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Strangely Brown View Drop Down
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    Posted: January 20 2023 at 7:54am
I have an O.A.L. question regarding Sierra MK's, or rather to save any misunderstandings a C.B.T.O. (Case Base To Ogive) query.
Do any of you remeasure C.B.T.O. for every batch of bullets you get or just hope there're all the same measurement?
I've just acquired 4 boxes of .311 174gr SMK's and the packaging suggests 2 boxes are of earlier manufacture, there's an average of ten thou difference between them and newer ones.
Mick
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Goosic View Drop Down
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Points: 7847 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Goosic Quote  Post ReplyReply Direct Link To This Post Posted: January 20 2023 at 8:05am
Geoff might certainly have an answer for you regarding your question but, that particular question does sound like it lays within the boundaries of a BR shooter IMHO...
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Originally posted by Goosic Goosic wrote:

Geoff might certainly have an answer for you regarding your question but, that particular question does sound like it lays within the boundaries of a BR shooter IMHO...
 

I blame my new Hornady O.A.L. gauge! 
Mick
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Goosic View Drop Down
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Points: 7847 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Goosic Quote  Post ReplyReply Direct Link To This Post Posted: January 20 2023 at 8:57am
I have that case gauge too.
The issue I was initially having was not realizing I was trying to get the same numbers using two different types of bullets. That being the Tangent and Secant ogives. The Tangent ogives in my 7.62mm No4Mk1* when set to an averaged OAL of 2.830" imitate the military M852 Match Round pretty much dead on. Using Hornady 150grn bullets with the Secant ogives in my 303B produced less than desirable results until I seated them out beyond normal conditions which left me with the firm belief to just keep using the 174grn BTHP/FMJBT Tangent bullets seated to an OAL of 3.035" Set It And Forget It!!...
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I do, but by inference only.
The original setup I used was worked out measuring with the OAL gauge, but then I measured COAL, creating an "offset".
I have "master cases" for all my loads & check the seating die with those by simply backing off the micrometer stem, inserting a case & raising the ram. Then screw the seating die stem to a "firm contact".
Then I seat a couple of "real" rounds & fine tune if needed.
The results seem to tell me I'm doing something right!
Don't shoot till you see the whites of their thighs. (Unofficial motto of the Royal Air Force)
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Points: 110 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Sapper740 Quote  Post ReplyReply Direct Link To This Post Posted: April 16 2023 at 4:06am
Yes.  I keep a bound book with load data.  One of the first things I do when I buy a new milsurp is use the Hornady Lock n Load bullet comparator to get the C.B.T.O. of any bullet I plan to shoot in that rifle.  It's necessary due the great differences in ogive design among the various bullets, especially the LeHigh target solids.  
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I don’t bother trying to seat the 174 SMKs close to the lands on my No. 4 rifles, primarily because they must be able to be loaded into the magazine for rapid fire stages in competition.  I seat the bullets to OAL of 3.05 inches, and that is it.  I’ve checked the CBTO dimensions for the 174 SMKs, to touch the lands the bullet is about out of the case neck at that point on my Long Branch Mk 1/3 that I use for competition.  It’s a BSA 5 groove with about 8 to 10 thousand rounds thru it.  The 174 gr SMK will tolerate a LOT of jump and still be very accurate, at least to 600 yds. 

My 7.62 conversion with LB barrel is similar, tough to get a 168 SMK close to the lands.  For mid range (up to 600 yds) I load to magazine length.  For long range, I do try and get secant ogive bullets (168 and 175 TMKs) to within 0.020 to .030 inches from the lands, but they have to be single shot loaded. 

For my AR, I do seat the bullets to within 0.020 from the lands for the 600 yard prone slow fire stage.p, short line ammo is loaded to magazine length.  I generally do not recheck CBTO dimensions every time I open a new box of bullets, but do set up the bullet seater die to get the correct CBTO measurement each time I load up my 600 yd ammo.   I will also check periodically, as the throat erodes, perhaps every 1000 rounds, and reset the CBTO dimension.  
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