I wouldn't worry about it. Yours wasn't the comment that caused the physics debate.
The tthree main problems with rail guns is that, due to Newton's First Law they would have monstruous recoil that would require extremely powerful compensation, that the energy required to accelerate any object larger than a subatomic particle is colossal, and that the magnetic forces that accelerate the slug put the rails under steel-bending stress. How the Tau figured out how to solve these problems is anybody's guess.
You know we have rail guns now right? It's just they are bulky as heck because we do not have access to 40k batteries. Also capacitors.
Yes, I do. They are the size of a small warehouse and need to replace the rails after a couple of shots because of the aformentioned problems.
While that would indeed pith a baneblade it's also a Naval Cannon! That's your prototype for macrobatteries, not an infantry weapon. They can be made smaller. 40k batteries would be the difference between a bb gun and a lethal weapon.
If you're over stressing your rails, it means you did a poor job of stress analysis on the design. It's not like newtons 2nd law of motion is news.
On topic the Backpack Ammo supply is clearly a terrible, terrible option. It's NEVER better that simply carrying a sack of ammo around and therefore pointless. It should at very least hold as much ammo as the equivalent weight in charge packs/magazines/shot pouches/what-have-you. It should probably also give you some bonus (like 10% extra ammo) to pay for the inconvienience of not being easily broken down into smaller and more sensible loads. Or it chould provide some other benefit like an improved rate of fire. Technically it does I suppose, since you don't have to stop and reload, but frankly I've never seen a 40k combat go on long enough for a gun to need reloading, not even when we fought the Bloodthirster. Maybe my group just doesn't use full auto enough? Heck I designed my characters custom gun and gave it 5 shots!
(Imagine the Tarus Judge carbine blown up into a sniper rifle and designed to shoot bolts. And then given a lever action because it's 40k and everything is a little bit stupid. Barneto M-8 Boltrifle, Basic, 1d10+5X Pen 4 S/-/- 200m Range Reload 3 full, Accurate, Tearing, 15kg.)
Maybe the Imperium did have efficient, man-portable rail guns that skirted the issues discussed here. I mean considering what else has been done it's not completely unfeasible (Also, actual legitimate magic), but like many things Imperial we just forgot how to build/operate such device.