I would stay away from issue packs, having used them throughout my army career I always preferred a military version of a Civvie sac. You can't go far wrong with a berghaus as a first try, they are basic but very tough and can be picked up very cheaply on ebay if you can wait a while.
One option is a cyclops roc approx 60 litres, this is based on a climbing sac and has no side pockets. This means it is a slim rucsac and will not easily get caught on branches and bushes. you can extend the capacity by adding side pockets or strap roll mats, bivis etc to the sides.
The other option is the crusader 65-80 litres, pretty much the roc with zip off pockets that convert into a daysac. I have been using these for well over 20 years and they work for me. I have had plenty of other rucsacs, issued to me and bought by myself and keep going back to the crusader. sold my DPM one on ebay and bought a green version in great nick for £30, 2 weeks later.
The final option is the vulcan, 100 litres and a monster. Great sac but then you are tempted to pack all you own and regret it very quickly. Plenty of these come up on ebay.
The most important thing to be aware of is the fact that they come in different frame sizes 1-4, depending on the model.
Size 2 - 5'3" to 5'8".
Size 3 - 5' 9" to 5'11".
Size 4 - 6' to 6'6".
size 1 is found in the sacs designed specifically for women.
They fit close to the back and the frame can be shaped to your back. The plus of this is the stability over rocky ground, the minus is a sweaty back, only a problem if you are stopping and starting and don't like the cold feeling.
When buying, look for a small white tag sewn on the back, between the shoulder straps with a little number on, this refers to the frame size.
I could probably knock up some photos of my crusader if you want.
Hope this helps and prompts some more posts on rucsacs.
The added beauty of the roc or crusader is the fact that they have side compression straps that enable you to reduce the size of the pack and the crusader has extra compression straps on the lid to pull in the sac even further. The problem with a 45 litre sac is that you then have to buy a larger sac for longer trips, meaning more cost.
I have used my crusader without side pockets as a patrol day sac and so long as the back length is not too long it makes a great overnight sac. I admit it means the pack is slightly heavier than a true 45 litre sac but you have the added versatility of chucking some fire wood under the lid for a quick and easy fire towards the end of the day. Some people place their roll mat inside first in order to bulk out the pac and to keep it dry in the rain.
Others may offer other opinions on the merits of this but it works for me.
