There's no such thing as a "jump gap " measurement. There is a throat erosion though. However, a .30 calibre throat erosion gauge will tell you nothing about a .303 Brit chamber.
Dunno if you'll be able to get to this, but it's a BILINGUAL CF manual.
http://photos.imageevent.com/badgerd...%20Summary.pdf
The .303 British headspaces on the rim. Military spec Max OAL is 3.040". Accuracy depends on the diameter of the bullet vs the groove diameter. Most commercial ammo and bullet makers use .311" or .312"(Montana Bullet makes some larger diameter cast bullets). Accuracy will be poor if you use the wrong diameter bullet(like a .308" bullet) in an oversize barrel.
Any No. 4 Mk 1* still around is highly unlikely to have seen any W.W. 2 service. No. 4 was in use up here with Army Cadet Corps until the mid 90's, as I recall. Still used as DP rifles with colour parties.
All remaining Savage made rifles, parts and machinery got sent to Long Branch when the contract expired. Had one on my MIU long ago.
Lithgow Lee-Enfields are NOT No. 4's. They're No. 1 Mk III's. Aussies didn't use No. 4's. Ishapore didn't make 'em either.
"...original 303..." That was the bore diameter. Groove diameters were .311" to .315", even at Long Branch.
You used to be able to order a Criterion No. 4 Rifle barrel chambered in .308Win. It is not a .308" barrel chambered as a .303 Brit. No longer available.