The U S Military allows up to 57,000 CUP maximum deviation for M118 LR.For example, 7.62mm NATO ammunition that has been subjected to 125°F to -65°F storage conditions can have an average pressure that shall not exceed 55,000 CUP (Copper Units of Pressure).
A number of Long Range target loads for the .308 exceed the max deviation of M80 Ball.
Not all 7.62 ammo is created equal, and same goes for the .308.
Rifles proofed for use with 7.62 M80 ball or its interchangability equivalent may not be proofed to standards for either .308 or 7.62 Long Range ammunition.
The heavier the bullet the longer it will be unless given a thick body and round nose. When a longer bullet is loaded to the same OAL then effective powder space is reduced.
A long streamlined boat tail bullet will intrude into powder space more than a near cylindrical round nose bullet of the same weight. To achieve even the same velocity will require an increase in pressure. To achieve a significant increase in velocity will require an even greater increase, powder type being the same.
Sometimes use of an alternative powder can allow extra high velocity with little or no increase in chamber pressure, at least if the cartridge company specs are accurate.
No.4 Rifles converted to 7.62 NATO in the late 50's or early 60's, including the L42 sniper rifles, were not proofed to euther SAAMI or CIP standards for the .308 Winchester.
The NRA UK has banned use of converted No.4 rifles with the NRA supplied .308 long range target ammunition unless re-proofed to modern CIP standards.
Those converted No.4 rifles not re-proofed are limited to 3650 BAR which translates as 53,000 PSI by transducer measurement, far below the working pressure of either 7.62 M118 (52,000 CUP) or most .308 Long Range Match Grade Ammo.
Indian Ordnance Factory 7.62 Ball is NATO compliant with a pressure of 48,000 CUP-50,000 PSI.
Theres no direct correlation between any Copper Unit of Pressure measurement and any of the present Electronic Tranducers methods of measurement.
Military organizations use the EPVAT standard for ammo marked as interchangable, with CUP as a sort of back up. M118 ammunition is tested only with CUP methods, no EPVAT PSI measurements are given in the manuals I've found so far.
As of yet I've seen no reliable information on the methods used to proof test the 2A rifles, only what appear to be wild guesses or third hand stories with no provenance.
I would expect that a rifle imported into Gret ritian would have to meet some proof requirements, but I've read in old Winchester literature that Winchester's commercial proof mark was accepted without question by British proof authorities. This was in relation to the Winchester 1895 rifles, and we now know that those rifles later proved to be unsuited to the pressures of some post WW1 .30-06 commercial ammunition and the military M1 Ball.
I'd be very leery of using any heavy bullet .308 or 7.62 ammunition in a 2A or converted No.4.
The Enforcer rifle built on a No.4 action is an exception. These were bulit using only hand picked and tested actions and all parts subjected to strenuous testing, more parts rejected than were used, and proofed to .308 commercial specs when new.
The NRA UK allows the Enforcer to be used with any .308 or 7.62 ammo with pressures up to the max of 62,000 PSI, but only so long as the rifle has not been altered since leaving the factory, and its original proof marks are intact.