I would say this is not very difficult to make, but it does take a bit of time and effort.  It also depends on whether you want just a plain hi cap mag or whether you want it to look the part as well. 
   You can either leave it as the standard 300 round M16 hi cap magazine it is, or add a few custom touches to make it look right.  After all an M16 fires a 5.56-mm bullet, which is a lot smaller than a 12-gauge shotgun cartridge, hence the shotgun magazine would be bigger.
   Iíll leave the choice up to you, but as I like things to look as they should, Iíve shown here how to make a life size 7 round shotgun magazine.
What your need to make the basic magazine:
           1 x M16 300 round hi cap magazine.
           1 x Marui shotgun shell.
           1 x piece of plastic pipe 7 mm internal diameter. (I cut the end 60 mm off my MP5 mag
                 filling tube as its perfect.)

Things you will need to make it look authentic:

           Piece of acrylic plastic sheet, 25 mm x 20 mm x 4 mm
           Chemical Metal car filler by Plastic Padding
           Plastic Padding Easy sand filler

Other things you will need:
           Sharp knife.          Masking tape.          Narrow chisel, 1/4" wide or less.
           Pair of wire cutters.          Hacksaw.          Hot air paint stripper gun.  
           Sandpaper medium (600) and fine (1000)grade.          Electric drill and assorted drill bits.
           Quick setting Araldite.          Matt black spray paint.          Humbrol Matt varnish.
     The first thing to do is cut off the small rectangular section on the top of the mag thatís directly behind the outlet for the bb's. This bit houses the sprung catch that stops the bb coming out while you wind it up for normal operation. This is so the shotgun shell can fit on top easily.
     There are two ways to do this, depending on how skilled at modelling you are.
(1) With the hacksaw, saw along the bottom and down the top, and itís off, however if you are a bit heavy handed you might find you cut too deeply into the mag housing, which you don't want to do. (2) This way takes longer but is safer. Just slowly cut away at the plastic with a sharp knife small bit at a time, until itís all smooth.
   Also at this point remove the little sliding door that holds the bb's in, as itís not needed anymore.  With the shell in place the shiny back part is right over the filling hole and stops the bb's coming out
     Next bend the bit of plastic pipe 45 degrees so it fits on the top of the mag using the hot air gun. I say to use one of these as if you use a flame it tends to burn the pipe in one spot where the hot air gun heats it all up. To bend it successfully you need to heat it up and gently squeeze each side as you bend it, otherwise it tends to kink. You don't actually need the entire pipe, itís only the actual 45 degree bend you need, the rest either side can be cut off. When itís bent and cut to length get a thin strip of sand paper and pull it back and forth through the pipe to make sure a bb will roll through it without getting stuck.
     When this is done, sand the end of the pipe that goes in the mag until it fits in the hole in the bb outlet, then making sure it points straight forward, glue it in place. Use only a small amount of Araldite and get it fixed right, then when thatís dry stick it more securely. Try not to get to much glue around the mag outlet sidewall, as this is where the shotgun shells fixes on.
     A hint to remember when gluing with Araldite is just after its dry, which is a couple of minutes you can remove any you don't want with a knife. I chose Araldite as it goes to a sort of hard jelly before it goes really hard and itís this point you can trim off any excess.
     The next thing is to do is the shotgun shell. You need to take the pin out the shiny end of the shell, which will allow the spring and bb guide to come out. The best thing I found for this is a small drill, which you just press the pin out with. Then remove outer sticker and undo the two small screws. When you open it up make sure the plate with two springs on it doesn't get lost, as it will be needed. As for the rest of the internals they arenít needed anymore.
     From the picture to the left it shows the inside of the shell. Ignore the grey stuff; it was filler from an earlier try. You will see if you hold a new shell up to your computer screen what has been removed, and this is whatís to be done next. With the wire cutters gentle cut out small bits of plastic at a time. WARNING!  If you try to take out a big bit because you think it will get it done quicker don't. This shell is not very strong and will break quite easily. Also if you remove a too big a bit, it will cause a white mark to appear where the plastic is strained. Take my word for it, there is no rush so do it slowly and make a good job of it.
     When you have got out most of the plastic with the cutters, get the narrow chisel and smooth off the cut marks left by the cutters. Again be careful and trim a bit at a time. If you push to hard and get carried away the chisel will quite easily go through the outer wall of the shell, which means you will have to start again.
     With the inside trimmed out, you need to drill a hole. First reassemble the shell without the sprung front bb retaining plate. The find a drill bit that is the exact same size as the bb outlet on the mag, and drill a hole 33 mm in from the front of the shell. Be careful not to press too hard to quickly or it may split.
     Hopefully if all is done correctly you should now have something that looks similar to the picture.
     Its best to test the hi cap before going any further to make sure it shoots the bb's out ok. So fill it up with a few and wind it until the bb's start dropping out the end of the pipe. If you bent and sanded the piece of pipe correctly, they should now be falling on to the floor. If not that means you didn't check it and sand it enough.
     Anyway I'll presume all working as it should be, and the bb's are dropping out. The next test is you put your finger over the hole and wind the mag up again. The bb's should now be under slight pressure and when you remove your finger they should spray out.
     With that done, itís now time to fix the shell onto the mag and see if everything works as it should. You need to place one half of the shell onto the hi-cap mag as shown in the picture, and see if it lines up. It doesn't have to be dead inline, as the bb's tend to go where pushed, and if itís a bit out of line that shouldn't matter to much.
     Then insert the sprung front plate that holds the bb's in, and then put the other side back on and screw it together. With the front screwed up tight, place the shiny end cap back on. Itís now best to stick a few strips of masking tape over the shell and stick them to the mag as the shells not glued on yet, it might move around.
     Again wind the mag up and test to make sure the bb's are pushed up to the exit of the shell. As the reassembled shell is now in place the bb's should stop at the sprung plate, as they should do.       Then slide the sprung bit to one side and let the bb's spray out.
     If I've explained it well enough, and you have followed it all right, you should now have a working hi cap mag for your shotgun.
    Itís now time to fix the shell on more securely. With the shell fitted on top of the magazine with masking tape, fit it into the gun and see if it locks in place ok. If it does thats fine, if you find it won't lock in place properly, loosen the tape a bit and try again. This is the most important part as when the shell is fixed in place, you can't move it around again, so do make sure its in the right place.
   If it all fits and your happy with it, its now time to fix it on. This is where the Chemical Metal filler cones into its own.  It sticks anything to anything.  Mix up just a small amount, and by small I mean about a 1p-size amount. This is because at this stage your just holding it in place so the tape can be removed, and also this dries quite quickly in a warm environment.
   Place a small amount of at the front where the shell over hangs the hi cap and a bit either side. Try and make this as neat as you can, as this saves having to tidy it up later. However if you make a bit of a bodge of the gluing don't worry as this has a sort of nearly dry stage where it can be removed. You can use a sharp knife but you might find you cut into the plastic, as the filler is not that hard.
   When this is set hard, which takes about an hour, carefully remove the masking tape.  Then make up about a 2p-size amount and carefully poke it in underneath the shell, making sure not to get it on or in the way of the sliding door where the bb's go in.
   Wait for a couple of hours to make sure is well and truly set, then you can try it in the gun. This is where all the checking and making sure the shell was the right distance from the magazine to lock into the gun properly, as if all has been done well, it should lock into place with a click.
    Itís now time to finish fixing the shell on very securely, as itís got to hold the entire weight of the hi-cap, and cosmetic bits if you go that far.
How you go about fixing I'll leave up to you, but if you look at the picture on the right you will see how I done it, so use this as a guide. It needs to be spread all round both sides and the front. If you followed the rest of my instructions you should have already pushed some underneath, so this new lot will just go round the outside.  Some people might say this looks a bit messy having all the glue showing, but when itís locked into the gun it doesnít show.
   With this done, you now have a hi cap magazine ready for use with your shotgun, that lasts a lot longer than the normal shells, and now in the heat of battle you won't run out of ammo.
   As I said you now have a working hi cap that you can leave at that, or if you want it to look more authentic keep reading as I'll now go into how to make it the right size and length.
   To load the bbís you still drop them in via the hole where the sliding door used to be.  Unlike before the shiny bit of the shell isnít fixed in place with the pin.  I found this to be the easiest way around the filling up problem.  The shiny cap fits on quite well anyway, but I found if you use a largest lump of blue tack and stuff it in the end of the shiny bit, when you press it on, it holds it firmly in place.  I know this may sound a bit strange, but it works well.  When you take the cap off to reload, roll the blue tack into a ball again and stick it back in the end.  Simple but very effective.
   Now on to making it the right size. As I said you can leave it just as the plain hi cap as I was going to do when I first set out to make it, but then I found when you laid a shotgun shell on the hi cap magazine its actually bigger than the mag casing. It was at this point that I decided to make it the right size. Mainly because it just didn't look right. It was far too small.
   To do this I laid 7 shells next to each other on a piece of paper, and then drew a line around the outside so to get the curve they made. If you only have one, or two, its best to draw round each shell, then move it, then draw round it again, until you have seven of them, in a slight curve.
   I then made two copies of it and stuck it to the back of the sheet of plastic using pritt stick, as it doesn't stick permanent, then cut round them. I have got several electric tools, one being an electric jigsaw and the other a bench sander. These do tend to take the hard work out of it, however if you don't have access to this sort of thing, youíre going to have to cut and sand them to shape by hand.    When you have two sides, hold them together and now sand them so they are exactly the same. This is mainly so the magazine when finished is even both sides.
   When you have both of them done, stick them to the magazine with blue or white tack to get them in the right place. I found the easiest way is to insert the magazine into the gun then stick each one in place. When they are both held in lace remove the magazine and check it goes in and out easily. If not try moving them around until it does.
   As and when you are happy with them, press them both firmly to the magazine and remove it from the gun.
   Now make several makes on the plastic so you know where it has to go on the magazine, and then remove one side.
   Mix up a fair amount of Araldite, and run three lines down the magazine, making sure there is enough there so it will spread out nicely as you place the plastic side back on and squeeze it firmly into position.
   With the side glued on, place the magazine back into the gun and check it is in the right place.  I found I put it in the right place, but when I pushed it down, it moved slightly. You need to make sure itís in the right place before the glue dries as it can't be moved once set.
   Leave it for several hours to make sure the glue has dried properly and then repeat the process with the other side. I can't emphasise enough how important it is to get it in the right spot, as if itís slightly out itís a real bugger to fix.
   Anyhow if for some reason, you find yours has moved without you knowing, you need to get a file, bit of flat wood and medium grade sandpaper. If itís a case of it doesn't fit into the gun properly, carefully file the top bits of the plastic until it does. Then la the sandpaper on the wood and sand both sides together to make sure they are even.
   Once this is done and you are again happy with it, its now time to just add a bit more glue around the edges to make sure itís fixed really well.
   Make up a small amount of Araldite, and spread a thin bead along the edge of the magazine where it meets the plastic, and smear it into the joint.
   Then leave the entire thing to dry overnight.
   Now with the outer plastic bits in place it now time to fill the edges with filler. To give yourself plenty of time to work the filler nicely into the edges, I always work in a cold shed or garage or outside if you have neither. The reason for this is the filler takes ages to set when itís cold, so this gives you maximum working time.
   I would say do it in several stages to avoid having huge lumps of soft filler to contend with, as it tends to sag easily. First lot spread into the corners and smear over the inside. When this is done place the magazine into a large plastic bag, and tie the top. Then take it back indoors and put near a radiator. It then dries really quickly. The reason for the bag is it smells very strong, and will otherwise make the house sink for ages.
   The next layer build up until itís just under the edge, let it set then finally fill until itís just proud of the edge.
   When this has set, take it back into the garden and with a bit of medium grade sandpaper on a sanding block or piece of wood sand the edges until you can cleanly see the plastic. If done right you should have a very smooth start edge.
   You might find there will be a few spots of marks in the filler, in which case, just mix up a bit more and fill them in. Again sand and repeat if you find anymore.
   When itís smooth and youíre happy with it sand it again with the fine sandpaper until its really smooth and you can see any marks at all.
   At this point you can also sand both sides of the piece of plastics to give the paint something to key to.
    With the sanding all finished its time to paint it.  However I did think it looked a bit plain, so I made a few grooves on each side which just seem to make it look a bit better, or I think so anyway.
   To paint it and get the best results I used a small can of matt black car spray.  You can get this from any car spares shop foe around £2.
   Itís best to spray it with several light coats instead of one thick covering as this help prevent runs.
   Find a piece of old stick to hold the magazine on to make moving it around easier.  The spray each of the sides that you filled first as they need slightly more coverage to get them looking right.
   If itís cold out use a hairdryer set to warm and on a low blow to aid the drying.  After an hour or so go back and give it another coat.  Do this several more times until your happy its black all over and then stand it somewhere out the way to dry properly.  This normally takes around a day.
   With it now dry you need to give it one final coat of clear matt lacquer.  I always use Humbrol paint for this as you only need a small amount and it comes in small tins.
Paint the four sides and the top, and then leave it to dry and paint the bottom.  This doesn't take long to dry either but its best to then leave it for a day or two to let it dry really well.
    All being well you now have a hi cap magazine for you shotgun, but being home made remember the more time and care you take over it the better the end results, and there is no reason it won't last for years.

In these last few pictures it shows the finished magazine from all angles.  The hi cap winder is accessible from the recess in the bottom, and to refill it you just slide the back of the shell case off and drop the bb's in.  There in no door on the magazine anymore, as it used to have to slide where the feed tube now sits, but the back part of the shotgun shell now fills the void when in place and stops them falling out.