Not sure if this belongs in "Bugs" or "Arsenal" but...
Problem: 4* Dark Thorn Blade's charge attack is more powerful than the 5* Barbarous Thorn Blade's charge attack.
I first noticed this in FSC, as I've done at least 40 runs with DTB before upgrading to BTB. Since upgrading, it has reduced my efficiency while killing trojans or when attacking Vanaduke.
This is a comparison between the 4* Dark Thorn Blade and the 5* Barbarous Thorn blade. (Yes, heat levels are both at 10.)
I did a little bit of work and based on the following,
Charge attack consists of a slash and a number of projectiles:
4*: Slash + 6-7x projectiles (assuming 6 for this comparison)
5*: Slash + 8x projectiles
these are my conclusions:
The 5* slash is more powerful compared to the 4* slash, but only slightly.
The combined projectile damage (6x) of the 4* weapon is significantly higher than the combined projectile damage (8x) of the 5* weapon.
The negative difference in projectile damage is not requited by the positive difference in the slash damage.
Here are some example calculations based on damage table information:
Top floor of stratum 5 vs beast/fiend:
DTB: 147[*6] (projectile) + 293 (slash) = 882 + 293 = 1175 total damage
BTB: 89[*8] (projectile) + 312 (slash) = 1024 total damage
Difference: The 5* blade hits LOWER than the 4* by 151 damage. (-151 damage)
Top floor stratum 1 vs slime/construct:
DTB: 5[*6] + 9 = 39
BTB: 3[*8] + 10 = 34
Difference: -5 damage when upgraded to 5*
This is true throughout all damage gradients.
Also, for the hologram:
Contrary to what the wiki states, the hologram is purely aesthetic and does not contribute to damage at all.
Some sort of proof can be derived from this video. I have also tested to see if this isolated hologram deals any damage; it doesn't.
TL;DR: 4* Dark Thorn Blade has a stronger charge attack than the 5* Barbarous Thorn Blade because of the 5*'s reduced projectile damage. The 2*->3*->4* upgrades show increases in projectile damage, but ->5* weakens it. This hurts the 5* blade especially when using the charge attack to hit enemies that are further away as there is no slash to help buffer the difference.
Solution: Increase the damage of the 5* blade's projectiles. 800 energy and 30,000 crowns is a lot to pay for a weaker weapon.
Is your 5* BTB fully heated yet?
Especially with a 5*, since it needs to be lvl 10 to be upgraded.
When it's upgraded, the 5* will have about the same damage as the 4*, except it is at level 1
It's probably lack of heat that is causing your problems, get the BTB heated to lvl 10, then compare damage