CBTO mystery

[uncle-buck]

I recently picked up a rifle which had not been shot in several years. Loaded some rounds using a load I developed almost 10 years ago and took it to the range. It shot OK, but not as well as it used to.

Decided to re-work my load and found that the distance to the lands had actually decreased by 0.057".

Am using the same bullet, a Sierra .30 cal. 190 gr. HPBT, but it is from a different lot than the bullet used in the original load development.

Can there possibly be that much variation from lot to lot?

 

[jr600yd]

Yup!

 

[Chaotik]

Same comparator?

 

[Bill K]

Often is.

 

[K22]

I have seen a lot of variation even within the same lot, in one of my favorite Sierra bullets, the 6MM, 85 BTHP but not as much as your reported but I wouldn't be surprised.

Nevertheless, despite this variation, this bullet continues to shine in my rifles seated at least 0.020 off the lands.

 

[Straightshooter1]

I recently picked up a rifle which had not been shot in several years. Loaded some rounds using a load I developed almost 10 years ago and took it to the range. It shot OK, but not as well as it used to.

Decided to re-work my load and found that the distance to the lands had actually decreased by 0.057".

Am using the same bullet, a Sierra .30 cal. 190 gr. HPBT, but it is from a different lot than the bullet used in the original load development.

Can there possibly be that much variation from lot to lot?

While there can be substantial variation from lot to lot, I doubt it's just due to that. I would guess it's mostly due to a different comparator insert.

Like, there's ~.060" difference in dimeter between my .308 Hornady comparator and my Sinclair one; the Sinclair one being the smaller and results in a measurement suggesting being closer to the lands, when it's not.

 

[Goodcat]

Comparator inserts can differ, but in my experience, measuring lands can be pretty finicky by what method you use, with various repeatability. I have not come across any method that measures lands within any consequential margin of error for me to call it “the lands” as an absolute, that I trust. For me, I measure the lands a few times, back off 20 thou from the shortest measurement, and then find a seating node depth. Works for me.

 

[uncle-buck]

Thanks for the thoughtful replies.

I think I was using a different seating depth gauge and a different comparator when the original load was developed years ago.

The new measurements were taken with a Sinclair seating depth gauge, comparator inserts, and Mitutoyo calipers. And repeated on two different days with the same results

Looks like a new load will need to be developed...

 

[fkimble@charter.net]

A carbon ring build up in the throat can also change this measurement and cause a pressure increase.

Frank

 

[uncle-buck]

A carbon ring build up in the throat can also change this measurement and cause a pressure increase.

Frank

Good point. I will post some pics of the throat this weekend and perhaps you or someone else can tell whether or not there's a carbon ring. And, if there is, how severe it is.

If there is a carbon ring, what size brush for a rifle chambered in .308 Win. would you recommend for scrubbing it out? Am thinking a nylon brush soaked in solvent and a power drill could be effective tools for removal.

Thx

 

[tominboise]

No power drill. Wet a patch with c4 or acetone and let it soak in the throat awhile. Then, if there is still a ring, try a few strokes with a 9mm pistol brush and some solvent. Repeat until it is gone.

 

[drop_point]

Either a carbon ring or lot to lot variation.

 

[uncle-buck]

This looks like a carbon ring. Will give it some good scrubs this weekend and work on getting rid of it.
Thanks for the helpful responses.

 

[mikecr]

Decided to re-work my load and found that the distance to the lands had actually decreased by 0.057".

This is not true, right?
And CBTO doesn't change, until you change it, regardless of bullets or barrel.

A dummy round made & kept helps with changes in tooling or measure.
Then you never have a 'CBTO mystery'

 

[Dave M.]

I recently picked up a rifle which had not been shot in several years. Loaded some rounds using a load I developed almost 10 years ago and took it to the range. It shot OK, but not as well as it used to.

Decided to re-work my load and found that the distance to the lands had actually decreased by 0.057".
Am using the same bullet, a Sierra .30 cal. 190 gr. HPBT, but it is from a different lot than the bullet used in the original load development.

Can there possibly be that much variation from lot to lot?

Never seen more than a grand total of .027” when sorting 3000 Berger 180 hybrids and these 6 boxes were comprised of 4 different lots. My records show another 6000 180g hybrids were within the above .027” extremes.
.057” seems way too large for a single type of bullet, even over dozens of lots, unless the comparator was changed, or the manufacturer change the bullet dimensions intentionally.
Dave