Seating Depths Affect on Velocity?

[ArtD]

I’m doing load development for my Cooper in .22 BR. I am pushing a 50 grain V-max with 30.9 grains of Benchmark. Average velocity is about 3600 fps. This weekend I was testing bullet seating depth. I shot six different OALs. OAL ranged from the recommended length to just touching the rifling. Velocities remained constant through the first four groups. My fifth group, .0075 off rifling, however, dropped to 3200 fps, while my sixth group, touching the rifling, jumped to 3700 fps. I am absolutely sure of my charge weight because they were all thrown at the same time and each round was weighed. How can seating depth cause this dramatic a velocity shift? FYI, my fifth group was also my tightest.

Thanks,

Art

 

[Lane]

By changing the bullet seating depth you have changed the amount of free travel or lead before the bullet makes contact or engages the rifling. All factors being equal as you assume the greater distance a bullet has to travel before engaging the rifling, the lower the pressure. Some unrestricted bullet movement allows powder gases to expand early and ease the pressure peak. If bullet travel is excessive some term this condition, Free-bore). Very little or restricted bullet movement,free travel), pressure goes up. You have in one case permitted very little unrestricted movement of the bullet as it engages the rifling. You have through bullet seating depth changed the pressure peak with the amount of lead or free travel before the bullet engages the rifling. With this condition pressure goes up with your particular load,component,chamber combination. The issue is with the throat or leade with your particular case in my opinion. Wish you the best Lane.