From TF2 Wiki
The Buff banner (also known as the BB, Bugle or just Banner) is a secondary weapon for the Soldier, unlocked by completing 17 Soldier Achievements. It appears as a tattered battle standard, mounted on a small backpack. Equipping the Buff banner makes visible to the Soldier his "Rage Meter", which he can fill by dealing damage. The Rage Meter becomes full after the Soldier has dealt 600 damage in a single life, at which point he can sound a bugle to rally his troops. During the 10 seconds following this rallying call, all hits made by allies positioned within 450 units of the Soldier (including his own) become Mini-crits. Buffed players within the radius emit team-colored rings of light from their feet, and their weapons brighten similarly to a crit boost.
Though players only receive the buff while positioned within the buff radius, the source(s) of their damage need not be within the radius, so things like in-transit rockets, pre-laid sticky bombs, Sentry guns, and afterburn will deal Mini-crit damage as long as their creator is buffed when the damage is actually dealt. Likewise, a damage source created within the buff radius will not gain a Mini-crit benefit if its creator no longer has the buff when the damage is dealt.
Switching classes or dying empties a Soldier's Rage Meter. Unequipping the Buff banner does not empty the Rage Meter; it merely hides it. In fact, a Soldier can build Rage without equipping the Buff Banner at all, tallying up damage with his weapons before changing to the Buff banner at a resupply cabinet for a pre-filled Rage Meter.
None. Rage Meter requires 600 damage to fill.
- Attack Interval: 10
- Trumpeting Time: ~3
All times are in seconds. Times are approximate and determined by community testing.
- The bugle was originally issued to American monkeynaut Poopy Joe as a means of calling for help before it was acquired by Mann Co. in entirely innocent circumstances. 
- The Buff banner uses the previous taunt (Juggle) of the Shotgun, used by the Soldier. After the WAR! Update, Valve have replaced the Shotgun's "Juggle" taunt with "21 gun salute" taunt.
- The Buff banner has separate, unique calls for both the RED team and BLU team.
- The glow effect of the Buff banner is a different color for both teams, but neither is the same as their team colors. RED has a gold buff glow, while BLU is teal or mint (greenish blue)
- If in range of a friendly Pyro, the Buff banner will cause enemies on fire to take Mini-crit damage, even if the fire was started before the Pyro was buffed.
- If in range of a friendly Engineer, that Engineer's sentry will do Mini-crit damage.
- On The Buff banner's bag there is a logo with an Eagle on with the text over spelling Screaming Eagles. This logo is similar in design to the unit insignia of the US Army's 101st Airborne Division, which is nicknamed the "Screaming Eagles".
- Mini-crits from the Buff banner are not assigned to projectiles, but rather the players that projectiles originate from. If a Soldier is next to you during an active Banner and fires a rocket, but you either run out of rage or that Soldier moves out of banner range before the projectile makes contact with an enemy, that rocket will not be a Mini-crit. In order for a projectile to be a Mini-crit, the person who fired that projectile must be within range of an active Banner at the time that the projectile makes contact (or is detonated, in the case of Sticky bombs) with an object.
- Telefragging an enemy results in a fully charged rage meter, regardless of what class the enemy you telefrag is, and how much charge you have initially.
- There are two unused textures for separate "attack" and "defense" banners in the game files, which depict an explosion (with an orange background) and a shield (with a green background), respectively.
- You can play the Buff banner underwater, which in real life would cause someone to choke.
- Holding the "Fire" button while the rage meter is full will allow you to hold the Buff Banner ready until you let go of the button to activate it.
"The Emperor's Trading Advice" As a Vintage weapon, the Buff Banner is very cheap and easily the same value as any weapon.
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