Throat testing

[nksmfamjp]

I'm interested if anybody has done any testing of throat parameters vs accuracy.

The key parameters to me seem like:
Diameter - At bullet diameter, I suppose there may be some pressure potential, but I think some folks are using 0,001" over bullet diameter. Up to 0.005" over bullet diameter. Results?

Freebore length - Maybe SAAMI is the minimum, but has testing been done with 0.1-0.5" of free bore?

Throat Angle - I think 1 deg 30 min is std, but have folks tried 30 min through let's say 5 deg?

If I understand right, the revolutionary concept with 6.5 PRC and 300 PRC is tighter freebore.

Any thoughts or successful endeavors you have had with changes to the throat?

 

[Mikecr]

It would be destructive testing with fleeting results.
It all changes throughout a barrel's life.

 

[rammac]

In Harold R. Vaughn's book "Rifle Accuracy Facts", he talks briefly about chambers and throats in chapter 3. Basically he says that any throat half angle of less than 0.75 degrees would increase the chances of stripping the bullet (removing material from the outside surface of the bullet). He states;

I prefer a steeper slope (2.5 degree half angle) on the throat of 6mm bench guns because the short bullets seat too far out in the case with a shallow angle throat.

So obviously the optimum throat angle would depend on the bullet shape and length.

I don't remember if he talks about diametrical clearance between the bullet and the throat but he does talk about neck and chamber clearance, which I'd assume that he wouldn't want any more bullet/throat clearance than whatever the neck/chamber space is. He says that military grade rifles have about 5 mils of neck/chamber clearance and he expects a 6mm bench gun to have only 0.4 to 0.7 mils of clearance.

As for throat length, I only saw where he talks about throats being non-concentric with the bore axis. He says that an error that creates an offset of the throat length of 0.18 inches is unacceptable for a precision cut barrel.

He may mention these features to a greater extent in his book but it's been a while since I read it and I don't have to time reread it now but hopefully this sheds some light on your questions.

 

[MagnumManiac]

I haven't tested with actual barrels by changing them dozens of times, but I have changed throat/leade angles and lengths when I have set back and re-chambered.
When changing my first 300WM barrel from SAAMI to the A191 chamber, accuracy improved straight up, it was more consistent and easier to tune. Since then, leade angles on reamers have been played around with but length has not. The angle needs to transition smoothly in my limited experience, but have not nailed that 'best' spot yet. My Weatherby chambers run .0005" clearance while most other throats run .001". Although .0015" is also mentioned on some reamer prints.
I also lengthened the throats on all my Weatherby chamberings, except my 270Roy, from .375" to .500" and had an improvement with accuracy, but also a better curve under MAP while gaining velocity.
Hope this limited knowledge gives you an insight.

Cheers.