the incredibly annoying cocking bolt that works free after the first five minutes of a game...
plenty of araldite my lad 
The tiny screws on the cocking handle do work loose just by looking at them. I tried locktight (well, my favourite nail varnish actually) but I'll have to resort to araldite. As for the thing sliding back and forth exposing the gubbins - well, I'm not keen on an external elastic band but I did try an internal one so that you can put the bolt back to adjust the hop then it snaps back when you let go, but as usual my bodge-it attempt was doomed to failure.
I haven't noticed any free play of the body parts but the lure of a better airseal seems tempting and being the proud possessor of a box of miscellaneous O rings (that didn't contain ONE that fitted the leaky tap job I had to deal with 
) I shall give it a go!
let us know how this goes chommers, i too have an o ring set .
Nice tip.
Now please tell me how to properly secure the incredibly annoying cocking bolt that works free after the first five minutes of a game...
Ah, I had this too-as mentioned above threadlock or a drop of superglue/Araldite will hold the 2 small screws that hold the bolt handle onto the "bolt", but I found that the bolt itself can be pulled further back in the receiver if the back edge is bent VERY SLIGHTLY away from the stopper welded inside the receiver, enough so that the bolt can be pulled past it- doing this leaves the bolt held snuggly by friction from the stopper, and its now in just the right position for a realistic "cocked" appearance - but it can also be pushed forward with a little effort if you want to close the bolt cover over the hop unit when it starts raining, or for storage. I painted the gearbox black so that it isn't too obvious through the small gap in front of the bolt that results when the bolt is pulled all the way back too.
But has anyone got a simple cure for all the folding stock movement when the stock is open?
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
painted the gearbox black! kudos for the effort. i used a strip of black insulation tape.
painted the gearbox black! kudos for the effort. i used a strip of black insulation tape.
Doh! Yeah that would have been easier if I'd thought about it! 
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
Here's a pic of my AGM MP40 as modified above - plus fire selector removed and stock refinished & distressed. Just got hold of a £10 repro leather sling from Airsoft World for it - back in stock at long last!
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
that looks nice, good work .
Nice tip.
Now please tell me how to properly secure the incredibly annoying cocking bolt that works free after the first five minutes of a game...
Ah, I had this too-as mentioned above threadlock or a drop of superglue/Araldite will hold the 2 small screws that hold the bolt handle onto the "bolt", but I found that the bolt itself can be pulled further back in the receiver if the back edge is bent VERY SLIGHTLY away from the stopper welded inside the receiver, enough so that the bolt can be pulled past it- doing this leaves the bolt held snuggly by friction from the stopper, and its now in just the right position for a realistic "cocked" appearance - but it can also be pushed forward with a little effort if you want to close the bolt cover over the hop unit when it starts raining, or for storage. I painted the gearbox black so that it isn't too obvious through the small gap in front of the bolt that results when the bolt is pulled all the way back too.
But has anyone got a simple cure for all the folding stock movement when the stock is open?
Exactly what I've done with mine, As for the stock wobble, a piece of thin cardboard apparently is the answer, Haven't done it myself, but according to those who have it works a charm. I've been able to figure out ho to remove the stock. Anyone got any ideas?
Mine came loose at the last game with otto, I removed it drilled out the small screw and tapped a larger bolt in it, then locktite the whole thing. Works for me!
Lt. Morris Schaffer: "Second rate punk," eh?
Major John Smith: Sorry. All I could think of on the spur of the moment.
Lt. Morris Schaffer: Thanks, that makes it even worse.
Would anybody be able to post up a quick pic showing exactly where to put the O-ring?
You've got nothing to ein, zwei, drei, vier
It's simple Andy. Takes literally seconds to do mate.
Just remove the upper receiver & place the O ring on the rear end of the hop unit where the nozzle slides into. There's a rim moulded into it at this point that will stop any forward movement of the O ring. Just ensure the ring stays in place as you re-assemble the weapon.
Hope that makes sense.
John.
Couldn't have put it better myself! I tried taking a pic but my camera wasn't up to the challenge sorry guys. 
Glad you found it of use John, thanks for taking the time to provide some chrono results!
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
