I am trying to find the frictional force with which the cartridges of the 6.8 SPC are held within the top of the magazine; I am designing an entirely new design of assault weapon, and I need to know the minimum amount of force necessary to remove the cartridge from the magazine while in the firearm.
Michael - The friction force will depend primarily on the surface conditions of the cartridge and the retainer. I will refer you to the Engineering Tool Box website: http://www.engineeringtoolbox.com/friction-coefficients-d_778.html. This will give you the basic equations to solve this problem - you should probably start with the assumption of dry/clean surfaces (mu = 0.35) and then see if you can make it work with greasy surfaces (mu = 0.19). This may very well be good enough, but know that surface conditions outside of this range are possible. If you're trying to size a spring, it may be best to start with what someone else did and then vary the spring length/stiffness based on some test results. I'm sure you'll want your clip to work within reason for a range of surface conditions (wet, greasy, dry, dirty, smooth, rough, etc), so experiment a bit with surface conditions.
