Savage 12 F-Class: Thermal Stress, Erosion and Deposites
The three barrel profile videos on this page show the inner surfaces of the barrel of the following weapon:
- Weapon: Savage 12 F-Class
- Caliber: 6.5-284 Norma
- Barrel length = 30 inches
- Number of grooves/fields = 6
- Twist length = 8 inches
- Year of manufacture: 2019
- Shot load: approx. 1,530 shots
The distinctive feature of this weapon is the obviously high thermal stress, the associated erosion of the surface and the deposits from the projectile jacket on the fields – even though the firing stress is not yet very high.
History
The weapon has had two owners to date, and its firing history and loads are well documented. The loads were always designed for Pmax -25%, which is approximately 3,100 bar for this calibre. However, this calibre is a .284 Win (7 mm calibre) reamed to 6.5 mm. However, the base of the case corresponds to a .308 Win (7.62 mm calibre). The following values result from a comparison of the opening ratio, which is calculated from the ratio of case volume to bullet cross-section:
- .308 Win: 7.5 cm
- .284 Win: 10.7 cm
- 6.5-284 Norma: 12.1 cm
The higher the opening ratio, the higher the wear. At the same time, a high-energy powder (RS60, 3,990 KJ/kg) was used in the loads to achieve a high V0.
Measurement of non-rotational free flight (erosion transition zone)
The original owner had designed his load with a 130gr bullet for a non-rotational free flight of 0.5mm. A measurement of the non-rotational free flight at a firing load of approx. 1,100 shots revealed a significantly higher distance to the fields than originally designed. However, it was not possible to extend the bullet further, as it could then no longer be held in the case. By switching to a 147gr bullet, the non-rotational free flight could be adjusted back to 0.5mm.
This suggests that the weapon is already significantly eroded in the transition cone area.
LA-L (linear axial barrel profile video)
In the LA-L, deposits from the bullet jacket can be seen on the fields shortly after the muzzle. After approx. 200 mm, the copper-coloured deposits recede and disappear almost completely. After approx. 440 mm, the deposits reappear and intensify up to the transition cone area. From approx. 560 mm, ‘white thread-like lines’ can be seen on the surface, which intensify up to the transition cone.
LR-L (linear radial barrel profile video)
The LR-L shows that from approx. 400 mm onwards, the fields and then also the grooves turn dark blue to dark grey. This is a clear indication of high thermal stress from this point to the transition cone. The ‘white thread-like lines’ from the axial view appear to be cracks in the barrel surface, which are probably due to high pressure in addition to thermal stress. It is also noteworthy that cracks are also visible in the area of the cartridge chamber.
360R-L (360° radial barrel profile video)
The 360R-L shows 5 X positions at a 360° view:
- X = 2mm (muzzle): no abnormalities
- X = 108mm: first deposits of the bullet jacket on the fields
- X = 608 mm: fields completely covered with copper-coloured deposits, rifling discoloured dark grey
- X = 693 mm (transition cone): rifling and fields discoloured dark grey, surface cracked
- X = 773 mm (locking): no abnormalities
