Jamming vs. barrel life

[fatelvis]

The 6mm 107 SMK is well known for shooting really well in the lands. They also shoot great off but in the lands where the engraving marks are square is always an easy button. It may not be the best but if I’m lazy I can load to 2920 or 3030fps in a BRX at that seating and go to a match and be competitive.

I often hear about loading jammed into the lands with "squared" rifling marks. What exactly does that mean?

 

[swd]

I often hear about loading jammed into the lands with "squared" rifling marks. What exactly does that mean?

When you push the bullet into the lands, the lands will leave marks on the bullet. When the marks are as wide as tall they look like square marks on the bullet. They frequently shoot well there but not always..

Also jammed and in the lands are very different things. Many guys assume as soon as the bullet touches the lands they say “jammed”. That’s not true jam. Jammed is when the bullet is far enough out of the cases that when you close the bolt the bullet gets pushed back in the case. That’s jammed or full jam. There is off, in and jammed.

 

[RabbitSlayer]

When you push the bullet into the lands, the lands will leave marks on the bullet. When the marks are as wide as tall they look like square marks on the bullet. They frequently shoot well there but not always..

Also jammed and in the lands are very different things. Many guys assume as soon as the bullet touches the lands they say “jammed”. That’s not true jam. Jammed is when the bullet is far enough out of the cases that when you close the bolt the bullet gets pushed back in the case. That’s jammed or full jam. There is off, in and jammed.

I have a gun that shoots good when jammed so you can't open the bolt. Shoots just as good when I can open the bolt and not dump powder all over though lol.

 

[steve210]

The 6mm 107 SMK is well known for shooting really well in the lands. They also shoot great off but in the lands where the engraving marks are square is always an easy button. It may not be the best but if I’m lazy I can load to 2920 or 3030fps in a BRX at that seating and go to a match and be competitive.

 

[JFrank]

Advanced beyond touch is often referred to as jamming, how much is called out as + .00x .. when a bullet is pushed back into the case it is often called soft seating, caused by not enough neck tension to hold the set seating depth. Solution is to add hold or use less depth.

 

[Greg@wfcustom]

“Also jammed and in the lands are very different things. Many guys assume as soon as the bullet touches the lands they say “jammed”. That’s not true jam. Jammed is when the bullet is far enough out of the cases that when you close the bolt the bullet gets pushed back in the case. That’s jammed or full jam. There is off, in and jammed.”

Best explanation I have seen since I posted my definition of “Jam” several years back trying to enlighten someone who was unclear on the concept. LOL
I had to dumb it down to “when ya can’t make it any longer without it getting shorter, that there is full jam”
Your definition is much more elegant Shawn, especially the simplicity of the last part!
Hope all is well up on the hill.
G

 

[Webster]

jamming bullets seems provides best results in 6br and 6.5cm in my experience. Usually I get 0.3-0.4 moa 5-shot group shooting 140gr or 105gr hybrids without any load development at 100m. Just quick obt modelling re powder charge and off we go.

The only question I am askin myself is how much this strategy affects barrel life. Does barrel life expectancy decrease by e.g. 30% or more? My load is usually 30gr N140 for BR and 42gr N555 for NeedMore.

Barrel erosion (hot gas erosion) is mostly related to the wt. of powder compared to the bore diameter the hot gases at high pressure travel down. Things like powder burn temp should be farther down the list. Powder burn temperatures are determined in a standard size cylinder that doesn’t resemble a chamber and barrel. All you can say is the were all determined the same way with the same powder charge. No barrel, no chamber, no bullet. Erosion isn't in the chamber as mentioned in a reply, I am sure he meant barrel. About all you can do is shoot lighter loads and don't shoot fast. Barrels are meant to wear out.

MY 6BR has about 4200 shots on it. Still shoots .350” groups. I try to space shots about 1 minute apart.