Received an interesting e-mail from Forgeworld this morning, in response to a couple of inquiries. I'll just quote the relevant bit;
The Mortis Autocannon is currently the most popular kit we make and we just cannot make them fast enough to keep up with demand. So they have been removed from sale to allow the casting department to catch up with back order and to create some more for stock. What this means is that it is likely to be 1-2 months before it is available to order once more.
For reference, this was the left arm autocannon I had been asking about. So, heads up if you're in desperate need for a rifleman dread, you're going to have to convert something up.