Dimensions are in inches * The M40A3 4, 5 & 6 we supplied to the USMC are identical contours but are finished to different lengths (A5 is 12 twist and A6 is 10 twist). ** The SR60 contour is very popular with law enforcement and often cut to 19-inches on 30-caliber for urban applications.
Note: there are many varieties of tactical barrel contours. Both the straight taper versions and stepped versions as depicted above. Application considerations have been dictating the choice.
The straight taper contours have an advantage in high rate of fire applications and also where recoil is perceived to be a factor. Example, we are the supplier of all the USMC M40A3, 4, 5 and now the A6 sniper rifle barrels. The straight taper is a natural fit.
Where heavy magnum calibers are used the straight taper is invariably the choice (but not always).