0.636" No.4 Bolt Head

A SAAMI Forster field gauge is .070" and would get you a better reading then your current setup. Before you condemn the existing bolt head. Case stretch as you have described is very common due to the very generous chamber area. Remember.  This rifle was built to shoot people originally and were never intended to be reloaded for. If the the case separated completely, no one cared as long as the bullet reached its target.

Your current bolt head is a #0 at .0632" if a no go gauge failed,it would then have the next size up #1 at .0634"/.0635". Please invest in a no go gauge of proper thickness and then start with the next number sequence. Your original question had you asking about a bolt head at .0637. That's wandering into the realm of the elusive #3 bolt head and a possible worn chamber. 

I have been machining my own .074" NoGo gauges and as you say,it would be a onetime use issue,until you need it again when your headspace starts to slacken up as it will with use. I have four bolt heads that range from. 0636" to .0640". PM me and we can discuss a price.

I think the gage nomenclature was stated in reverse.  Should be .064 GO, .070 NO GO and .074 Field.  The No Go is used only by armorers for setting headspace when assembling the rifle.  Field is used, well, in the field as the maximum acceptable headspace.  If it failed the field gage, it went back to the armorer for correction.

I set the headspace of my rifles to close on a .064 GO but not on a .070 NO GO.  I think this is part of the reason why I get many reloads on the brass until case web cracking appears.  SAAMI standards will be slightly different given as 0.064 Min, 0.071 Max.  Field is usually Minimum + 0.010, 0.074.  

Using a case with brass shims is a bit “spongy” for feeling when the bolt closes.  The camming action of the bolt lugs is quite strong, use very light finger pressure for when you feel the bolt close. It’s also not easy to get a precise rim thickness measurement.  If your bonding shims on the case, the bond line will have a thickness to it.  Perhaps we can get Armorer to chime in and straighten us out here...

Let me jump in here for a second if I may.

The .303 is a rimmed case. Because of that headspace is between

0.064"

  (minumum, any less you may not be able to chamber a round)

0.067"

  (optimum between too tight & too loose)

&

0.074".

Sloppy maximum (unless theres a war on.)

Now lets not confuse that with the infamous "oversized chamber" which allowed filthy, mud covered cases from seating into the neck area of the chamber & tying the rifle up in combat.

They are frequently confused & this is where a lot of the misunderstanding comes from. In a rimmed .303 case ANYTHING FURTHER IN FRONT OF THE BOLT FACE IS NOT HEADSPACE! It a "generous chamber" problem!

It is mostly a problem for reloaders trying to reuse brass multiple times. The original design was "fire & forget", lets keep that in mind.

No worries.  If someone cares enough to educate me (a stranger) I appreciate the intent and gesture. 

But let me 'splain.  I am a reloader (since 1976) and a shooter/hunter.  I have no doubt my No 4 Mk 1/2 left the FTR in mostly the shape I have it.  Relatively sound, new (1955) barrel with lovely bore.  I know and appreciate the rifle was designed to chamber a sandy, muddy, iced up round.  BUT.  I want minimal case disruption on that all important first shot of new cartridges that opens the brass to a generous "service" chamber.  Major George C. Nonte, Jr. in my treasured volume of MODERN HANDLOADING (1972) calls it "case space" rather than headspace.  If I can minimize the amount the case must open up with the all important first firing I can thereafter neck-resize or even "bump" (I use sets of Redding 0.002" sequential she!! holders in other calibers that headspace on the neck) to put as little stress on a case during firing as possible.  I won't be neck turning, checking run-out, or other "tricks" for the .303 British.  But I will appreciate adding maybe two additional reloads before retiring the brass.  The esteemed Maj. Nonte refers to that as removing the "slop".

That said - I have made arrangements for the desired bolt head and a No Go gauge so I am in tall cotton.

i have put up a post on how i head space my smles it might not be to everyone’s liking but it works

batch your brass to the rifle and neck size only helps for case life