The British NRA banned the tight bore .308 barrels due to dangerous high chamber pressures shooting issued military ammunition.
Other way round. Tight bore was developed for and used with issued ball ammunition primarily Radway Green Arsenal's 144gn 'Black Spot' standard ball, and the slightly improved 146gn 'Green Spot' sniper variant both in use with UK forces until the late 1980s. (The 1984 Falklands War showed up how poor the then standard Lee-Enfield No.4 based L42A1 sniper rifle and its ammunition were and as part of the postwar review induced changes saw urgent respecifications and tenders issued for new improved models.) Both of these pre-1990s RG 7.62mm bullets were significantly under standard 30-calibre diameters and benefited from tight bores and also because of concentricity / balance / uneven jacket thickness issues very slow rifling twists, 1:14 being the norm. RG then developed a 155gn FMJBT bullet for its sniper ammo which although still smaller than a commercial 308 match was closer and although the new sniper round (eventually packaged as 'RG Bisley Match' for the civilian market) could be shot in the tight very slow twist barrels, a move to only a little under SAAMI dimensions and 1:13 twist pitch took place.
Although the 155gn RG round was better than the 146gn 'Green Spot' both in long-range external ballistics and in production quality, unhappiness over its consistency grew especially after one of two very poor lots got onto the market. RG now privatised as RORG Ltd (Royal Ordnance Radway Green Ltd) and part of the British Aerospace Industries Defence division also announced that from a quoted date it would not supply any civilian customers and only deal with governments.
The GB NRA then sourced ammunition by competitive tender on the international market for ammunition to its own specifications. 308 Win loaded with the original 155gn Sierra MK (#2155) and which would generate CIP MAPs within the slightly undersize barrels, therefore downloaded against standard SAAMI / CIP specs where test barrels are 0.300/0.3080. This gives around 2,925-2,950 fps MV from such a (tighter) barrel of 30-inch length, but less in custom rifles with standard SAAMI-spec barrels or equivalent factory 308s.
The initial supplier was RUAG under its RWS brand, later Lithuanian company GGG which still supplies the NRA under contract, both 223 and 308, a standard and match grade for each.
Because of the use of the typically 0.3083" dia. 155gn SMK, both makes of 'NRA Match' ammunition can cause over-pressure problems in surviving very-tight barrels and as many of these are found on first generation TR rifles built on adapted [Lee-] Enfield Number 4 rifle actions which are considerably weaker than modern front-locking models such as the RPAs, safety considerations caused a reported ban.
Interestingly as to how this ammunition is tailored to barrel dimensions which are still under standard specs (for whatever reasons as I cannot see a technical need these days but US Palma shooters often use the same specs too), the final year of the RUAG contract saw a year's supply of RWS ammo that turned out to be over-pressure in many TR rifles and there were hard extraction and blown primer issues in warm weather. The unused ammunition was returned to RUAG as not compliant with specification. RUAG / RWS tested it in a standard specification CIP test barrel and found it was still within CIP 308 Win pressure spec so repackaged it as plain 308 Win Match and put it back onto the market some of which ended up in the UK. The UK NRA then had to put a ban on its use in TR rifles as being over-pressure!
