I feel real silly asking this but whats the supposed starting ammunition for players like the devastator and tactical marine. Me and my friends have been looking through our books and cant seem to find the answer. Appreciate the help

It depends. The Devastator starts with a Heavy Bolter and backpack ammo supply (250 shots, in the HB's case) he can always use. The Tactical Marine gets a free clip of specialty bolter ammo for each mission.

If you're wondering how much basic ammo players have for their weapons, the book says to assume that their weapons come with "a sufficient supply of standard ammunition for the weapon" (pg 158). The basic rule is that you don't have to worry about running out of regular ammo for weapons. A lot of people don't like this, especially for longer missions without resupply, so other suggestions have been made (there have been quite a few threads on it). If the rule of not keeping track of basic ammo works for you, great. If not, I've seen suggestions that range from roughly 3-9 clips per weapon.

I run my grp with 8+1 Mags for their Bolters and 3+1 Mag for their Boltpistols. Worked out good so faar and made the group often change their tactic from simple Run&Gun to more the intressting Sneak-in&stab-them

Either don't worry about it, or be fairly generous. Soldiers shouldn't knowingly be walking into combat with woefully pitiful ammo supplies. If *I* was going to hold off a tyranid invasion and I was given three magazines, I'd make the QM eat them.

6-to-8+1 for main weapons and 3-to-4+1 makes sense, with perhaps a dozen grenades. For the heavy bolter, the Dev should have his backpack load, plus a regular magazine, and then everyone else in the kill-team should also be carrying a regular magazine for him.

Thanks a bunch, you really did help me out alot. I will try out the 9 clips for bolter and 4 for pistol and see how that runs.

Regular ammo has no requisition cost, so your players can take as many clips as you deem "reasonable" - I'd say the number of clips they are likely to need depends entirely on the length and type of mission they'll be doing.

More importantly to me, where do they keep all these extra clips? The models themselves never appear to carry ammo, although a few have a long pouch that might fit 1 extra clip.
The backpack provides power, life-support etc. and isn't actually for storage. Judging by the models and artwork, 1 clip of regular bolter ammo is almost as long as the upper leg of the power armour, so you could maybe fit 3 on each upper leg with a squeeze, then perhaps 2 on the chest and 2 on the back that would still be in easy reach for reloading. So 8 slots total, maybe 2 pistol bolter clips per slot. However you'd want at least one slot for the bolt pistol itself, maybe one for the combat knife. The forearms might be able to fit one slot comfortably, although probably only for a knife or bolt pistol ammo.

I do prefer to have Space Marines as cinematic types who don't need to bother about ammo, so the easiest thing is to assume they cache their extra gear before a fight, or have some poor recruit running around after them with huge bags of clips