Do these weapons follow standard SP ammunition costs? Or are there rules for specific purchase of these weapon ammunition? An autocannon in particular is a bargin as a sniper weapon as well as a squad support weapon. If they buy standard SP rounds, it will be incredibly cheap.

Is this right?

Why not? But we can see the word "shells" in the description but the cost is similar. I think it is right becose you may see their Rof and clip and their own costs.

Just keep in mind that the bore of autocannon shells is significantly larger than any other SP you might be looking at, to the point that they are in fact called shells . They're often used as the primary weapon on tanks, and the shells themselves are about the size of your forearm. That, coupled with the fact that the most reliable way to come obtain their ammunition is through the Munitorum (as they are, indeed, military weapons outside the scope of the general populace), would lead me to believe that the cost would be significantly higher, so I'd count them as at the very least Exotic (20 thrones/round on Table 5-11: Ammo (pg. 142 DH core rulebook) as they are also still considered to be Very Rare. When it lists "Shells" on that same table, it is referring to shotgun ammunition, not light artillery.

Moriartus said:

It does depend on the source. The recent (as in, out this month) novel Gunheads (dealing with an Imperial Guard tank company) depicts a Chimera's turret-mounted autocannon (or the twin autocannon of a Leman Russ Exterminator, I can't remember which) as essentially being 30mm cannons (and thus at least superficially the same as modern-day autocannons).So, rather than the shells being the "size of your forearm", they'd be a little under 6 inches/15cm long...

Lol that works, too. Guess I was going from the scale of the models.

I consider assault cannon to have a smaller bore than autocannons (about 12.7mm to 15mm), so they use rounds that are 5 times the cost of normal shells (yes, shells not bullets, they're explosive after all). Autocannons I consider to be other either 20mm or 30mm. 20mm autocannon (the man-portable ones) use shells about 10 times the cost of normal shells, and if they're 30mm shells (I go by the ones for the GAU on the A-10, about the size of a milkbottle, for tanks/Chimeras/Sentinels etc), they're twice 20 times the cost. My players have already looked into half-inching a tank, so I've made sure I'm prepared...

Remember that if you buy in bulk you will often get discounts!