Extending the throat leade and its impact on harmonics

[A A Ron]

Curious how close the relationship between the free bore and barrel harmics is?

For example in my rifle a 190 ABLR shoots best at .140" jump. That jump causes the bullet to be seated deeply in the case using up capacity and it is way past the neck shoulder junction.

So if the throat was cut .140" deeper or so could you move the bullet the same jump to get back to node? My guess is the changing pressure, changing velocity, and changing harmonics timing would necessitate a complete new work up.

 

[Bob Wright]

Curious how close the relationship between the free bore and barrel harmics is?

For example in my rifle a 190 ABLR shoots best at .140" jump. That jump causes the bullet to be seated deeply in the case using up capacity and it is way past the neck shoulder junction.

So if the throat was cut .140" deeper or so could you move the bullet the same jump to get back to node? My guess is the changing pressure, changing velocity, and changing harmonics timing would necessitate a complete new work up.

There is risk, but there are a lot of options to get it to shoot again with different components.

 

[Radman]

Curious how close the relationship between the free bore and barrel harmics is?

For example in my rifle a 190 ABLR shoots best at .140" jump. That jump causes the bullet to be seated deeply in the case using up capacity and it is way past the neck shoulder junction.

So if the throat was cut .140" deeper or so could you move the bullet the same jump to get back to node? My guess is the changing pressure, changing velocity, and changing harmonics timing would necessitate a complete new work up.

I like your thinking & I'm betting that you're right.

 

[Tiny Tim]

I agree a new workup would be in order, even if it only confirmed your existing data. So many things at play.

Assuming existing throat and freebore was cut by a chamber reamer. So using a throating reamer to extend freebore may cause the bullet to engage the rifling somewhat differently, and may result in a change in jump in the modified chamber.

I know very little about this and this is all based upon supposition. Just my opinion and reasoning.

 

[Varmint Hunter]

In all likelihood you would need to rework the load. However, .140" off the rifling probably isn't the only good node. It sounds like a little more experimentation may also be helpful.

If you extend the bullet out another .140" will it still fit in the magazine?

 

[milo-2]

I agree a new workup would be in order, even if it only confirmed your existing data. So many things at play.

Assuming existing throat and freebore was cut by a chamber reamer. So using a throating reamer to extend freebore may cause the bullet to engage the rifling somewhat differently, and may result in a change in jump in the modified chamber.

I know very little about this and this is all based upon supposition. Just my opinion and reasoning.

I'd also worry about how work hardened your throat is, like how many rds down the barrel. Just say you let the throat or barrel get hot, you most likely have retempered the steel, especially the raised rifling. It may come out in chunks.
I've been down this rd with a case most likely using half the powder charge.

 

[A A Ron]

I'm not 100% sure which reamer was used to cut the chamber originally. If it was a PTG it should be 1.5 degree leade for a 30-06 AI.

I've opened the magazine and feed ramp a little. It will be very close to fitting the magazine with a 190 ABLR. Definitely no with a 200 gr HBT.

This is a Bartlein barrel and I've been careful not get it too hot. It has around 800 rounds down it. I've never thought about changing barrel steel on a bolt gun. A crew served machine gun would absolutely do it. Not sure I'm working the bolt that fast enough with a 5 round capacity to get there. The barrel at the chamber has never been more than warm. The suppressor is by far the hottest part of the system. Too hot to touch.

 

[Tiny Tim]

Depending upon throat condition at that round count. Setting barrel back .5-1" may be more beneficial if you wanted to try that.

 

[milo-2]

Depending upon throat condition at that round count. Setting barrel back .5-1" may be more beneficial if you wanted to try that.

 

[Lou270]

No idea, but if it ain't broke don't fix it. In my experience seating a bullet deep is a very mlnor ballistic sin. If you do the math there is very little to be gained volume wise seating a .3" cylinder .1" furrher out and that translates to few fps not something significant

Lou

 

[A A Ron]

Leaving it alone is probably the best advice, but it us always fun to ponder and want more or better.

 

[milo-2]

Leaving it alone is probably the best advice, but it us always fun to ponder and want more or better.

It will give you something to ponder on your next barrel.
Now, I am not a 30 cal guy, but I have friends who swear, jumping 30 cal bullets .100" or more improves performance, and seems to lower pressures at the same time than say a bullet 15 off. Again, with a grain of salt, they are shooting magnums of some sort, not any versions of 30-06.

 

[Tiny Tim]

It will give you something to ponder on your next barrel.
Now, I am not a 30 cal guy, but I have friends who swear, jumping 30 cal bullets .100" or more improves performance, and seems to lower pressures at the same time than say a bullet 15 off. Again, with a grain of salt, they are shooting magnums of some sort, not any versions of 30-06.

I have a couple of rifles that doo well in that .090-.110 area. Glad it works because they fit in the mag at those jumps.

Found when doing a seating test with Berger Hybrids starting at .015 and going in .015 increments, lost about 10 fps each change until somewhere between. 060 &.090 off. Once there, erosion of the lands may not affect velocity (at least not as critical as when close to the lands).

 

[milo-2]

I have a couple of rifles that doo well in that .090-.110 area. Glad it works because they fit in the mag at those jumps.

Found when doing a seating test with Berger Hybrids starting at .015 and going in .015 increments, lost about 10 fps each change until somewhere between. 060 &.090 off. Once there, erosion of the lands may not affect velocity (at least not as critical as when close to the lands).

I misspoke some, concerning lowering pressures with more speed, not sure that is possible, lol. What I should have said was, a sizeable jump seems to reduce a hard initial pressure spike, which can hinder initial load development.