10-22-2022, 10:19 PM
The relative width of the lands and grooves and their associated diameters is what the cross sectional area of a bore. The current thinking is that certain types of bullet work better with either more or fewer lands. Barrels with odd numbers of lands are thought to help center the bullets rather than allowing it to stay pinched slightly to one side.
A few months ago I got curious and calculated the land/groove ratios of some cartridges based on SAAMI specs. The SAAMI spec has both a suggested rifling configuration and a the cross sectional area of the bore. The Grendel and 6.5 Creedmoor land/groove ratio is about 33/67. Interestingly, the 260 Rem specs wider grooves and a slightly larger bore area with a (roughly) 30/70 ratio.
It turns out that the 270 Win and most other .277 cartridges (6.8 SPC and .277 SIG Fury included) are spec'd with a 25/75 ratio. A lot of .277 barrels made in the early 2000's, did not follow that recommendation, however, and had 33/67 or smaller. It was eventually discovered that many of the barrels that caused pressure issues with the 6.8 SPC were undersized so it is no wonder that cartridge had pressure issues. Harrison's "narrow land" barrels are just the SAAMI spec. I wonder if Winchester realized that many barrel makers were not following the 270 SAAMI spec when they decided to use 33/67 for their new 6.8 Western.
A few months ago I got curious and calculated the land/groove ratios of some cartridges based on SAAMI specs. The SAAMI spec has both a suggested rifling configuration and a the cross sectional area of the bore. The Grendel and 6.5 Creedmoor land/groove ratio is about 33/67. Interestingly, the 260 Rem specs wider grooves and a slightly larger bore area with a (roughly) 30/70 ratio.
It turns out that the 270 Win and most other .277 cartridges (6.8 SPC and .277 SIG Fury included) are spec'd with a 25/75 ratio. A lot of .277 barrels made in the early 2000's, did not follow that recommendation, however, and had 33/67 or smaller. It was eventually discovered that many of the barrels that caused pressure issues with the 6.8 SPC were undersized so it is no wonder that cartridge had pressure issues. Harrison's "narrow land" barrels are just the SAAMI spec. I wonder if Winchester realized that many barrel makers were not following the 270 SAAMI spec when they decided to use 33/67 for their new 6.8 Western.