This is the projectile code
Code:

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		bullet_speed.x = 100 * time_step; // adjust the speed of the bullet here

		vec_set(pos1.x, my.x);
		c_move(me, bullet_speed, nullvector, IGNORE_YOU|IGNORE_PASSABLE|IGNORE_FLAG2|ACTIVATE_SHOOT);
		vec_set(pos2.x, my.x);
		vec_sub(pos2.x, pos1.x);
		pos2.x = pos2.x / time_step;
		pos2.y = pos2.y / time_step;
		pos2.z = pos2.z / time_step;

		vec_inverse(pos2);

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This is the particle, remove the remark with if use with 2nd method.
Code:

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function tail(PARTICLE *p)
{
	p.bmap = tail_image;
	p.size = tail_size;
	//set(p,BRIGHT|MOVE|BEAM);
	set(p,BRIGHT|MOVE);
	p.alpha = 100;
	p.red = 50;
	p.green = 255;
	p.blue = 50;
	p.gravity = 0;
	p.lifespan = tail_life;
	p.event = tail_func;
}

function tail_func(PARTICLE *p)
{
	// erases the tail after a while
	p.alpha = 100*(p.lifespan/tail_life);
}

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Method 1:
If I compute the tail by manual, I can get the expected particle tail, this method is found in the advance particle tutorial by Thomas Gisiger.
Code:

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		// * manual compute without beam option		
 		// first compute velocity length
		pskill_a = vec_length(pos2.x);

		// then compute the number of particles necessary for each frame
		pskill_b = ceiling(pskill_a*time_step/(tail_size*tail_p));
		
		// finally the tail is constructed here
		pskill_c = 0;

		while(pskill_c < pskill_b)
		{
			tail_position.x = my.x - time_step * pos2.x * (pskill_c/pskill_b-1);
			tail_position.y = my.y - time_step * pos2.y * (pskill_c/pskill_b-1);
			tail_position.z = my.z - time_step * pos2.z * (pskill_c/pskill_b-1);

			effect(tail,1,tail_position.x,nullvector);
			
			pskill_c += 1;
		}

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Method 2:
But I got very wrong particle if I just pass the velocity to effect with beam option:
Code:

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effect(tail,1,my.x,pos2);

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