I want to shoot my .308 bolt gun & AR10  off a backpack (not at the same time). I have a place all to myself where I can shoot steel out to about 800 yards from a number of directions. I want to walk out with the gun and backpack for the day and shoot at different distances. The backpack would need to hold magazines/ammo, range finder, IFAK, binoculars, knife, food, water, etc. for the day. I may supplement it with a chest rig for the AR10.

Right now I have an old LAPG 3 day pack I was thinking of using,  but it may be too big and too cheap for this use.  What backpack do you guys recommend?

I'd start with the 3day bag to see how things go.
I've got a box full of "perfect" bags that ended up not doing what I wanted.

I like ALICE packs and external frame packs in general.  Mind your muzzle, gas cuts can shred bags before you know it.

Good advice. Did you ever find one that worked, assuming you were using it for the same thing?

Both rifles have short barrels so I'll keep this in mind. Thanks.

I was speaking in generalities.
I have packs that would work for shooting off of. I just don't

I'll give you an example. I bought this CamelBak day pack years ago.
Turns out I don't care for bladder. So back to water bottles, but the CamelBak bag doesn't have bottle pockets.
Into the box.
I bought a bag with Molle webbing on it and added a couple insulated bottle holders.
Same compartment layout, even has a spot for a bladder if I change my mind. But I can add any kind of bottle holder, or other pouch, I want on the side.
Sometimes you just need to find what works for you.

I am trying to keep weight to a minimum and only carry what I really need for a couple of days in the woods/field. Given some of the ranges I will be shooting I will need a decent rest, so the backpack will pull double duty as both storage and a shooting platform.