Kurtz - Sorry, it was my impression you were talking land ordnance - you mentioned 105mm. I'm no expert of sub guns, sorry. Unless you are talking of that dead-end concept, the submarine cruiser, with big guns, I would go with thannon's comments.
Howitzers and mortars have a high trajectory, which offers flexibility to the gunners as they can, in effect, shoot over hills. Most land artillery fire missions use indirect fire, in which the round lands in a place where the crew has not a hope of seeing it. Such shoots depend on an observer to correct the fall of shot or else, recently, some highly impressive (and expensive) wiggly-amps kit. To give the flexibility in trajectory, the propellant charges need to be variable, which leads one back to the charge bag concept, allowing the gunners to vary the muzzle velocity. Mortars and howitzer muzzle velocity is generally pretty low.
Standard naval gunnery has generally been more direct fire in nature, with the gunners (or at least somebody in fire control) being able to see the fall of shot. As with tank guns, a high velocity is most useful for this. The main reason for separate ammo would be simple weight (the big boys are heavy enough without having to try to load everything at once) and size (imagine having to design a battleship ammo handling system for 16" fixed rounds, with the projectile permanently fastened into the casing!).
Of course, really big naval guns could fire very long distances and aircraft were often carried as spotters. In a shore bombardment role, observers again might be necessary. In general however, what I said above is, while not universal, almost so.
In short, with smaller-bore high-velocity rounds, fixed ammo is standard as, among other things, it gives you a higher rate of fire. With small-bore howitzer, semi-fixed is the norm. Anything above a certain size (much above 5" or so) tends to be separate due to handling constraints. Again, there are exceptions to every rule.
As to our starting video clip, they were shooting in the direct fire role, something land gunners practice but rarely get to do for real. (In general, this is usually considered a Good Thing as if the gunners can see the target, the Bad Guys have been allowed to get waaay too close. There are exceptions to this too.) Fire a round, watch where it lands, correct your own fall of shot. Lotsa fun.
piersyf - I understand your arguments and have sat in on the odd meeting on whether or not to provide protection. From my point of view however, blast pressure (and thus danger) drops off very quickly (the cube root of the distance) and is thus a very short-range threat, whereas fragments are dangerous to much greater distances. To my way of thinking, if you have the time, it is therefore a no-brainer if you are concerned about your troops and assets. The dangers of containing a blast are far outweighed by the dangers of not containing shrapnel. However inaccurate the bad guys may be, Chicken Little only has to be right once. As an old sergeant instructor once told me (as I was sweating my first trench-digging lesson), all the sweat you expend in your entire career only has to save your life once to be worth all the effort. I 'm not much concerned with trying to maintain appearances - macho posing gets people killed and I'm with Patton on such things. Different case if it's shoot-and-scoot, of course.
WRT the rate of fire of mortars and such. While the Taliban have used them, sitting in one place and firing a sustained barrage against an established base is a good way to meet 72 young ladies in a hurry.
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