Been awhile, but i had a touch of epiphany while looking around on Forge World's site. I came across a re-release of the old Space Marine piece, and they had the original advertising picture there. The stance shows the Marine reloading the bolter, and it shows clearly the actual cartridge breach. Low and behold, while only half paying attention, i noticed something. The rounds are placed in two columns.
A dual feed would account for the claims that the Bolter Clips hold 20-30 rounds without making them absurdly long. They would almost certainly have to be slightly broader than they are on the model, but its an answer to one mystery. How the rounds would feed without causing the weapon to jam is another matter, but its 35,000 years in the future, i feel we can assume they've found a solution.