Bearing surface and pressure relationship

[archerross]

I have a good target load using H4831sc and a target bullet. I'm looking to try a hunting bullet of the same weight, but with a longer bearing surface (0.472 vs. 0.546 in) and would like to use the same powder. My current target load is just under max pressures. I realize that the optimal charge weight for the new bullet will likely be different. Out of curiosity, will the bullet with the longer bearing surface inherently create higher pressures if all else is the same?

 

[jonbearman]

I would think with more friction it would bump the pressure up and you should back off 10 percent and work up to optimum charge weight.It is always safer to start low and work up.Just load a cople of each charge to find your pressure sign and stop.

 

[TonyR]

I have found variations in the average bearing surface in two lots of the same bullet (and in one case two groups from the same lot purchased from different suppliers) of about 0.020 inches. Although I didn't need to adjust the load, one group printed in a different place on the target than the other. I was shooting sighters at 1000 yards and used seven from one group to warm the gun up then 3 from the other to set the elevation for the record string. The difference was about .5 MOA so something was going on. I've never directly compared two groups of bullets with much more difference than that.

 

[Larryh128]

The longer bearing surface will most definitely bump the chamber pressure. Bullets like the Swift Shirroco that are basically a cylinder with a tapered point will absolutely jack pressures up. More bearing surface = more pressure. That's one reason that Barnes started putting 3 grooves in the bullet.
Then everyone else "invented" it a few years later.

 

[bheadboy]

to prove the point that bearing surface increase, increases pressure, go to the Hodgden loading data,
look at the 139gr load data on a 7-08 or 7mm rem mg and compare to the 140 gr load.
the 139 is th shortest bearing surface and the 140 is the longest.

1 gr of weight will not make the differance


Bob

 

[BoydAllen]

Any time that you change a component it is a good idea to do a new load workup. A long time ago, an impatient friend changed a hot varmint load from Federal to Winchester powders and blew primers out of cases. Take your time, and do it right.

 

[spclark]

Yes, increased bearing surface will most assuredly lead to increased friction & thus higher pressures.

One can infer the probable bump by calculating the percentage increase in surface area, then adjust your initial test loads downward accordingly, always with a mind towards keeping pressures down until you know how the new bullet is behaving. Increased bearing surface vs. pressure increase isn't a linear relationship, but knowing what the difference is at least gives you a point of reference when making adjustments in load levels.

As BoydAllen states, anytime you change a single component or measurement parameter (like bullet seating depth) in a known working load you need to use caution & common sense to determine what effect the change has with regard to safe operation.

 

[archerross]

Thanks for all the information fellas! I appreciate the reminders to follow safe loading practices. My question was more to satisfy my curiosity than to try and shortcut the load development process. I figured greater bearing surface (i.e. friction) would increase pressures assuming all other variables were constant. With the wealth of experience on this forum I thought I would get confirmation and additional details. Now all I need is my rifle back from the "shop" and a day without rain, then its game on.