It's a bit late, I know, but in case you haven't finalised your plans for this rifle, here's a few suggestions for you.
---
Keep in mind that helical magazines become dramatically less worthwhile the longer/wider the cartridge becomes. The Bizon-2 holds 53 9x19mm rounds in a mag that basically runs from the trigger guard to the flash suppressor (~25cm). The same magazine can hold 45 rounds of 7.62x25mm ammo (and probably roughly the same were it to be converted to either 4.6x30 or 5.7x28).
Obviously feeding common assault rifle cartridges into a magazine of that length would largely obviate the usefulness of helical magazines in the first place. Having said that, the rifle's going to be a fair bit longer, so we can assume that the magazine can afford to be longer as well (in the case of your gun roughly 50% longer). If that were the case, I figure you can go one of (at least) three ways:
1. Consdier the weapon a lead-spitting carbine, and have it fire something fast and effective like the 5.7 or 4.6mm ammo. A long helical magazine with that kind of ammo could probably hold upwards of 70 rounds.
2. Use an existing rifle calibre - something relatively small and light. The obvious choice would be the 5.56x45mm cartridge. It's about 50% longer than the 5.7mm cartridge, so I'd guess magazine capacity would sit somewhere around the mid-40 area.
3. Invent your own rifle cartridge to help boost ammo capacity. Perhaps something caseless to save on space?
Hope this helped. I've put a couple of links at the end for size comparisons.
Img296.imageshack.us - 4.6x30mm (left) vs 5.7x28mm comparison.
En.wikipedia.org - 5.7x28mm vs 5.56x45mm comparison.
LTrain
Linc joined
Blah blah blah