I've converted an AGM standard 50rd locap using a Marui "Uzi" hi-cap mag; the pics show the idea but the following notes should help anyone contemplating the same; if using all new parts it works out about £35 per mag in the UK, or £25 if you already have the MP40 mag to hand. BB capacity is around 250-260. Parts needed are the AGM mag body and top (feed) assembly, and the Uzi internal hi-cap mechanism ("mech") and mag base, plus tube, bits of plastic sheet and basic modelling tools - knife,rule,hacksaw,file,glue etc.
Once split apart as per the above photo the top of the Uzi mech needs cutting/filing back so it will slide inside the MP40 mag body (compare mech with photo below). The Marui mag base can now be slid snuggly onto the MP40 mag body (NOTE that the front edge as fitted to the Uzi-curved corners-needs to be at the BACK on the MP40 mag as the Marui mech is reversed to fit the MP40). A 44mm piece of suitable tube (eg old airsoft brass/aluminium barrel/lo-cap loading tube etc) needs to be superglued between the recess at the top of the mech feed and the MP40 mag top feed assembly, ensuring all is square; make sure no swarf from the cutting processes gets inside the mech (or superglue . . . !) Photo below should explain;
Once this is done you need to consider how you want to fill the mag with BB's once complete - I decided for ease to file back the lugs which extend down from the MP40 mag top feed, allowing the whole mech to be pushed up a couple of cms from the base of the mag so that BBs can be poured in from the top. This being the case I reinforced the modified mech with 2mm plastic sheet as per the photo below to make it more rigid. Lots of test fitting ensures everything fits as it should.
The completed mech should now be a snug push fit into the mag body and will look just like the lo-cap it once was, except for the tell-tale winder at the base of course. Further plastic edging was added around the top of my mech to stop BBs going astray when pouring them in, and Araldite added to the joins for extra strength; I will try to add extra pics in another post below this one as they explain it all SOOOOOO much better than my rambling can! 
Feel free to post any questions though folks.
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
Above shows the extra plastic sheet "anti BB spill" sides (in white) fitted to aid filling (as mentioned in thread above). Note they were added after I'd araldited and painted the assembly as shown in the last pic above as they were an afterthought . . . . but at least it shows where they go!
. . . . And the Marui mag base showing the mech winder.
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
Excellent very clear guide Von Straffham - now stickied for reference!
boooooo hi-caps suck, the tools of the devil 
( just kidding mate, nice work)
Nice one, but a bit late. This guide would have gone down a storm about a year ago!
The King Arms mid caps are a lot less trouble and give a decent amount of rounds.
I got mine from airsoft armoury, I think. Yup they're in stock. £45 for 5. Well worth it in my opinion.
Nice one, but a bit late. This guide would have gone down a storm about a year ago!
The King Arms mid caps are a lot less trouble and give a decent amount of rounds.
I've never had any trouble with the KA midcaps for my XM177 and M14, as cheap mags they are great, personally I prefer metal mags in all honesty though. Using this mag and my 5 standard lo-caps lets me now carry 500 rounds ready to go, rather than the 300 it would be with just lo-caps, which was the reason for the conversion. I can also now use my MP40 at open days up at my regular site on a more level playing field! 
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
What I mean by "less trouble" was that you can just buy a set of KA mid caps off the shelf rather than have to heavily modify other mags...
What I mean by "less trouble" was that you can just buy a set of KA mid caps off the shelf rather than have to heavily modify other mags...
But where's the fun in that? 
I've as little conversion (that has probably be done before) that you, Yith, may be interested in. I'll post it up soon.
A Proud Member Of 'Team Spleen!' who play mainly at Gunman Airsoft, Tuddenham, Suffolk.
heh... as good as my new double-stacked AGM shotty mag?
heh... as good as my new double-stacked AGM shotty mag?
Shotty mag???? Is it for an M1897 trench gun?? 
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
Nope it goes in my M1 garand/M14
Nope it goes in my M1 garand/M14
Wondered what it was for - does it involve a lot of work to mod one to fit the M14 mag well? Do like the Garand and quite fancy doing an M14 conversion myself if I get round to doing a US loadout.
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
There's a good article on the mags here:
UPDATE on the MP40 mag conversion.
Having now field tested the magazine in the gun I've found there can be some feed issues; this generally happens when using 9.6v batteries rather than standard 8.4v ones due to the slightly higher rate of fire. 
There's no problem with single shot, but being an MP40 with a hi cap we don't want to bother with that do we?! 
Basically the small clockwork wind-up spring which pushes the BBs up the lengthened mag tube isn't beefy enough to push them up quickly enough to cope with the higher ROF. (Who'd have thought an extra 7 or 8 BBs would make such a difference? Maybe leaving the mag loaded weakened the spring?) 
A search through my bits box yielded an old plastic Cybergun M4 mag - the spring was removed, shortened slightly (as its much bigger) and grafted into the TM mechanism in the MP40 mag; hey presto a huge improvement! Not totally perfect but much better nontheless.
Like any conversion this one has not been without its foibles!
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
A MP40 is more fun with the standard mags
get 2 3 cell mp40 pouches + 1 mag in you gun & your away
Steve.D
Can't argue with that matey, got 5 standard 50rd mags too to fill my mag pouches so no problem there - locaps do feed more reliably theres no doubt. 
Only if theres a 500 rd limit per game I can carry them all in mags rather than a speedloader, and it can even the field a bit against the hicap Thompson brigade!
Now, realcaps would be even better!
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
Now, realcaps would be even better!
But I assume from his posts that the hi-cap wouldn't be an issue for realistic rate of fire.
I meant if EVERYONE used realcaps really. Horses for courses I guess . . . . . .
The first casualty of war is innocence.
The first casualty of airsoft is your wallet!
E Hobby asia sell these mags too, come in fives for £28 posted.
