I thought boat tail mean that the rear end of the bullet was open. But today i figured out that it apparently does not mean this. What does "BT" mean?

boat tail refers to a taper at the rear of the projectile.




see the three examples on the right...



ar-jedi

Oh ok, what is it when the bottom of the bullet is open and has a lead end.

I ask because all of the M193, M855, and silver bear ammo i pulled was like this, but the hornady bullets i bought to reload have a closed bottom.

it depends on which way the copper jacket was drawn over the lead core.  for example, open tip match (OTM) bullet construction, with the copper jacket drawn from the base, leads to a more accurate round.  this type  of ammo is not "hollow point" in the sense that it is designed to expand after impact.  instead, the more defined and better controlled OTM base geometry provides a more homogeneous match with the muzzle crown at time of bullet egress from the barrel.  this minimizes the inital upset of the projectile and results in a truer flight path.





see also,
http://www.thegunzone.com/opentip-ammo.html

ar-jedi

"Boat Tail" means the tail of the bullet is tapered.  A full metal jacket bullet will usually have an open base; this is due to the method used to make the bullet.  FMJ is closed at the nose.  There's a type of high-accuracy bullet called "open tip match" (OTM) that is manufactured in the opposite direction from an FMJ, leaving a tiny opening in the jacket.  THIS IS NOT A HOLLOW POINT BULLET.  Far from it.  OTMs perform very similarly to FMJs.  But since their bases are VERY consistent and smooth, and that they are made with extra care, they are MUCH more accurate than generic FMJ bullets.

Hornady makes a style of bullet they call "Encapsulated".  In their "encapsulated" bullets, there is no exposed lead anywhere.  They do this because some places require no exposed lead on the bullets for one reason or another (some indoor ranges require it because exposed lead at the bottom can get into the air and their filtration systems aren't set up to deal with it).

But it could just be the way the jackets were drawn over the lead core.  For example, JHP and JSP bullets designed for revolvers usually have lead exposed at the front of the bullet, but none at the base.  To see the lead in a Hornady XTP pistol bullet, you have to look down into the hollow cavity, but there it is, plain as day when you do.  The bullets are pushed into the jackets base first on these.  In traditional FMJ bullets, the core is pushed into the jacket nose first.