Rifling query

**AJR** · 29-09-2022, 13:02

I slugged my 7x64 (twice) by carefully shoving a well oiled, small cylinder of lead down it, after it was cleaned.

The standard for this barrel requires a diameter of 7.24mm for the grooves and a diameter of 6.98mm for the lands.

The slug measured 7.24mm for the grooves, 7.04mm for the lands (dial caliper).
Does that mean:
a) the height of a fresh barrel's lands are 7.24mm-6.98mm / 2 = 0 .13mm per land,
b) this rifle's lands measures 7.24-7.04 /2 = 0.1mm per land,
c) this rifle is left with 77% of its lands?

Would that be considered a shot out barrel?

(The rifle still groups around an inch (sometimes a bit less) if the soft stuff behind the gun does its bit.)

**Ds J** · 29-09-2022, 14:22

There are several things to keep in mind:

- the rifle might have been slightly out of spec from the start, yet in good condition

- the lead shows the tightest part of the barrel

- lead can compress and swell out, leaving you with a false reading (happened to me)

- different brands may differ slightly in measurements

- the ends of the barrel (throat and crown) are generally the first parts to show erosion.

Edit: question for the knowledgeable folks: it has been mentioned in other threads that European and US measurements differ very slightly. If memory serves correct it concerns bullets, but it might be the same case with barrels as well?

**driepootx** · 29-09-2022, 14:58

http://www.gsgroup.co.za/cip.html

**AJR** · 29-09-2022, 16:04

Ds J, I feel the measurement is fairly trustworty, as there was little compression required to move the slug through the barrel. The crown was measured separately (by 1stly only pushing it down +-2 inches), and rendered a similar result.

**Sean KZN** · 29-09-2022, 16:16

Originally Posted by AJR

I slugged my 7x64 (twice) by carefully shoving a well oiled, small cylinder of lead down it, after it was cleaned.

The standard for this barrel requires a diameter of 7.24mm for the grooves and a diameter of 6.98mm for the lands.

The slug measured 7.24mm for the grooves, 7.04mm for the lands (dial caliper).
Does that mean:
a) the height of a fresh barrel's lands are 7.24mm-6.98mm / 2 = 0 .13mm per land,
b) this rifle's lands measures 7.24-7.04 /2 = 0.1mm per land,
c) this rifle is left with 77% of its lands?

Would that be considered a shot out barrel?

(The rifle still groups around an inch (sometimes a bit less) if the soft stuff behind the gun does its bit.)

b) this rifle's lands measures 7.24-7.04 /2 = 0.1mm per land,

That sounds about right for the barrel in question. If it is worn .03mm per land that doesn't sound like a worry.

**treeman** · 29-09-2022, 18:49

[QUOTE=Ds J;1475749n.

Edit: question for the knowledgeable folks: it has been mentioned in other threads that European and US measurements differ very slightly. If memory serves correct it concerns bullets, but it might be the same case with barrels as well?[/QUOTE]

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Correct, I never really dug into the subject, but somewhere I recall thinking "How does one control this world scale" - also read that it was not really a safety issue, more of okay, correct and perfect kind of thing. Not the kind of thing to cause rifles to blow up, but the kind of thing that causes the rifle to be sold to guys like me, because it "just will not group"

This matter of rifling varies in a lot of the 7 mm offerings and kind of baffles the mind - do you ask for a 7 mm barrel when buying to chamber a 7x57 or do you ask for a 7x57 barrel.
If you buy a 7mm 08 and find it a bit slow for your self image can you chamber it to 7x64 or your 7 x 57 to what ever ?
Never heard a gunsmith mention you can not - seems any 7 mm barrel will do, but originally they were made to X spec.

Never heard one horror story regarding any 7 mm blowing up because of wrong barrel used.

This is a old interest of mine never finished, hoping for much input.

**A-R** · 29-09-2022, 21:54

Originally Posted by AJR

I slugged my 7x64 (twice) by carefully shoving a well oiled, small cylinder of lead down it, after it was cleaned.

The standard for this barrel requires a diameter of 7.24mm for the grooves and a diameter of 6.98mm for the lands.

The slug measured 7.24mm for the grooves, 7.04mm for the lands (dial caliper).
Does that mean:
a) the height of a fresh barrel's lands are 7.24mm-6.98mm / 2 = 0 .13mm per land,
b) this rifle's lands measures 7.24-7.04 /2 = 0.1mm per land,
c) this rifle is left with 77% of its lands?

Would that be considered a shot out barrel?

(The rifle still groups around an inch (sometimes a bit less) if the soft stuff behind the gun does its bit.)

A barrel gets "shot out" from the chamber end. It will not shoot straight any more long before the lands get worn down that much, that the grooves are noticeably shallower.

Land height or groove depth of 0,10mm is pretty standard for medium calibers. The 300 calibers for example have a .300" bore, .308" groove diameter. That gives 0.004" groove depth which is 0.10mm.

When a barrel maker chooses to deviate form the CIP standard and makes the groove depth the same as all the other calibers, I would not regard that as significant. What is does emphasize is that there are small differences between individual rifles of the same caliber and you need to develop a good load for your specific rifle.

The you should also compare the barrel groove diameters of the European 7mm's with those of the American 7mm's. There are differences too. Then there are also differences between the diameters of bullets from different manufacturers...

**AJR** · 29-09-2022, 22:09

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?

**treeman** · 29-09-2022, 23:43

Originally Posted by A-R

A barrel gets "shot out" from the chamber end. It will not shoot straight any more long before the lands get worn down that much, that the grooves are noticeably shallower.

When a barrel maker chooses to deviate form the CIP standard and makes the groove depth the same as all the other calibers, I would not regard that as significant. What is does emphasize is that there are small differences between individual rifles of the same caliber and you need to develop a good load for your specific rifle.

The you should also compare the barrel groove diameters of the European 7mm's with those of the American 7mm's. There are differences too. Then there are also differences between the diameters of bullets from different manufacturers...

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Pretty much the full answer.

**treeman** · 29-09-2022, 23:45

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?

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been thinking about this, ? Would a .270 loaded up to 160 gr bullets not be CM candidate ?