My step-sister's son visited yesterday. He brought with him a N. 4 Mk. I* SMLE sporterized that appears to be poorly completed. I'm only concerned with information he received upon purchase. He was told the rifle has been converted to 308. I have never seen such a conversion. The bolt face is a N2 that still has a flat bolt face that appears to be OEM. Barrel appears to have excellent rifling and is clear of wear and debris. I cannot verify whether chamber remains OEM or has been rechambered. Further "flags" are that the rifle has no magazine and magazine box appears to be OEM.

Is there a way to determine its chambering without traveling 85 miles to a Missoula gunsmith?

If, in fact, the chamber has been altered while leaving bolt face OEM, can barrel be cut or sleeved at breech and chambered for 303 safely?

You can make a chamber cast using sulphur quickly, simply, and cheaply. That's what I'd recommend. Web search should bring up a ton of hits I'd suspect.

I believe some of the late model Enfields were converted to .308 by Ishapore using new heat treatment and barrels.

Canada Rifle Association also converted some No4 Enfield for sporting use.

India used N. 1 Mk. III actioned rifles during World War II which were made at Ishpore arsenal. It was this design that India heat treated differently when 308 version was made. As far as I know, N. 4s did not improve heat treat on converted rifles or on newly manufactured 308s. I have no idea whether Ishpore magazines fit N. 4 actions, regardless of Mark.

Do you have a 308 round? (or a 45acp round?)

It's larger in dia than the 303.

See if the 308 will even chamber.
or for that matter even a 45acp round. That has the same base dia.
The 308 won't chamber if it's still in 303B

It should give you some idea of wether the rifle has been rebarreled.

Any markings on the bbl to indicate arsenal (re)work to 308.

Ishapore made the 2A and 2A1 in 308cal/ 7.62Nato
starting in the 60's
These are No1MkIII* rifle design.

I can't recall that they made any #4 rifles

A simple chamber cast for ID purposed here can be made from candle wax poured into the slightly oiled chamber. Stuff a cleaning rod w/ tight patch just up front of the chamber before pouring to push it backout.

The closest I have are 8x57 mm snap caps. Can you confirm that 308 bolt face of converted N 4s use a flat face, as do 303s? If not, I'm uncertain what the 8x57 snap cap will demonstrate.

The barrel is OEM. I understand that wartime N 4s had such wide variances of bore diameters that OEM barrel might shoot .30 caliber bullet reasonably accurately.

Ok, here are some facts.
1. Ishapore 7.62s are all new made; yes they had to change the heat treatment from the 303.
2. Ishapore never made a #4.
3. #4 bores do not vary enough to worry about.
4. See if a 308 cartridge will go into the chamber; if it will, it is a 7.62.. You don't need a cast nor a gauge.
5. All bolt faces are flat; they can't work any other way. Only the extractors are different; longer for the 7.62 case; to push it over against the ejector.
6. 7.62 magazines are square; not triangular; easy to tell the difference from a 303.
7. This rifle should be restored to original military configuration; the sporterize it will reduce the value by 75% at least.
8. Forget the 8mm gauge; it will tell you nothing.
9. What do you mean the barrel is OEM? Is is a 303 or 7.62?
7.62 barrels are Not converted from a 303. That is impossible.

Here is why you can't rechamber a 303 to a 308/7.62, even if the bore was the same which it isn't.

If all you have is the 8x57 snap caps...

Take one of the 8x57mm Mauser snap caps.
Turn it around and try to insert the BASE of the snap cap into the chamber of the rifle.

If it goes into the chamber even a little ways,,then it's something other than 303Brit which has a smaller base diameter (approx .460) than the 8mm Mauser (approx .470).

Not saying what it is exactly, could be anything with a base dia close to 30-06 or one of the x57mm Mauser cartridges.

Based on what I've read on this thread, plus Cartridges of the World, I realized that the shape of each cartridge's taper and it rate of each taper allowed me to drop the snap cap directly in the chamber. If snap cap descended until its shoulder was stopped by chamber's shoulder, chamber is 308 Winchester. If snap cap is stopped before meeting chamber's shoulder, chamber is 303 British. Rate of taper of 8x57 mm is less than 303 in the sense that diameter at 8x57's shoulder is stopped partway down because it exceeds diameter of 303's. To make a tedious reply less tedious, the rifle is not rechambered. It is still 303 British, and the young man is relieved.

Many thanks to everyone for furnishing such useful information.

The .308 SMLE conversions I have seen did use the flat bolt face, but the extractor claw was replaced with one having a longer tip.