They do Impact damage because they're SP weapons. This means that the solid slugs are likely the "default" ammunition type. Obviously, if you want them to fire explosive shells, change the Damage type to (X). I don't see what's so difficult about this. Bows are capable of doing (X) damage even though they're statted up to do (R) damage, by virtue of explosive arrows.

Besides, weapons are perfectly capable of accepting different kinds of ammunition, and these different kinds of ammunition may alter a weapon's default stat line in a way. You're arguing over nothing.

Apocalypse, Pg.107

The Hydra Flak-Tank is a specialised vehicle used for mobile air defence, based on the Chimera chassis. The Flak-tank can often be seen defending Imperial Guard columns and artillery positions from arial attacks, its long-barrelled autocannons sweeping the skies and unleashing a hail of explosive shells

Imperial Armour Volume One - Imperial Guard and Imperial Navy, Pg. 171

…, its four long barreled auto-cannons sweeping the skies and unleashing a wall of explosive shells.

Imperial Armour Volume One - Imperial Guard and Imperial Navy, Pg. 205

(Shows various Autocannon shells, no further details given)

Multiple descriptions of Autocannons firing explosive rounds AND having multiple rounds for different effects. And guess what? There is no issue with them using solid rounds, solid rounds have superior effects in some situations, like penetrating armor, while explosive rounds would work better in other situations.

On the heavy stubber being a small caliber, doesn't it look like this? (http://en.wikipedia.org/wiki/M2_Browning)

Which, like the Heavy Stubber is used, is a heavy machine gun and a pintle weapon (though as an infantry weapon Stubbers are used less, mainly due to the heavy bolter)

Cymbel said:

Nope, to me, it's more like this:

Or, for man-portable (with the option for a tripod) like this:

Those are both .30 cal M1919s (an A4 for the first, and an A6 for the second).

Really?

They seem pretty big to me

The second image you show is most certainly more appropriate for an autocannon, but here's a picture from Lexicanum (I loathe to use them, but it'll do):

As for an autocannon, look at this:

That sure looks similar (aside from a different ammo cassette) to the twin-linked weapon the Death Korps were using in your image.

First off, pictures without scale don't help us, so let's stick to official ones where they are holding them

Heavy Stubber (4)

VS

For a direct comparison in size with the same model, the same trooper, same perspective. What we can see is:

HEAVY STUBBERS ARE HUGE

Which fits the fluff, they are massive machine guns who use the antiquated stubber ammo (so have to be bigger to compensate).But they are used because they are cheap and hit pretty hard still, though not the most effective weapon.

Back to how big autocannons are

Just for comparison's sake, here is a Heavy Bolter

Cpt. Harkonnen said:

50 calibre weapons are common in 40k.