Of course, they do need some work as to how much you get and more specifically if any of it is compatible. Bolts up to heavy bolts should be, but unless you get the proper clips how will you load your gun with them? lots of minor stuff I can see rule lawyers sink their teeth into.
Ammunition requisitions far too vague, exploitable
A lot of guns also don't use clips or magazines. Revolvers, Pump Shotguns (some combat shotguns?), Bows, Crossbows, etc. Others have "alternate" mags, like the Heavy Bolter in most books can use a "clip" that is quick to load or the longer fed belt.
Make players buy ammo by the magazine, it punishes guns with small magazines. Make players buy ammo by the box, it punishes guns with large magazines.
Well, that is why you also look at the average magazine for a gun and make sure it at least matches that. But base it off real ammo box sizes first. Gets the best of both worlds and introduces the MUCH better quirk for SP guns...Caliber. Not every gun is chambered for the same round and even though you may find "autopistol ammo" you may not find YOUR autopistol ammo.
I liked how Gamma Slayers handled the different ammo types, just a letter (A-F) for each type, a side bar with some explanation as to what caliber that represents and what availability they have. Sure, in Warhammer 40K we have many more ammo types but the letters should suffice. It does probably simplify the ammunition types and merge some together but it would probably make it easier for some players to just note ammo as A: 36, C: 54 and so on, especially when looting ammo off enemies.
You could just make a table with the ammo quantity you get for each ammo type letter and the availability, plus the weapons it's used for, if you want a quick overview and - in my opinion - a nice compromise between realism and easy-to-handle. Like:
A 50 average Flamer, Hand Flamer, Heavy Flamer
B 50 scarce Bolt Pistol, Boltgun
C 100 scarce Heavy Bolter, Storm Bolter
D 100 common all Las Weapons
Just an example, I'm not sure which weapons would fit best in which category as I'm rather new to WH40K and don't know much about weapons we haven't used in the game yet.