The key areas are chamber neck section and leede diameters. PT&G's '.308 Win Bisley 150 Rule' chamber reamer drawing shows a nominal 0.3435" neck dia. and 0.3085" leede dia. That's actually marginally smaller at the neck than 'minimum SAAMI' which is 0.3440". Other popular chamber designs are in the same situation, the still popular if superceded (since the #2156 Sierra Palma MK appeared) 'Palma 1995' chamber yet tighter at 0.342" neck dia. this undoubtedly used in many custom 308s built by our gunsmiths here too.
Here's what Stuart Anselm (Osprey Rifles) who builds a lot of FTR GB League / ICFRA internationally compliant rifles posted recently on another forum about this subject so far as FTR goes. (TR will be the same as TR, FTR, and Match Rifle technical and match specifications are regulated by ICFRA, the International Federation of Fullbore Rifle Associations, the GB NRA our member of the group.)
I have recently been through all of this and the Official line is that for an F T/R rifle to comply with ICFRA rules it must be chambered to CIP with the exception that there is no limit to throat length. The [GB] national league follows the ICFRA rules in so far as it ensures that all GB competitors are compliant with the rules when they compete at the World Championships. The main issue with regards to chambers is the neck dimensions, tight necks are not allowed. Technically you require a minumum .344' neck - Birmingham Proof House have some new gauges and if the neck is not .344" then it wont get stamped as 308 Win. There is quite a bit more background to this that would take me forever to type so if you wanted a bit more info give me a call.
Note the proof house reference - this has caused a lot of problems recently with marginally 'tight-neck' 308 Win (and other calibre) chambers where intially they wouldn't provide proof at all. They will now but it is stamped up to show the
chamber is 'non-compliant'.
Because the 150 Rule only looks at 'minimum' dimensions, the test obviously doesn't pick up on excessive headspace and/or where the competitor thinks he's going to get a bit of competitive advantage by having a near Ackley Improved chamber or whatever and handloading fireformed large capacity brass for a bit of extra velocity. Using very thin / high capacity brass (US favourite is Winchester, in Europe some lots of Norma) is allowed and can provide a small but useful MV increase. However if the Rule 150 / ICFRA chamber regs are complied with, such brass has extra clearance at the neck and this may impact adversely on precision and MV spreads. (In the US, many competitors run chambers with 0.340", or even 0.338" necks to get around this.)
Our NRA Rule 150 goes back to when 'issued' milspec 7.62mm NATO ammunition was the norm in UK 'Target Rifle'. Some of this stuff was terrible and gunsmiths took all sorts of measures to reduce the downsides and produce enough velocity to shoot it to 1,000 yards - 'tight bores', 1-13 or even 1-14 inch rifling twist rates and suchlike. A few started reducing the chamber neck dimensions too .... And then one sunny day, the NRA bought a large amount of imported 7.62mm which had an outside neck dia. that was right on, or maybe a bit above, the maximum allowed tolerance, and the rifles with tight and/or slightly short chambers produced excessive pressures. Since Musgrave and Swing actions are pretty (very!) strong, I don't think anything broke or anybody got hurt, but there were fired cases that didn't want to be extracted and people using cleaning rods on the firing line to get them out. It soon became obvious that there was a potentially dangerous situation developing, so Rule 150 was introduced and inert cartridge gauges used before major matches to ensure everybody's rifle had enough clearance.
PT&G has a 'Bisley 150' reamer that meets the minimum spec and which is nevertheless a very good chamber for 308 match use with the original 155gn Sierra MK. Looking at my PT&G produced 'The Gunsmith Book of Chamber Prints', I see it's officially called "308 Palma Bisley (Bisley 150 Rule)" and the neck is shown as 0.3435" dia.
Sierra TMK 195gr
