Let's get one thing clear!
NOT everybody can get the ass to the grass when they squat, so stop telling those who can't that they are wrong. Here follows a short explanation of why.
1. Femoral Neck Angle Variations.
Let's take a look at this first picture. Here we have two femurs from two different people. One points more upwards, the other points more downwards.
Do you think these people will squat the same when they have that much bony difference? This is an example of femoral neck angle variations.
2. Length of Femoral Neck.
In this second picture, we see an almost identical femoral neck angle, but the length of the neck is different between the two images, which can affect how much of the head/neck is covered by the socket of the hip joint.
There can also be differences in the size of the femoral head. There is a relationship between the size of the femoral head and the depth of the hip socket.
The more 'full' the socket of the hip is with the head of the femur, the less overall range of motion there will be in hip motion.
3. Torsion of the Femur.
Look at the next photo, specifically at how different the angle is that the ball is pointing between these two femurs. One of these people will have a bony block when they try to squat narrow while the other can squat narrow like a champ. One literally will have to toe out substantially so that both femoral heads are in the socket in the same orientation (left person and right person). This view represents torsion of the femur. This is typically a genetic result, although it could be due to repetitive motions when the skeleton is still malleable and growing during youth.
Maybe your piriformis isn't the limitation after all.
Read the full explanation here www.themovementfix.com/the-best-kept-secret-why-people-have-to-squat-differently/