Is it safe to use NATO 7.62 ammunition in civilian sporting rifles,
considering that there are sometimes differences in tolerance and
maximum allowable pressures?
I believe that NATO is intended to be a lower pressure (50000?), so
will be ok in civilian guns, but non-mil ammo (308 Win) is not safe in
service firearms.
Gun is a Remington 700 SPL, and the round is a 155grain FMJ,  British
surplus,  Radway Green, (known in the UK as RG).
Not fussed about loss of accuracy due to different chamber tolerances,
just interested in whether the barrel will blow up if I keep using the
stuff.
I am aware of the issue with 5.56 NATO and it's incompatibility with
civilian .223, but lots of people seem to use 7.62 RG as a cheap
alternative to 308Win factory loads. An ammunition data sheet would be
nice.

You are right to be wary of using an ammunition other than what your
chamber is stamped with.  You will, of course, find people on both
sides of this fence.

http://en.wikipedia.org/wiki/7.62_mm_NATO
"To summarize, while it is not unsafe to mix .308 and 7.62 x 51 mm
ammunition, fewer problems will be encountered if the correct caliber
designation is used."

http://www.thegunzone.com/30cal.html
"The .308 Winchester and the 7.62mm NATO (nee T-65) cartridges are not
the same, nor should they be considered interchangeable"

As you can see from the pressure ratings in the link above the .308 is
rated to a much higher level than the NATO 7.62.  But the military
rating is still above the standard .308 pressure level of 55,000.
http://www.smellysmleshooters.net/ammopressure.htm

This link has a table the British military specs for NATO 7.62
ammunition.  Ammunition coming for the RG arsenal will meet one of
these specifications.  These rounds will create approximately 50,000
psi.  Pressure will not be a problem for you rifle.  The .308 "proof
pressure" is 83,000 psi!!!
http://www.cruffler.com/trivia-June01.html

The other factor to consider is the difference in chamber size and
head thickness.  The chamber on your .308 is smaller than that of the
military 7.62 rifles.  The NATO cartridges fit without problem
(obviously since you have been using them).  The military rounds will
usually have a thicker case that that of the civilian rounds.  This
could potentially be a problem if were shooting .308 through a 7.62
because the increased chamber size will give less support to the case
and it could split. This is a possible safety hazard and a pain to
pull jammed casings from your gun.  You of course have the .308 with a
smaller chamber, so this problem would not apply to you.

The difference between the .223 issue and the .308 is that the mil
spec ammunition "hotter" than the commercial ammunition where as with
the .308 it is just the opposite.

Summary:
Your gun will not suffer any ill effects of shooting NATO 7.62
ammunition because of any technical difference in the ammunition.  It
is not a safety hazard.  Will it be dirtier and less accurate than new
commercial ammunition? Sure it will.

I have a Savage 10fp chambered for .308.  I did this same research
when I bought it.  I am now shooting Turkish surplus 7.62 for my
"plinking? needs.

Just another interesting link about the different kinds of NATO ammo produced.
http://www.inetres.com/gp/military/infantry/rifle/762mm_ammo.html

I trust that this answers your question.  If you require any
additional information please post a request for clarification.

Happy Shooting!