On a rimless cartridge such as the .30-06, the stopping surface is the shoulder. On a rimmed cartridge like the .30-30 or 7.62X54R, the headspace is the gap for the rim. On a belted magnum, headspace is the gap for the belt.
You can basicly swap bolt heads, and as long as they are all within tollerance (and its very rare that they are not) your good to go.
Mosin Nagants, especcially ones fresh outta the cosmo are "combat Ready" brought into spec with refurbishment in an arsenal and then stored for WWIII.
Clean that cosmo!!!
Get it outta the chamber , or you will have issues with extraction. Cosmo after 60 years of drying can look like shine clean steel. Use a solvent and a good chamber brush, rinsing with the solvent untill it runs clean. I use a 20 gause brush and gasoiline.
Be carefull with your stock, as the gas or other bore solvents can strip the varnish off the wood. Some who like to shoot and keep 'em shiney dismount the wood from the barreld action, thought this too can scrape the varnish, or they are carefull not to spill any solvents or alcohol on the varnish either.
The wood would normally be stripped with the cosmo when the rifle was isseud to the soldier,and a nice oil applied to both the stoick and the steel.
The varnish on the stock is to protect it like the cosmo is doing.
Clean it, shoot it and smile....then clean it good and put it away for the next days shooting
Be sure to use a water based cleaning solvet if you shoot milsurp ammo, its almost all corrosive.
A Mosin will tell you when you need to clean it, its action will stiffen up.
They come with a cleaning kit and an issued screw driver that also has a firing pin protrusion gauge set into its side.
Use 'em