Had a chat with some other folks about this. We came up with some optional or addendum rules to use for abstract ammo bearing rules.

Option to allow weapon slots you are not using be converted over to ammo for primary weapons. Such as if you do not have 2 basic weapons you may allocate the 3 reloads over to your only basic weapon and/or pistol. Like wise a pistol slot weapon may have its 3 reloads given to another pistol or be replaced with 1 basic reload.

Heavy weapon reloads are also on a case to case basis From what I earlier said carrying a single heavy weapons with pistol weapon. 1 heavy weapon and 1 belt or 3 drum reloads in the case of a heavy stubber seems fairly reasonable. But a missile launcher would perhaps get 3 extra missiles due to bulk. 2 lascannon batteries or 2-3 extra plasma cannon flasks would not be out of line as well.

Carbines and some smaller shotguns count as pistol slot or basic slot weapons. Thus a las carbine or some short pattern autoguns may replace a hip slot pistol allowing a character to trade the longer range of a carbine over the close combat potential of a pistol.

Backpacks having 6 points of reload slots. With 1 slot carrying a basic or pistol reload, 2 for any magazine deemed odd such as a quiver or grenade launcher drum and 3 for a heavy weapon reload. Getting stuff out of a pack is a full round to take off the pack and at least one hand must be free and another full action to get the item out and in hand. As well a final full action to get the backpack back on.

Backpacks can be replaced by a number of weapon specific tanks or belt feeds. Such as hellgun capacitors, flamer and melta tanks or something more esoteric. I would go with the Macharian handbook which details those in depth especially for hellguns. As you lose backpack space for not just ammo the gain should be equal to roughly 8 reloads. Giving the basic flamer 24 shots without having to reload. At the expense of only being able to carry a limited amount of misc equipment.

If the idea of using extra equipment to simulate bandoleers and ammo belts does not appeal to you. You can apply the backpack rule to the characters themselves giving them either 4-6 points of extra ammo carrying capacity like the backpack. A further option of instead allowing them Strength Bonus in extra reloads with heavy weapons counting as 2 reloads.  This of course will favor stronger characters.

Quality ideas for previous posts ammo bearing gear. As well each character should also have access to a free common quality one at character gen.

Gunslinger belts/shoulder rigs: Good quality holsters are universal for whatever pistol or ammo. Best allow a user with the quick draw talent the ability to holster a pistol, sword or other one handed weapon carried on it as a free action once per round

Bandoleers and other generic ammo gear: Good: gains 1 free slot for any basic or pistol reload or 1 grenade or 1 quickdraw item as weapon/gear storage. Best: The flexible design allows this setup to carry 3 points of any weapon reloads like a backpack by either expanding mesh pockets, adjustable straps or just ingenious construction.

Weapon/Gear storage: Good quality allow one item such as a rebreather, auspex, medkit or knife to be drawn or replaced via dummy cord/quick release pouch as a free action once per round. Best allow for either 2 such items rigged as per good quality. (As well the option to use the common quality universal slot for this)

The following is an example of a character geared with this slot style. Followed by comparing ammo weight tracking in simulation vs abstract styles.

Confesser Shanks carries as a basic weapons flamer and a pump action shotgun. His secondary weapons being a hammer and autopistol. As well 2 frag and 1 krak grenades Each gun with 3 reloads stored about his person.

As he is a militant sort he wears a good quality guard issue weapon storage rigging.  With 2 flamer and 1 shotgun reload with 2 autopistol mags tucked into the extra slot. He has chosen to carry his medikit for quick access. His backpack contains 4 flamer and 2 shotgun reloads one being along with a couple ration packs, a dateslate and his good robe.

So all total he has 10 flamer fuel tanks 4 of which are normally not easily accessible. For 30 shots of fuel with a lot of reloading. His shotgun assuming its not a custom job with a fire selector has 8 total reloads for 64 shells some of which could be exotic ammo. His pistol has 6 mags for 108 shots. As well as 3 grenades, a warhammer and a small knife tucked into his belt

if we add up the above weapons and any basic equipment as well as armor.  We can easily figure if he is inside his basic carry weight. Assuming he is average strength and toughness bonus 3 we get 36 KG as our max weight.

Using the base games ammo system 10 flamer tanks is a reasonably hefty 6kg as much as the flamer itself. We can either use this, Kaihlik's method of ammo calculation or possibly omit ammo weight entirely.  The shotgun being 5kg and 4kg for ammo. The autopistol totals 3.75kg, his grenades 1.5 and 4kg for the hammer and .5kg for his knife. As well 2kg for his medikit and .5kg for a dataslate and finally 1kg for his weapon/gear storage webbing and 1 kg backpack.

Total is 35.25 kg for all his equipment and ammo minus any odds and ends such as a nice robe or foodstuffs. Though with so many weapons he will obviously be passing anything other then a single piece of mesh armor. 11.25 kg of this weight is strictly ammo for 2 basic weapons and 1 pistol.  However if he was a ability bonus higher in both toughness and strength he would instead have 56k total. Allowing him to carry all this while wearing Storm trooper carapace armor.

If we remove having to worry about ammo weight. Instead of simulated tracking of each magazines weight as new gear or plot related items present themselves. We allow characters with more average scores to carry a reasonable full combat load while not being able to break the abstractions limit on maximum reloads and carried weapons. Thus the 11.25 kg of ammo would instead have to be used on 11kg of guard flak or a combination of lighter items and misc equipment. While lower and higher characteristic players will not have to worry about having to balance either too many reloads going over limit or being able to take insane amounts of ammo with decent armor.

In summation we replace simulated bean counting of ammo weight with a system of abstracted bulk. With ammo weight on the bottom end we see weaker characters struggling with a pistol and a basic weapon. Along with whatever misc equipment and armor. While a hulking character is able to not only carry a larger weapon but a arsenal of smaller one potentially. As well the idea of specialized gear itself to bolster ammo capacity. Replacing bandoleers and webbing rigs with free points of extra ammo capacity much like a backpack. Whatever fits your groups play style.

Since I cannot edit the above post for some reason. I recently tested and found the previous 3 reload per weapon system flawed. I instead devised an ammo point pool which is much more balanced then the previous one.

Each character has limited number of slots they may use to carry weapons as follows.
2 basic or two handed melee weapon OR 1 basic/two hand melee and 1 heavy weapon,
2 pistols and/or one handed melee weapons
1 one handed melee weapon
4 small weapons such as knives, powerblades or tiny pistols and/or grenades of any type
Intelligence bonus in concealable small weapons I.E knives and hold out laspistols.
(2 basic weapons slots one of which can be heavy?)

Carbines including some small shotguns can be placed in either a pistol or basic weapon slot.

A character may carry a maximum of 12 basic or pistol reloads, A character may replace 3 of these reloads for an odd weapon "ammo pool reload" such as a quiver of arrows or lead shot w/ powder horn or 3 reloads for 1 reload of a heavy weapon.

Thus a maximum reload capacity of 360 standard autogun shells, enough charge packs for 720 standard power lasgun shots, a full 4 belts of heavy stubber ammo or 12 missiles and nothing else. Obviously most players will take a mix of ammo rather then rely on a single firearm. As well ammo already loaded into the weapon does not count against reloads.

A character may replace reload points for small weapons 2 for 1 to get more grenades or other weapons. These extra weapons weight applies to max carry weight as normal.

If the above feels lacking. of the following options is provided for further ammo carrying capacity. Characters add Strength bonus to reload points. Another option is giving all characters 3-4 extra points regardless of strength. The final option is to allow for wearing of ammo bearing gear which increases available extra reloads on the basis of what type and its quality.

Some weapons have small magazine sizes such as double barrel shotguns they instead draw from an ammo pool of a reload size of comparable weapon of the same type. In this case the pump action shotguns 8 magazine size. A single shot grenade launcher would use the stock models 6 round magazine to determine its reloads. Other weapons such as the Bow or Handbow. Have no comparable weapon to go off of instead replace 3 reload slots with a single quiver or boltcase containing 30 rounds. As it can be assumed the arrows or bolts are bulky and provide the same number of shots as a autogun mag but take up more space. In each case the GM has the final say.

Cetain weapons have multiple ammo sources such as hellguns. They can either replace a backpack for X number of shots from a capacitors pool, use standard lasgun packs at a reduced capacity or finally banks of small buttpack capacitor which take up 3 reload points each.

Weapons using fire selectors allow 3 magazines to be loaded to the weapon without cutting into max carried reloads. However this makes the weapon heavier then before.

Heavy weapons are unique in this way as each has a different number of extra reloads which depend on its type. A heavy stubber using 100 round belts would have only 1 reload while one using 40 round drums would get 3 reloads(the same as a basic weapon). A missile launcher 3 reloads. Las, plas and melta cannons 2 reloads.

Exotic weapons usually behave much like a basic or pistol weapon but certain ones may have an ammo pool magazine.

Backpacks can carry 6 points of reloads in the same way as bonus reloads for high strength or basic point pool. Taking a backpack off or putting it on is a full action. Taking ammo or items from a removed backpack is a move action. Once you take ammo from a backpack you can reload as per normal actions. Characters should be assumed to replace carried ammo from a backpack between combat if permitted by narrative time. I.E. Not being chased through the underhive or other tense situations.
Good backpacks only take a move action to remove or don and Best are environment sealed. (other quality options such as good quality having a concealed cavity to smuggle items are possible)

A ammo backpack such as a flamer, plasma fuel tank provides 8 reloads (if not specified) of whatever weapon it is attached too. However such backpacks preclude carrying additional equipment in a standard backpack.
 

I have worked on this more but I cant upload it to this board, its on the last post of this thread if you want a look. darkreign40k.com/drjoomla/forum/index.php

Its not finished at all, I would completely reformat it if I had the time/energy and I would be aiming to bring the weight and cost of ammo into one document and rewrite a fair bit of the costs to make them more consistent across the various sources.  There is also a couple of weapons in there from Rogue Trader that are there because they are relevant to my players.I have not gotten around to a fair number of weapons and would be interested in suggestions. I can explain my reasoning of anything you are unsure of why I set the values like that (plasma looks weird because you expect the fuel to weigh nothing compared to the container but its based off of an actual hydrogen container I found online).

If you have any other questions let me know.

Kaihlik