Throat erosion

[Rockhurr]

Shooters.....I got a pretty serious throat erosion with my 6.5 CM and wondered if any of you guys are experiencing simular problem. I'm shooting a Tikka T3X TAC A1 factory rifle.

My original Base to Ogive measurement was 2.316" with the Hornady 147 ELDM'S. I was shooting 43.4 grains of H4350 and 0.020" off the lands. No pressure signs (no sticky bolt, ejector marks, catering primers, blown primers, nothing). About maybe 200 rounds later I rechecked BTO and was still the same. This pasted July, I switched over to shooting coppers, mostly Barnes LRX 127, 44 grains of Reloder 16. Again, no pressure signs what so ever. After total of maybe 400 or 500 rounds which were 99% time shot slowly, my BTO now measures a devastating 2.346". This is a 0.030" erosion. Is any of you guys seeing this at all? Is my throat really gone or is there something going on that's messing up my measurement? I thought my chamber was dirty so I cleaned it with a nylon brush and solvent but still seeing the same thing. ?‍

 

[_Raining]

2 different bullets are going to have different CBTO measurements. Shooting will also change the throat, that is why you see the term chasing the lands. IIRC it is around a few thou every 100 rounds but it will vary depending on a whole list of things, the same list of things you will see about barrel life (freq of firing, heat, over pressure etc).

 

[Wheres-Waldo]

If the datum diameter on the bullet you choose to measure is the SAME diameter as the bore diameter (.256”) the bullet makes no difference.

When you start measuring a smaller diameter datum is when bullet style and ojive profiles will influence the measurement.

This is evidence that your comparator should measure the diameter of what is effectively the bearing surface.

(A SMK and a VLD will have the same BTO length for a given chamber, as long as the datum diameter of your comparator is the same as your bore diameter

 

[Milo 2.5]

If the datum diameter on the bullet you choose to measure is the SAME diameter as the bore diameter (.256”) the bullet makes no difference.

When you start measuring a smaller diameter datum is when bullet style and ojive profiles will influence the measurement.

This is evidence that your comparator should measure the diameter of what is effectively the bearing surface.

(A SMK and a VLD will have the same BTO length for a given chamber, as long as the datum diameter of your comparator is the same as your bore diameter

NO...………………...again, No
He needs to use his original bullet used to gauge anything correctly.

 

[Wheres-Waldo]

So different bullets are stopped by the lands/leade angle at different diameters for different bullets of the same caliber in n the same chamber?!

For all intensive purposes, excluding the minute variations in ojive profiles and their interaction with leade angles, how does seating depth at the start of the bearing surface change from one bullet style to another?

Serious question... not trying to talk shit, I just want to measure things correctly! But I’ll need it explained WHY it’s correct one way and incorrect another way.

 

[Milo 2.5]

Shooters.....I got a pretty serious throat erosion with my 6.5 CM and wondered if any of you guys are experiencing simular problem. I'm shooting a Tikka T3X TAC A1 factory rifle.

My original Base to Ogive measurement was 2.316" with the Hornady 147 ELDM'S. I was shooting 43.4 grains of H4350 and 0.020" off the lands. No pressure signs (no sticky bolt, ejector marks, catering primers, blown primers, nothing). About maybe 200 rounds later I rechecked BTO and was still the same. This pasted July, I switched over to shooting coppers, mostly Barnes LRX 127, 44 grains of Reloder 16. Again, no pressure signs what so ever. After total of maybe 400 or 500 rounds which were 99% time shot slowly, my BTO now measures a devastating 2.346". This is a 0.030" erosion. Is any of you guys seeing this at all? Is my throat really gone or is there something going on that's messing up my measurement? I thought my chamber was dirty so I cleaned it with a nylon brush and solvent but still seeing the same thing. ?‍

I had answered your question above, need your first bullet used to check these measurements.
I find it hard to believe that you had 200 rds down the tube with no erosion, and here is why. The leade angle going in the bore is most likely 1 degree, so the rifling at the entry point is extremely thin and prone to wear easily until the thickness becomes substantial. And at that time it becomes easier or more forgiving to tune your seat depth.
As for your throat being gone, it is really never gone, just pounded forward. When you get a pressure dump(velocity loss from your load) is the beginning of throat loss or ability to hold normal pressure.

 

[Milo 2.5]

So different bullets are stopped by the lands/leade angle at different diameters for different bullets of the same caliber in n the same chamber?!

For all intensive purposes, excluding the minute variations in ojive profiles and their interaction with leade angles, how does seating depth at the start of the bearing surface change from one bullet style to another?

Serious question... not trying to talk shit, I just want to measure things correctly! But I’ll need it explained WHY it’s correct one way and incorrect another way.

Just take 2 different brands of bullets and run you own test. I once had 65 thou difference in this measurement between 2 lots of 140 gr 6.5mm hybrids, bullets looked alike, but different shape. A slight change in ogive angle makes a big diff.

 

[Wheres-Waldo]

Milo,

Thank you, I’ll test it out!

 

[clcustom1911]

My BTO measurements for my Berger 7mm 168 VLD bullets are different from Hornady 175 ELD-x which are different from Hornady 139 GMX which are different from 168 SMK's in my Tikka 7mm RemMag.

Different ogive shape and bearing surface junction angle on different bullet designs

I used the BTO for my 168 Matchkings as my baseline when I first loaded my Bergers to "touch" the lands..... loaded a dummy round to check/confirm.... The Bergers were into the lands by .010". Now I do a BTO measurement for every bullet I load, record that for posterity and future use.

 

[spife7980]

So different bullets are stopped by the lands/leade angle at different diameters for different bullets of the same caliber in n the same chamber?!

For all intensive purposes, excluding the minute variations in ojive profiles and their interaction with leade angles, how does seating depth at the start of the bearing surface change from one bullet style to another?

Serious question... not trying to talk shit, I just want to measure things correctly! But I’ll need it explained WHY it’s correct one way and incorrect another way.

Thats the thing though the minute variations between bullets profiles arent minute with respect to what we are trying to measure. And its not directly at the start of the bearing surface that the bullets are first touching nor where we measure either.

Finding the point where the bullet goes from floating in space to tangentally touching to jammed at the maximum diameter is exceedingly difficult so we dont do it there.

Depending on the shape of the nose where they actually touch the lands can result in a different reading than where another different bullet design touches, hell even the same design can have a significant variance in between lots. You have a curved bullets ogive interacting a linear plane of the lands and getting a reading from a location thats different from the tool used to seat them.

The grooves are .308, the bore is .300 in a 30 cal barrel. There is .008" of difference between the start of the lands and the full profile of the lands and that .008" of difference is spread over .1637" (2.3025-2.1388) in a saami spec chamber with a 1.75° leade angle. And thats text book, not even accounting for real world deviation. And that bullet doesnt touch at the .308 max diameter, it touches somewhere further forward and down smaller on the nose than that.

If the lands were 90° perpendicular square to the bore instead of angled and the bullet were a cylinder rather than a curved surface so that the did contact at the max diameter then same then the same caliber/same location thing would hold up. But thats not the case.


Now, thats not to say that the coincidence of having one bullets reading end up close to one another is an enigma, its just far from a sure thing.

 

[Mordamer]

Everyone that said you have to use the same bullet as your first measurement is right. If you don't believe them, go measure to the lands with both bullets and you will most certainly get 2 different measurements.

That being said, .030" of throat erosion is not that much. That is exactly in the ballpark of what I would have predicted for the round count you have. If it still shoots good, who cares what the throat erosion is? I have had barrels with more than double that much throat erosion and they still shot good.