Rendering to integer formats

I'm now trying to render to an integer format as attachment 1

GLFormat = GL_RG_INTEGER;
        GLInternalFormat = GL_RG32UI;
        Type = GL_UNSIGNED_INT;
glTexImage2D( GL_TEXTURE_2D, 0, GLInternalFormat, Tex->Info.Width, Tex->Info.Height, 0, GLFormat, Type, NULL );

I have GL_NEAREST filtering on this texture.

And in the object's shader I have this :

layout(location = 1) out uvec2 VelocityOut;
//...
VelocityOut = uvec2( vec2( VelocityVertex.xy * vec2(0.5) + vec2(0.5) ) * vec2( 4294967295.0 ) );

VelocityVertex is -1..1 and I'm trying to pack it in 0..4294836225 (allegedly MAX_UINT)

The problem is that I don't get the results back to -1..1 in a following post processing shader where I have :

uniform highp usampler2D Texture1;

//...

vec4 curFramePixelVelocity = vec4( texelFetch( Texture1, ivec2( OutTexcoord * Texture0Size ), 0 )) / vec4( 4294967295.0 );
    curFramePixelVelocity = curFramePixelVelocity * vec4(2.0) - vec4(1.0);

The exact same shaders give the correct results on my Desktop AMD card. Using a GL_RGBA8 texture attachment that I just convert from -1..1 to 0..1 works as expected but I'm trying to increase the precision by using an integer format.

UPDATE : I think my problem is related to needing to convert to float from UINT32 inside the shader. I just tried using 65535 as the multiplier and with that it works fine. I'm curious what is the maximum value I could use in a shader since it says floats have 23 bits of precision, is 2^23 safe to use ?

UPDATE 2 : Trying to create a GL_RG32UI texture with Samples = 4 results in :

//glTexStorage2DMultisample TextureTarget=37120 Samples=4 GLInternalFormat=33340

GLDebugCallback Source=OpenGL Type=Error Severity=high ID=33350 Message=Error:glTexStorage2DMultisample::invalid number of samples

calling glGetError after glTexStorage2DMultisample returns GL_INVALID_OPERATION

I was looking at glTexStorage2DMultisample - OpenGL ES 3.1 Reference Pages and there is nothing that says integer textures should not support Samples > 1