Directly from Bushmaster:
We get rifles all the time for warranty service that have our lower receiver, but every other part is a gun show special. Before purchasing, look things over carefully. You are about to lay down a sizeable chunk of money, so take a minute to be sure of what you are buying.
Bushmaster's identifying marks are:
1. On the barrel, in front of the front sight, it will be stamped:
"B MP 5.56 NATO 1/9 H BAR" (exactly that way). On some barrels, it may be under the handguard, but it will still say that exact thing. The only difference might be a 1/7 instead of a 1/9 . This denotes the rifling twist.
2. Our front sights are held on by two taper pins, one through each of the bands of the front sight. We do not use hollow roll pins, never have - never will.
3. Our upper receiver has raised letters "FK" on the right side by the rear sight base.
4. Our handguards have aluminum heat shields in them.
5. Our barrels have the correct gray/green manganese phosphate finish, not black.
6. Our tele-stocks have a small roll pin holding the nut on under the sliding latch. If it's not pinned, it's not ours.
7. Our hammers and triggers have polished contact surfaces, and the hammer has the letter "P" on the left side, while the trigger has it on the right side.
8. Our bolt cariers have the gas key staked on by 4 V-shaped stamps.
9. We always use aluminum delta rings to retain the handguards.
10. Our buffers have a gold-colored finish on them, which is easy to see inside the stock.
11. Our aluminum parts all have the mil. spec hard anodized flat or matte finish to them, not shiny. This includes the receivers, the charging handle, and the delta ring. However, oil on the surface might make the normally flat or matte finish appear shiny.
The best insurance is to always buy from a local dealer, someone you can go back to if you have any problems. Most of them are reputable, and will at least help you contact the factory for service problems.