I've really liked some of the midlength and full length rail systems / handguards on 16" barrels. In order to do that, do I just get a low profile gas block? Does that fit under the handguards / rail system then?  Would it matter if the rail system / handguard was/wasn't free floating?

I'm assuming that is how it would be done, since you really can't move the gas ports on the barrel. I've done some reading on here, on custom barrels, and how having the gas port too close to the muzzle can make some ammo not cycle correctly.

I haven't seen a whole lot of "stock" uppers configured like this from companies like Bushmaster or RRA, so I'm assuming that most of these setups are custom. I was hoping to have some info shared, so I don't make some bad purchasing decisions when I go shopping for a new upper.

Also, what is the difference between an A3 and an A4 upper?

Thanks,
H.

Not only do you want a float rail, you also need one.  Non-float rails, by definition, have a point of contact at the gas block.

Larue and many others make a gas block that fits under a float tube.

Ideally you want a mid-length gas system for 14.5 to 16 inch barrels.  No, they can't be mover easily, although some people do try it from time to time.  People do run into problems when the muzzle gets 2 inches or closer to the gas block.  This can't be avoided (easily or practically) with the 10.5's.

I like stuff from MSTN, but ADCO, Dennysguns and a few others on this site are also good to go.

The difference between and A3 and an A4 upper is that one company is sufficiently inept that they named their clone the XM15A3.  Mind you, the clone is of the military's M16A4.  The M16A3 is actually an A2 upper with the auto feature instead of burst.

The M4 and M4A1 are differentiated, similarly, by the provison of an auto feature on the A1 and burst on the regular M4.  No M4A3 exists.

Excellent site, Mr. Rausch.