weapon view model projection matrix?

im trying to do seperate FOV for weapon models
this shaders flips the model vertically and also flips backfaceculling
how do i solve it?

shader_type spatial;
render_mode blend_mix,depth_draw_opaque,cull_back,diffuse_burley,specular_schlick_ggx;
uniform vec4 albedo : source_color;
uniform sampler2D texture_albedo : source_color,filter_linear_mipmap,repeat_enable;
uniform float point_size : hint_range(0,128);
uniform float roughness : hint_range(0,1);
uniform sampler2D texture_metallic : hint_default_white,filter_linear_mipmap,repeat_enable;
uniform vec4 metallic_texture_channel;
uniform sampler2D texture_roughness : hint_roughness_gray,filter_linear_mipmap,repeat_enable;
uniform float specular;
uniform float metallic;
uniform sampler2D texture_normal : hint_roughness_normal,filter_linear_mipmap,repeat_enable;
uniform float normal_scale : hint_range(-1,1);
uniform vec3 uv1_scale;
uniform vec3 uv1_offset;


uniform float FOV = 40.0;

void vertex() {
	UV=UV*uv1_scale.xy+uv1_offset.xy;
	if(FOV != 0.0)
	{
		float onetanfov =  1.0 / tan(0.5 * (FOV * PI/180.0));
		float aspect = VIEWPORT_SIZE.x / VIEWPORT_SIZE.y;
		PROJECTION_MATRIX[1][1] = onetanfov;
		PROJECTION_MATRIX[0][0] = onetanfov / aspect;
		POSITION = PROJECTION_MATRIX * MODELVIEW_MATRIX * vec4(VERTEX.xyz, 1.0);
		POSITION.z = mix(POSITION.z, 0, 0.999);
	}
	
}



void fragment() {
	vec2 base_uv = UV;
	vec4 albedo_tex = texture(texture_albedo,base_uv);
	ALBEDO = albedo.rgb * albedo_tex.rgb;
	float metallic_tex = dot(texture(texture_metallic,base_uv),metallic_texture_channel);
	METALLIC = metallic_tex * metallic;
	vec4 roughness_texture_channel = vec4(0.333333,0.333333,0.333333,0.0);
	float roughness_tex = dot(texture(texture_roughness,base_uv),roughness_texture_channel);
	ROUGHNESS = roughness_tex * roughness;
	SPECULAR = specular;
	NORMAL_MAP = texture(texture_normal,base_uv).rgb;
	NORMAL_MAP_DEPTH = normal_scale;
}