A kit of precision ground shims made in Czech Republic. 1/2" (13mm) or 5/8" (16mm)
It helps with right timing of you muzzle device. Suits most of the common thread types upto 16mm, like 5/8"x24.
The advantage of the shims is laying in it's ability to keep your muzzle device's backend always paralel with your barrel's foreend. This will come in hand especially for the suppressors where the alignment error could cause a serious damage or destruction.