There is also a smart card available that gives the complete timing and headspacing
Use of the rimmed case is an old and reliable form of cartridge headspacing.
The belt serves as a cartridge stop for headspacing as the chamber is reamed.
Here the mouth of the case, which is just marginally larger in diameter than the bullet, is used as a case stop or headspacing.
As final headspacing is close, I torque the barrel down to about 75-100 foot pounds to attain near optimal axial pre-load.
If everything checks out, your M2 is ready for headspacing and timing.
If the M2 fails to meet the headspacing test, it's unsafe to fire.
ONCE THE M2 PASSES HEADSPACING, IT'S READY FOR TIMING
There is now a laminated card available that contains the entire headspacing and timing procedure.
Chambers for rimmed-case cartridges are often cut rather generously, since the chamber's dimensions have nothing to do with headspacing the cartridge.
38 ACP used the semi-rim for headspacing, in one of the manners described above.
38 Super pistols originally had a small relief step cut on the barrel hood or in the chamber intended to interface with the semi-rim of the case and provide a headspacing point.