Yeah I would again bow to Running Wolf on this, but, 1) do you always carry the shipping tube? If so, no problem, a reinforced soft pouch should work. If not, will bare round tear up soft pouch? 2) If so, how about some combo of kydex and cordura? Kinda like a kydex holster insert with cordura cover. Or maybe hypolon and cordura? 3) How much does a single round weigh?
This is an interesting project because you are approaching the limits of what a soft pouch can/will carry without shitting the bed. When do you switch from soft goods to metal/polymer mounting system. I'm thinking of those new "skeletonized" pouch holders that seem to be all the rage with JSOC these days. This might be a good app for them.
One thing that MR did that is interesting is the "Overload" system like my buddy currently uses. With a few mods to a Crossfire system, you could have a pack frame that could work as: a stand-alone back panel for various crew served weapons systems (Goose, Jav, 240, etc); with a pack bag (with perhaps some kind of pass-through attachment system for heavy shit on the back wall); or with tab strap extensions, fit the bag (like maybe a DG-3) over the crew served stuff, ala "overload".
Not sure how much/often you want to do that, as I sit here with a beer, but if you have to carry all that shit, this might be a better way than laundry sacks.
Another idea from the GIF: What if you ran a zip around the hard panel, so you could add a pack bag, with the hard panel now acting as the rear wall, which can now still mount heavy shit. Kinda like the AWS Yote beaver tail. Yeah if you want to trust a zipper that much, WTF, put a center zip in in too! Ha, there's your ultimate "tri-zip".
We now return you to your regular program.