Most of those are much more challenging to get than a joyeuse and you still don't have a DNC roll and SAM roll that a COR would have.
Agreed 100%. I've done a lot of weapon breaking for for Coffinmaker, Gun of Trials, and Nyzul weapon on COR with NPC. Don't discount SAM and DNC rolls (or even SAM and WAR rolls for max TP gain if you think you don't need the DNC roll Regen), they help a ton. They hit NPCs, and if you get a friend to help out you'll be flying through the WS points.
Cost is a non-issue. If you're breaking weapons, use cheap bullets. I know this forum has scared some people into thinking you shouldn't even OWN anything but Steel, but this is the place for garbage bullets.
if you know anyone with Savage Blade or another WS on the Fragmentation slot, you can make Light with Heavy Shot so it goes faster.
This is the only reason to go with RNG over COR - ability to make a level 3 SC depending on your partner. If you don't have an appropriate partner though, you don't have this advantage. Once you have Detonator, COR gets Light so it's easier to break Quicksilver/Coffinmaker on COR than it is for Gun of Trials.
As for breaking the gun on COR, I started the quest on RNG and broke the gun on COR as I lvled it. It has the same base dmg as martial gun just without the TP bonus, so I didn't get many complaints using it in parties.
BOOOO. Don't f'ing break your weapons in exp/merit parties. Guess what, people aren't using Martial for the DMG - they use it for the TP bonus. A "Martial Gun without the TP bonus" is a gimpy POS. You'd be better off using a variety of other guns - Corsair's, Coffinmaker (unlocked), Shark Gun, Murderer (even without the paired earring, the R.Acc+8 outweighs 1 DMG).
You might not have got many complaints, but that's either because people were ignorant or afraid to lose rolls. You were still gimp though. Unless you're playing with friends who know you're breaking a weapon, don't pull it out in an exp party.