Rifling query

**A-R** · 30-09-2022, 22:02

Originally Posted by AJR

I never realized there is a difference Treeman / DsJ / AR, thanks! As per Driepoot's link to GS Customs' barrel specifiations, a 7mm08( also known as the 7Creedmoor?) and .280rem runs a 7.21mm groove, whilst the germans reamed 7.24mm out for the 7x57/64. Interestingly GRT models with the same amount of propellant & casevolume the .280 creates higher pressure than its 'ballistic twin' the 7x64: presumably because of its 'thinner' bore?

There is also a small difference in case capacity.

**A-R** · 30-09-2022, 22:11

Originally Posted by AJR

I never realized there is a difference Treeman / DsJ / AR, thanks! As per Driepoot's link to GS Customs' barrel specifiations, a 7mm08( also known as the 7Creedmoor?) and .280rem runs a 7.21mm groove, whilst the germans reamed 7.24mm out for the 7x57/64. Interestingly GRT models with the same amount of propellant & casevolume the .280 creates higher pressure than its 'ballistic twin' the 7x64: presumably because of its 'thinner' bore?

The 7-08 Rem and 260 Rem cartridges have their case mouths and shoulders roughly in the same place as the parent 308 Win cartridge. This results in them having COL's longer than what most short-action mags can take if loaded with the heavier bullets with longer ogives.

The 6,5 Creedmoor, 6,5x47Lapua and other similar cartridges have their case mouths and shoulders moved back by 3,5 to 5mm, allowing for the longer bullets to be seated properly and still fit a 308-lenght magazine.

A 7Creedmoor would need to have the same changes.

**AJR** · 01-10-2022, 13:16

AR, you should start your own channel. It seems 'thou speech be sooth. '

Your comment about deterioration of the bore irrespective a what the lands might measure explains a lot of what happens within that little chamber of heat, pressure and noisy horrors.

**Dick** · 01-10-2022, 18:52

I always thought that all 7mm calibres were 277 bore x 284 groove, but there seems to be two sizes, European and American. The venerable 7x57 and the 7x64 are 275x285 (6.98x7.24). So the OP's measurements show that his 7x64 is dead on spec. The 7mm08 and 7mm Rem Mag are 277x284 (7.04x7.21). 7x57 Grooves are 5 thousandths deep but 7mm08 only 3.50. Mystery solved.

**treeman** · 02-10-2022, 17:09

Originally Posted by Dick

I always thought that all 7mm calibres were 277 bore x 284 groove, but there seems to be two sizes, European and American. The venerable 7x57 and the 7x64 are 275x285 (6.98x7.24). So the OP's measurements show that his 7x64 is dead on spec. The 7mm08 and 7mm Rem Mag are 277x284 (7.04x7.21). 7x57 Grooves are 5 thousandths deep but 7mm08 only 3.50. Mystery solved.

*******************************

' yeeha ! Dick, you just made my day, not often I know something that you did not.
Treeman feeling rather clever at this moment.

**A-R** · 03-10-2022, 08:23

Originally Posted by Dick

I always thought that all 7mm calibres were 277 bore x 284 groove, but there seems to be two sizes, European and American. The venerable 7x57 and the 7x64 are 275x285 (6.98x7.24). So the OP's measurements show that his 7x64 is dead on spec. The 7mm08 and 7mm Rem Mag are 277x284 (7.04x7.21). 7x57 Grooves are 5 thousandths deep but 7mm08 only 3.50. Mystery solved.

Then we have the guys who build rifles. Gunsmith takes a 7mm barrel and chambers it to the caliber requested by the customer, not checking what the exact bore and groove diameters are. I have seen a beautiful rifle built in 7x64 using a Douglas Premium barrel and pretty Turkish walnut stock. The first reload, using the Somchem start load, blew the case, cracked the stock and locked up the action. In retrospect it was discovered that the proud owner and careful reloaded had used a .284" bullet in his .284" barrel, while Somchem had used a .283" bullet (PMP) in a .285" barrel. Big difference. After repairs were done, he backed down significantly and developed an accurate load 5 grains below the start load. Maybe and extreme example, but it shows what the potential issues are with dimensional variations. I see Somchem have made reference to this in their 7mm Rem Mag reload data table.

**treeman** · 03-10-2022, 14:16

This ?

Due to smaller diameter of PMP projectiles higher velocities are obtainable?

**A-R** · 03-10-2022, 20:33

Originally Posted by treeman

This ?

Due to smaller diameter of PMP projectiles higher velocities are obtainable?

Yes, less friction, lower shot start pressures, some gas leakage (and possible barrel erosion) and higher powder charges possible.

**Peter Wells** · 06-10-2022, 08:46

If you are getting 1MOA or less you are stressing about nothing!

**AJR** · 07-10-2022, 13:46

I wonder: "Does the differences in barrel size noticeably affect the projectile?"
If similar projectiles are released in different barrels with the same load, which barrel is more efficient?

A 154gr hornady interbond that starts off as 7.21mm in diameter, gets persuaded to grow to:
a) 7.24 / 6.98 in a rigby/mauser,
b) 7.21/7.04 in a savage /remington.

Does that mean in a 22.5inch barrel :
a) in the german barrels the volume is 7.24mm*22.5inches less the land's cylindrical volume 7.24-6.98mm =.26x22.5inches: ( 9.3990cm3)
b) in the american barrels the volume is 7.21*22.5inches less 7.21-2.04mm= .17mmx22.5inches (9.3281cm3)

The radius for a 22.5inch cylinder with 9.3990cm3 volume is 7.22353mm (german)
The radius vir murican barrels would then be 7.208mm.

The German barrels are 'dikker' + less friction, the american barrels are 'dunner' + slightly more friction (relative to the germans).

Does that mean anything?

Gemsbokke - obviously - would know the difference, but can we decipher whether a projectile geslinger from an american barrel has marginally less / more bc & sd than from a german one?