most probably been brought up b4, but can anyone advise me on which hop up unit will fit the agm mp40 an also which barrel? thanks
I`ve never attempted it as my MP40 works fine, but I believe it was the hop and barrel from the AK 
The HOP on the AGM MP40 isn't the same as an AK's unfortunately. If you want to modify it (with a new barrel for example), you have to chop it up and stick it all back together. I think it was Taffy over on ASCUK who put a "how to" guide together for this very job.
I've found the HOP on mine to be pretty good now that it's had a couple of thousand rounds through it. I also gave it a damned good clean as soon as I took it out of the box. These chinese guns come absolutely covered in grease in the barrel and the HOP.
When I want your opinion - I'll tell you what it is!
And not to mention the fire selecter switch too. My M14's fire selecter got jammed because a piece of polystyrene had become affixed to the grease aroudn the fire selecter switch.
Superficially, the hop-up looks the same as an AK unit. There are, however, some significant differences. Notably to the positioning of the screw holes.
Is it possible to fit one with some butchery? possibly. One of these days i'll get round to trying it lol
When we were a Kingdom it was run by a King
When we were an Empire it was run by an Empress
Now we're a country we're run by a..........
well when ya give it a go let me know how ya did it 
Also the air nozzel is a lot longer on the AGM mp40, so you may have probs if you changed the hop, once cut, butchered & glued with a stock hop rubber & better barrel it works well better, if anyone wants a 8meg word doc will pics & step by step on how to do the mod PM me with a email addrerss & I will send it to you
Steve.D
Its a tough one, I made some hi caps that miss feed, I recon it could be a prob with the air nozzle, its very long in comparison to a AK one, I recon as Kermit says a AK one may fit if you drill 2 new holes & shorten the feed tube, but full auto is the way I use it
Steve.D