Originally Posted by Nomercy448

For the civilian user, the difference in the profiled AR-15 upper and the A3 receiver (WITH the forward assist, brass deflector, and ejection port cover).

The "ejection assist" as you call it, is ACTUALLY a "forward assist", i.e. it assists in pushing the bolt forward in case the bolt failed to close fully into battery because of fouling or dirt in the chamber. If the bolt doesn't close in civilian use, it's mostly indicative that you need to clean your rifle, and you shouldn't necessarily force the cartridge into the chamber. Soldiers, on the other hand, needed a way to make their bang-stick go boom when SHTF.

The triangular shaped protrusion on the right side is a "brass deflector", so the hot brass is ejected further outwards, instead of backwards.

Neither of those attributes are really any REAL advantage for the civilian user. However, the advantage to the A3 upper is the ejection port cover, as it helps keep dust and dirt out of the action.

The smooth side AR-15 upper is SLIGHTLY lighter than the A3/A4 and has less parts.

I'm not entirely certain why you would "upgrade" to a bushmaster lower for either of these rifles? Yes, I would recommend a trigger upgrade for either model, but not an entire lower receiver upgrade. Keep in mind, that the lower receiver is what qualifies as the "rifle" itself, so the SMART route for building or re-fitting an AR-15 is to buy the LOWER that you want up front (since it requires the FTR Form 4473), and re-fitting it with the upper that you prefer.

Personally, of the two, I would get the LR-308. The LR-308B is a shorter barrel, but ALSO a carbine length gas system, which in my experience exhibits LOWER muzzle velocities even for equal barrel lengths, and increase action fouling (more powder coming back through the gas system instead of burning in the barrel). Then consider the barrel is 6" shorter for the LR-308B, the muzzle velocities, i.e. power factor, for the shorter barrel will be much less for the 308B. The LR-308 rifle is also about 1.5lbs heavier and 6", because of the longer handguard/gas system, and longer barrel, which is more stable for longer range shooting. Personally, I prefer a longer barrel and rifle length gas system for higher muzzle energy, and a heavier rifle for stability. Some people prefer lighter weight (then they should be shooting a light barrel, not a heavy barrel model).

As far as the LR-308L (or ANY AR-15 with a low-profile gas block) and the lack of a front sight rail, your options would be to mount the sight on your quad rail you say you're installing, or buy a $30 replacement gas block. The 308L is the Lightest (hence L) of the 3, because it sports a light contour barrel, so it would be the easiest to carry in the field, but on the other hand, it would be the least stable for precision shooting.

So again, out of the 3, I would get the LR-308, add a trigger kit, and go shooting.

Also, get a reciever rug to keep your trigger group clean, and bolt buffers... You'll thank me in the long run...