Mission Models offer two sets: MM-003 is a full ammo set and MM-004 is spent shells set. There are 25 pieces of ammo or shells in each set. Each set comes with two small photoetched brass frets with 25 ammo belt clips, so you get 50 clips in each set - twice as much as is needed. But those tiny parts are so easy to lose that it is a good idea to provide a lot of spares.
On the photo above you can see a close-up shot of one full ammo piece and one spent shell. Below them is the edge of metal ruler - distance between black lines is 1 mm. You can see that those shells are really tiny - spent shell is just about 3 mm long. Considering the size, the level of detail is just stunning!
Below you can see details of both ends of spent shell - as you can see the shell is hollowed from one end, just as it should be. Of course the shell is not empty inside - the hole is just about 1/3 to 1/2 mm deep, but it is enough in this scale. Add a tiny drop of black paint inside and illusion of real thing is just perfect.
As mentioned earlier, both sets come with ammo belt clips. .50 cal ammo belt disintegrates when the gun is fired - so wherever you see a pile of spent .50 cal shells on the ground, you will also find a lot of those little clips laying around. On the back of Mission Models ammo set packaging you can find instructions for bending / rolling those tiny clips to shape.
Once you manage to form enough clips to make an ammo belt, you can put ammunition in them - see photo below. Working with those tiny clips is not easy, but result are really great!
Once you have the ammo belt ready you may encounter a serious problem: finding the ammo box for your ammunition! If Mission Models ammo dimensions are correct for 1/35 scale, and I have no reason to doubt it, then all 1/35 scale .50 cal ammo boxes I know are too small... Even the pre-painted photoetched ammo box from Eduard (photo below) is too small - it is a matter of just a fraction of millimeter, but still the Mission ammo doesn't fit. Maybe this is a good idea for next Mission Model product: give us photoetched brass ammo boxes that will accommodate your ammo!
If you want to know how accurate the Mission Models .50 cal ammunition is, take a look at the photo below. It shows the real .50 cal ammo and ammo belt clips. You can also see that clips should be gunmetal / metallic black color.
At $16 for set, those products are not cheap. The price translates to some 64 cents for one piece of ammo. If you are superdetailing your model or diorama, then this extra touch of realism is definitely worth the price. But high cost and quite tricky forming and assembly of ammo belt cause that this is not a product for every casual modeler.
Still - I highly recommend those sets for all you crazy superdetailers!