over-all-length comparison

[Rich Coyle]

9/19/22 55*

Today I compared two overall lengths of 7mm Hammer Hunter 131 bullets firing two five shot groups at 300 yards. The maximum over-all-length to the lands is 3.731". One five shot group was fired with the over-all-length of 3.713" or .017" from the lands; and maybe a slight crimp. (I tried.) Their velocity was 3,703 feet

per second. The other five shot group was fired with an over-all-length of 3.64" or .091" from the lands. Their velocity was 3,627 feet per second. The velocity difference was 76 feet per second slower with the load farther from the lands. I liked the hold on every shot.

I discovered the throat erosion after 196 shots is .051".

 

[vancewalker007]

I've had about the same throat erosion with over 600 rounds in my 6.5 PRC. Seems like a lot of erosion, but the 7RUM is pretty overbore.

 

[Rich Coyle]

This one has the shoulder pushed back about .100" to get a longer neck. I heard the throat will last longer. On my next barrel I will use the same reamer modified to use a 40* shoulder. Maybe that will help.

 

[Mikecr]

I don't believe longer necks work to reduce erosion, and for sure it won't extend barrel life.
Going to 40deg shoulders would have a bigger affect in this regard.

A heat peak point in the bore changes with cartridge design, in that the more bottlenecked the design, the higher the percentage of powder burning inside the chamber, rather than some distance down the bore. A longer neck doesn't change this.
I know there is the flame point theory, but necks are just not going to be long enough to shield heat occurring beyond.
We're talking several inches of fire cracking and erosion, not just a touch point for bullets.

But your design ideas won't hurt anything. IMO, a higher shoulder angle is better no matter what.

 

[Rich Coyle]

Lonny, at Hornady, made two rifles for bench rest testing. They were identical except one had a longer neck than the other. The one with the longer neck lasted twice as long, at bench rest accuracy, as the other. Based on his experience I made the long neck. Also because I have to trim after every firing I'm going to the 40* shoulder on the next barrel.