All rifled barrels have a twist, right or left handed, and lands and groves.
These impart spin to the bullet and increase its range. However, they also serve to identify the type of gun that fired the bullet.
Let us say investigators find a bullet at a crime scene. It has 8
lands and groves with a right hand twist. Detectives can look into a
GRC or general rifling characteristic database and see which guns
can fire such a bullet.
In some cases, they will actually trace the ATF records and question
every such gun-owner in an area.
There are two ways to check your gun’s rifling: easy and hard.
The easy way is the stick some silly putty into the barrel and pull it
straight out. Then just count.
The harder way is to fire your gun into some water and recover the bullet.
One fires into water so the bullet isn’t damaged.
Gun manufacturers work with the Murkan police state. That is why they
put serial numbers on the gun along with a unique rifling in the barrel.
A DAA only has two friends: reason and suspicion. Use them properly.