Metric Mayhem

The metric system (or “SI” for those who wear a man bun) may be the standard throughout most of the world but it hasn’t made much inroad in the U.S. There are exceptions, of course. Scientists use the metric system for its “logic” and in order to be consistent with their colleagues in other parts of the world. During the ’60s Vietnam vets learned to express distances in “clicks” while college kids, at least according to legend, learned about grams and kilos from the drug trade.

Groovy

Barrels with bores above .22 cal. up to about .284 cal. generally have groove diameters 0.007″ larger than bore diameter, while those with above .284 groove diameter is usually 0.008″ larger than bore diameter. Remember there are always exceptions, and of course most factory barrels today are made by cold hammer forging or button rifling rather than with cut rifling.

Most of our centerfire .22 cartridges such as .222 and .223 Rem., .22-250, .220 Swift and others have a nominal bore diameter of 0.218″, a groove diameter of 0.224″, and fire 0.224″ bullets. For cartridges such as .243 Win., 6mm Rem., 6mm Creedmoor, .240 Wby. the bore/groove dimensions are 0.236″ and 0.243″. For American .25-caliber cartridges the numbers are 0.250″/0.257″. The various .270 cartridges have a bore diameter of 0.270″, groove diameter 0.277″. The .30 cal. cartridges have a bore diameter of 0.300″ and a groove diameter of 0.308″.

Some common metric sizes converted to inches (to three decimal places): 5mm = 0.197″; 6mm = 0.236″; 6.5mm = 0.256″; 7mm = 0.276″; 8mm = 0.315″; 9mm = 0.354″.

The 6mm/0.236″ diameter is particularly interesting as it has a long history in America. Way back in the 1890s the U.S. military developed and adopted the 6mm Lee Navy cartridge — very much over the objections of the “Old Guard” who were horrified enough at such small bores as the .30-40 Krag replacing their beloved .45-70.

The Other Direction

Now let’s convert the other way, changing popular bullet inch diameters to metric: 0.224″ = 5.69mm; 0.243″ = 6.17mm; 0.257″ = 6.53mm; 0.264″ = 6.71mm; 0.277″ = 7.04mm; 0.284″ = 7.21mm; 0.308″ = 7.82mm. A reloader who has taken the time to plow through these numbers is likely thinking, “Hold on, this can’t be right! The box of bullets I just bought says 6mm .243, or 6.5mm .264. or 7mm .284. Are you suggesting bullet makers can’t read a conversion table?”

Not at all, it’s just they have another goal, which is for you the buyer to get the right bullets for the cartridge you are reloading. If you have a 6mm Rem., Creedmoor or Lapua, just get bullets marked 6mm in the weight you want and don’t fret the actual bullet diameter is 6.17mm.

Speaking of bullet weights, some of the unusual bullet weights we have are due to metric roots. By “unusual” I mean bullets for 7mm cartridges with weights of 139 gr. or 154 gr. Why not just go to 140 or 150 gr.? Well, the European practice is to measure weights in grams. A 5-gram bullet is equivalent to 77 grains. Other examples: 6/93 gr., 7/108 gr., 8/123 gr., 9/139 gr., 10/154 gr.

Going The Distance

Standard barrel lengths on American-made rifles are usually 18, 20, 22 or 24″ long so why do European-made rifles have oddball barrel lengths? Tikka T3 rifles generally have 22 7/16″ barrels in standard cartridges, 24 3/8″ for magnum cartridges. They are even numbers, but in metric terms — 570mm and 620mm respectively. My much-loved Sako Finnlight .243 has a 20.27″ barrel and weighs 6 lb. 2.8 oz. In metric terms it has a 515mm barrel and weighs 2.8 kg. Actually I don’t care, I just think of it as a 6-lb. rifle with a 20″ barrel.

Twisting It Up

Barrel twist rates are one area in which American measurements seem to dominate. Twists are generally measured as one turn in “X” inches. A 1:10 twist is one rotation of the bullet in 10″. The system is so well known we just say twist is 1:8, 1:12 and so forth.

European makers do use metric terms such as one rotation in 220mm equivalent to 1:8.66. My Sako Finnlight .243 is shown as having a 1:10 twist (1:254mm) though I suspect it is actually 1:250mm (1.9.84). Since the U.S. is by far the biggest market for sporting rifle sales, it makes sense to use terms Americans are used to.

Subscribe To GUNS Magazine

Purchase A PDF Download Of The GUNS Magazine November 2020 Issue Now!

RELATED ARTILCES