A better option than the 11.5 barrel with the welded 5.5 flash hider is to buy a standard 16" barrel and use the screw on extended flash hider/fake suppressor.
The extension covers up the extra barrel length and the rifle still looks like a tricky CAR-15 and as an added bonus, should you ever decide you want a standard 16" barrel with a Phantom Flash hider or whatever, you just screw off the extended hider/suppressor and add whatever gizmo you might desire.
I now own a Bushmaster upper as described and like it quite well.
I had a Bushmaster with the 11.5 barrel and welded suppressor and was not pleased with the accuracy or the functioning.
The only thing the CAR-15 'look' upper will allow you that the others won't is the ability to mount a standard bayonet.
You can alter a bayonet to fit the 16" barrel with the extended flash hider/supressor if you open up the bayonet mounting ring to fit the larger diameter of the extended hider.
I agree that it is better to spend the extra dough and get a fitted bolt and carrier for each upper you use.
One bolt, several uppers, means the bolt is going to fire more rounds through different platforms with slightly different tolerences and this is indeed going to accelerate wear on the bolt and the carrier.
Remember, the Mil-Spec mean rounds fired before failure is set at only 3000 rounds for the M-16 bolt.
Semi automatic bolts tend to run longer because they don't suffer near the abuse that a full auto bolt does but this is muted by constantly switching a bolt from one weapon platform to another.
Collapsing stocks look tricky and I confess I own a lower with a collapsing stock installed, but there is absolutely nothing wrong with the standard A2 fixed stock, and there are a lot of things right with it too, strong materials, right length for most shooters, storage space in the stock, an excellent shoulder gripping buttplate, the ability to mount accessory combs and cheekpieces though you can do all these thing to a collapsing stock too, the ones that have these features cost about 100 times more than a good old standard fixed stock.
