Can't understand this data

[Texas Solo]

I'm trying a new bullet in my .308, Hornady 168 BTHP Match, part# 30501. Rifle is a Savage #11.
My CBTO seems really short as measured with the Hornady OAL gauge and a comparator. I've measured 20 times, all within .005 of each other. My average is 2.085 with an OAL of 2.695. (much shorter than the Nosler 168 CC I started with) I'm loading to 2.080 CBTO, so it's not jamming. I don't so much care about the OAL, but my concern is that of increased pressure with the bullet seated that deep. I'm using LC brass & 44.0gr Varget. I'm getting great groups of .390 and no sticky bolt or other pressure signs.
My concern was only raised due to a chronograph reading (5 shot avg.) of 2901. Hodgdon site says 2750 with 46.0gr with a Sierra MK, which I know has a totally different ogive than the Hornady. I'm only at 44.0gr. AFAIK, the only way to increase velocity is to increase pressure.

Not sure what to make of this data. I've talked with folks using the same 44.0 varget load, 168's, and LC brass. None report getting 2900 FPS, but none were shooting the Hornady bullet either.

If anyone is using the same bullet/powder combination, I'd sure appreciate some feedback. Despite good groups and no pressure signs, I'm still concerned. I can't understand my results, something is weird here.

 

[CatShooter]

I'm trying a new bullet in my .308, Hornady 168 BTHP Match, part# 30501. Rifle is a Savage #11.
My CBTO seems really short as measured with the Hornady OAL gauge and a comparator. I've measured 20 times, all within .005 of each other. My average is 2.085 with an OAL of 2.695. (much shorter than the Nosler 168 CC I started with) I'm loading to 2.080 CBTO, so it's not jamming. I don't so much care about the OAL, but my concern is that of increased pressure with the bullet seated that deep. I'm using LC brass & 44.0gr Varget. I'm getting great groups of .390 and no sticky bolt or other pressure signs.
My concern was only raised due to a chronograph reading (5 shot avg.) of 2901. Hodgdon site says 2750 with 46.0gr with a Sierra MK, which I know has a totally different ogive than the Hornady. I'm only at 44.0gr. ASAIK, the only way to increase velocity is to increase pressure.

Not sure what to make of this data. I've talked with folks using the same 44.0 varget load, 168's, and LC brass. None report getting 2900 FPS, but none were shooting the Hornady bullet either.

If anyone is using the same bullet/powder combination, I'd sure appreciate some feedback. Despite good groups and no pressure signs, I'm still concerned. I can't understand my results, something is weird here.

Loading manuals are only approximate. They do NOT tell you what you will get.

Develop your own loads using manuals as a guide, and using sensible pressure signs.

 

[jonbearman]

+1 for catshooters take.The manuals dont take into account to your actual groove variation ,tight or looser,your barrel length,the primer you use if you dont have what they suggest,the lot of powder can vary and on and on. So it is only a laboratory book to go from and to be safe in all rifles(sporters).

 

[TroyMN]

My 168 Hor BTHP is LC07 brass, Br2, 46.1 varget, 2.255 ogive, about 2715
That is with 20" barrel

 

[Texas Solo]

My 168 Hor BTHP is LC07 brass, Br2, 46.1 varget, 2.255 ogive, about 2715
That is with 20" barrel

Thank you Troy, that is actually very helpful. What is your OAL?

 

[bigedp51]

Texas Solo

Catshooter hit the nail on the head, when reloading manuals use a specific firearm in the testing a strain gauge is glued to the barrel and is used for the pressure readings. Normally a stain gauge must be calibrated with a known type of ammunition and pressure. And this can make the data very ballpark because no two firearms are exactly alike.

When you look at the Lyman manual you will see they use a universal receiver and pressure test barrels with pressure transducers. This is more accurate but in many cases these test barrels are made to minimum chamber and bore dimension to get the highest possible pressure. Meaning a standard factory rifle will probably have a larger chamber and bore.

I have two AR15 5.56 rifles and a bolt action Savage .223 rifle and the throat is longer on the bolt action .223 than the military type AR rifles. So NOTHING written in the reloading manuals is written in stone and there are thousands of variables.

Even the pressure signs you can see on your cases an vary, Federal brass is noted for being soft and you will see ejector marks of the base of the case at much lower pressures than other makes of brass.

 

[southtxhunter]

From what I read it sounds like you are loading Hornady at the same seating die position as the Nosler with out adjust the die is that correct?

 

[Texas Solo]

From what I read it sounds like you are loading Hornady at the same seating die position as the Nosler with out adjust the die is that correct?

No, not correct. CBTO data was achieved using an OAL gauge with comparator. Die was then adjusted to the CBTO figure, minus .005".

I've re-measured the chamber with a different bullet and now I get a more reasonable CBTO figure of 2.198. Puzzled, I checked again using the original bullet. Guess what? It seems that I picked the one defective bullet in 100 to measure with. I then used 3 other bullets and those figures matched the most recent findings.
So if I'm correct, the test loads I shot WERE loaded way short and therefore over pressure.
Yes, there is a difference between the Nosler and the Hornady, which is to be expected. But not .118" difference.

Looks like the chrony may have saved my butt. Glad I questioned the results.
Of course I realize that loading manuals and published data are a general guide, but sometimes things are just too far off not to be questioned.

Thanks for all the replies.

 

[tiny68]

Having sthot that bullet with Varget, RE-15 and 4064 for 1000s of rounds, I am fairly certain you have a chronograph problem. With a 26" barrel, I could get 2750 as a hot load. 2800 fps was over pressure. I am have repeted with 3 different 308w barrels.

I would get some Fed Gold Metal Match 308 168 SMK ammo. All 3 of my barrels shoot these right at 2700 fps. That is my reference to verify my Chrony is working.

Luck, Tim

 

[CatShooter]

Having sthot that bullet with Varget, RE-15 and 4064 for 1000s of rounds, I am fairly certain you have a chronograph problem. With a 26" barrel, I could get 2750 as a hot load. 2800 fps was over pressure. I am have repeted with 3 different 308w barrels.

I would get some Fed Gold Metal Match 308 168 SMK ammo. All 3 of my barrels shoot these right at 2700 fps. That is my reference to verify my Chrony is working.

Luck, Tim

And I have had two 26" barrels that shot them at 2550 fps and 2560, one 24" that was 2685 fps - same box of ammo.

You can NEVER use advertised velocities to check or verify anything. Same ammunition will give different velocities in all rifles, regardless of barrel length.

 

[drumcorpschamp]

Alright, I'll ask. What in the hell is CBTO?

Jerry

 

[Pres100]

It's a base to ogive measurement.
More accurate measurement than the over all length, due to bullet length/shape variances

 

[Erik Cortina]

Alright, I'll ask. What in the hell is CBTO?

Jerry

Case Base to Ogive is how I interpreted it.

 

[Bamban]

At the range one time, there were 3 chronos, a Beta, a CED, and a Oehler we were playing with. The Beta Chrony consistently registered 100+ FPS higher than the other two.

 

[tiny68]

And I have had two 26" barrels that shot them at 2550 fps and 2560, one 24" that was 2685 fps - same box of ammo.

You can NEVER use advertised velocities to check or verify anything. Same ammunition will give different velocities in all rifles, regardless of barrel length.

CatShooter - I totally agree with you about the variation in speed with barrels. My point was I don't think he is actually driving the 168 SMK that fast.

Tim

 

[finnaddict]

Alright, I'll ask. What in the hell is CBTO?

Jerry

http://www.bergerbullets.com/effects-of-cartridge-over-all-length-coal-and-cartridge-base-to-ogive-cbto-part-1/

http://www.bergerbullets.com/effects-of-cartridge-over-all-length-coal-and-cartridge-base-to-ogive-cbto-part-2/