in the weapons macro in the XPath
//bullet/@class
is what you want to change ;) (XPath/sel path should apply to any Weapon ;) just use it in the right file )

i make a mod of a mod, so actually i have this :

<?xml version="1.0" encoding="utf-8"?>
<!--Exported by: 192.168.3.101 (192.168.3.101) at 30.07.2013_13-26-21-->
<!--macros>
<macro name="bullet_turret_small_mg_macro" class="bullet">
<component ref="mg_bullet" />
<properties>
<bullet speed="900" lifetime="1.8" range="1620" amount="1" barrelamount="2" timediff="0.2" angle="0.25" maxhits="1" ricochet="0" scale="0" attach="0" />
<reload rate="18" />
<damage value="50" repair="0" />
<effects>
<impact ref="mg_impact" inside="mg_impact_inside" />
<launch ref="mg_muzzle" />
</effects>
</properties>
</macro>
</macros>-->

<diff>
<replace sel="/macros/macro[@name='bullet_turret_small_mg_macro']/properties">
<properties>
<bullet speed="1200" lifetime="20" range="13000" amount="2" barrelamount="2" timediff="0.5" angle="0.1" maxhits="1" ricochet="0" scale="0" attach="0" />
<reload rate="0.8" />
<damage value="150" repair="0" />
<effects>
<impact ref="mg_impact" inside="mg_impact_inside" />
<launch ref="mg_muzzle" />
</effects>
</properties>
</replace>
</diff>

so i need to change the component ref "mg_bullet" in "plasmaemitter_buller" but i'm really dumb in command...

ah, now i understand what you want.. you want to change the properties if the bullet, not which bullet is used...

your diff file looks about right, but the comment containing the original is a diffrent version from what i have here.. which file do you want to change?

muh.. i don't know, i just want the small mg turret, shot small fighter plasma emitter blue shot, "plasmaemitter_bullet" can i do that in this XML file ? beetwin the "diff" ? like.. in "effects" or something.

what file under what path is this in your Mod? (this defines what core game file you want to change, it is not something chosen arbitary)

i don't know.. for now i just change the value in the mod's XML files. in effect if i put "plasmaemitter_impact" i work perfectly, i think i just need to change the component ref mg_bullet to plasmaemitter_bullet, in the bullet_turret_small_mg_macro, but i don"t know if it possible. i already try lot of thing like put the component ref line beetwin "diff" but not working, this XML can change all properties and effect of the bullet but impossible to change the bullet aspect itself, i'm kind lost..

ah, thats what you meant.. (for me "modding another mod" means changing an existing mod with a new one, you, but you meant "playing around in an existing mod")
what you think you need to change is inside a Comment (notice the <!-- marking the beginning of it and --> for its end) - these are just there for Documentation Purposes or helpful hints, but will not actually be interpreted. to be precise its is the Content of the Original File the Author changed here, very likely for easier reference.

to actually change this entry for the Game you add a new line between the <diff> and <replace ....> with the following content: (or put it after the </replace> and before the </diff> - whiche of these two options doesnt matter)

ok i will try it, thanks !

meeeh... i try lot of things with this line but still doesn't work. anyway, thanks for your help. i try to change this f***ing bullet since yesterday.. i'm done.