Add this to your effects file
<tracer_blue ClassName="TracerEffect" TracerLength="1.7" TracerWidth="1.7" TracerRorationSpeed="500" TracerBlendMode="Add" TracerSideTexture="TracerSide29.dds" TracerEndTexture="tracerEnd29.dds" SmokeLength="600" SmokeWidth="7" SmokeRorationSpeed="1200" SmokeBlendMode="QuadSprite" SmokeSideTexture="tracerSmoke4.dds" SmokeEndTexture="black.DDS"/>
<fx_tracer_blue ClassName="GroupEffect" Effect0="tracer_blue" />
and the xdp: AvHistory_gr_Mg151-20L_round.xdp
<?xml version="1.0"?>
<UnitData>
<General Allegience="" LongName="" ShortName="" ModelName="" BlastDamageScale="1.0" ImpactDamageScale="1.0" FireDamageScale="1.0" Priority="2" Type="weapon" EnteredService="01/01/1944" BlastDamageMod="0" ImpactDamageMod="0" FireDamageMod="0" AllowSpawn="n" Category="weapon" Country="germany" Mass=".2781"/>
<Description String="20 x 82 x 115 g"/>
<Weapon WeaponType="shell" ImpactDice="1" ImpactDieSize="1" ImpactOffset="38.22" BlastDice="1" BlastDieSize="1" BlastOffset="31.96" BlastRadius="0.8" FireDice="1" FireDieSize="1" FireOffset="7.8" ExplodeEffect="fx_grndexpl_t" GroundEffect="fx_bltgnd_m" AirEffect="fx_bltpln_m" WaterEffect="fx_bltwtr_m" TracerEffect="fx_tracer_blue" />
<Loadouts/>
<GunStations/>
<DamageBoxes/>
<Systems/>
</UnitData>