Throat Length Variance

[rebel107]

Why would I have an .080" variance in throat length after using the same 7mm SAUM reamer on two different 7mm barrels?
Both barrels are Benchmark, 1 is a 9 twist and the other is 8.5 twist, I have measured with the same Stony point gauge and projectile a number of times with the 9T being longer each time I measure.
Is it maybe barrel diameter being different although I think the same pilot was used on the reamer for both.
What am i doing wrong??

 

[Dusty Stevens]

Did the longer one get throated out or been shot a bunch?

 

[rebel107]

Did the longer one get throated out or been shot a bunch?

Yes the older long throated 9T barrel was used for 900 rounds then an inch cut off then rechambered. Interesting to note that after rechamber it measured almost exactly the same as it was cut from new.

 

[Dusty Stevens]

Its gotta be different bore diameters if it measures the same after the rechamber as it did new

 

[Tozguy]

If the same reamer gave two different throat lengths, the difference being .080'', and if it really is for the same reamer I gotta wonder about your measurements.

 

[Ggmac]

What are you using to measure it with?

 

[savagedasher]

Have you polished the chamber ? Larry

 

[tightneck]

Did you use the same bullet to do all measurements? I mean the exact same bullet, not just same model. I've seen Bergers vary .060" BTO, from lot to lot.

 

[rebel107]

Ok i haven't polished the chamber, not sure why I would do that?
same projectile on the threaded modified case pushed in to chamber then pushed up the projectile locked the brass bolt. Done this 4 times and got the same results each time. Remember, the 9T measured the same when new, but the 8.5 T barrel throat is shorter by .80" It must be different bore size.

 

[savagedasher]

Ok i haven't polished the chamber, not sure why I would do that?
same projectile on the threaded modified case pushed in to chamber then pushed up the projectile locked the brass bolt. Done this 4 times and got the same results each time. Remember, the 9T measured the same when new, but the 8.5 T barrel throat is shorter by .80" It must be different bore size.

I assumed one was shot and the other wasnt . That was why I responded that way
The last 4 I had done there was only 5 thousand difference . I even can changes cases from one to the other . Larry

 

[dkhunt14]

If the one throat is new and not shot much, it has little marks from the reamer. I have seen throats move a lot from new till they get broken in. Then they settle down and not move as fast. Matt

 

[Tozguy]

Ok i haven't polished the chamber, not sure why I would do that?
same projectile on the threaded modified case pushed in to chamber then pushed up the projectile locked the brass bolt. Done this 4 times and got the same results each time. Remember, the 9T measured the same when new, but the 8.5 T barrel throat is shorter by .80" It must be different bore size.

Are we talking .80'', .080'' or .008'' ?
The original post mentions .080'' which seems to be a lot to explain by bore diameter variation. I would expect that such a difference in bore diameter would be noticed when fitting the tightest pilot bushing.

Does the rifling mark the bullet in the same way on both barrels?

The idea of polishing a fresh chamber is possibly to remove burrs that might affect the reading and/or reduce the chances of the bullet getting held in the leads and pulled out of the case on extraction. If you are using a long ogive bullet then I would try seating the bullet backwards in the tool and repeat the measurements.