8208xbr pressure signs with 308?

[Sdupontjr]

I was looking into a different powder for my 308 load with 155 scenars. Currently using 47.5g Varget, CCI-200, R-P Brass, CBTO 2.216" or 2.835" COAL. This gives me .050" jump on a fairly new Tikka CTR 308 20" barrel. That load is compressed pretty good and yields 2840 avg and a 10 shot moa or less groiup at 100.

When I looked on Gordons, it looks like I'm not getting 100% burn in that short of tube. So I decided to give 8208xbr a shot. Have a lb of it and its starting to show back up. Loaded 44, 44.5, 45 & 45.3 (book max) and went shoot for pressure. Saw nothing. Primer looked fine, very easy bolt lift, no swipes or ejector marks. So loaded up a few just to see. Neither were near compressed loads, so I liked that.

45g 8208, 2.216" CBTO, .050" jump
45.3g 8208, 2.216" CBTO, .050" jump

What I noticed was the SD/ES were ridiculously low and velocities were up. It was +90* when I was shooting and humidity was like breathing pea soup. When I tested for pressure, I didn't see anything to speak of, but when I shot these 3 rounds each to get some velo numbers, I noticed a few things on the case head.



Of course, only 3 shots, but 45 was .617" and 45.3 was .412" at only 100 yard (I was limited by this range)

I did notice a slight ejector mark on 1 of each set. So should I back down to 44 / 44.5 and work back up to 45? I have .050" to play with. Do I seat longer to relieve case pressure but closer to lands will build pressure back up? Any thought yall have would be greatly appreciated.

 

[37L1]

I run 44.6 of 8208 in my 24" Brux 1/10, Hornady brass, .020 off. Berger 155.5

Last time I shot it at 600

 

[Doom]

GRT say 43.6 gn is max @2.800" COAL. The Hodgdon data for 8208 is in Winchester brass which has greater capacity than R-P. [EDIT] Probably on the order of a grain or more of powder.

 

[Sdupontjr]

Yeah i saw that. On mine, 43.9g is 60,298 psi in a 20" barrel. Hell, hodgdon site has max is 45.3 at 60,900 psi. And I saw where book max (cant remember where) was also 45.3. So who knows.

I know my H1000 load in my 7 mag is 3g over book max and still not a hint of a sign of pressure. But WTH do I know, thus me asking. Ha

 

[Evintos]

Yeah i saw that. On mine, 43.9g is 60,298 psi in a 20" barrel. Hell, hodgdon site has max is 45.3 at 60,900 psi. And I saw where book max (cant remember where) was also 45.3. So who knows.

I know my H1000 load in my 7 mag is 3g over book max and still not a hint of a sign of pressure. But WTH do I know, thus me asking. Ha

Did the GRT estimated velocity match up with the real world obtained velocities? If not then the pressure simulation is very likely off as well and there's a factor in your input that needs to be changed in order to bring it more inline.

 

[Sdupontjr]

No it wasn't.

GRT - 45.3 was 2826
Actual - 45.3 was 2882, 2886, 2875

GRT - 45 was 2808
Actual - 45 was 2857, 2857, 2856

I'll have to play around with the factors and see where the issue is.

 

[Evintos]

No it wasn't.

GRT - 45.3 was 2826
Actual - 45.3 was 2882, 2886, 2875

GRT - 45 was 2808
Actual - 45 was 2857, 2857, 2856

I'll have to play around with the factors and see where the issue is.

Load up your GRT 308 file and click on OBT

Click "Create grtload file copy with the calibrated values" and this should then give you a new generated page with calibrated values.

GRT new calibrated value will give you 2880.6fps for 45.3gr and 2863fps for 45gr.

Massage case volume slightly to 56.5gr H2O (up from 56.3) and you'll get new calibrated values of FPS at

45.3 gr - 2875.4fps, 69780 psi
45gr - 2857.9fps, 68321 psi

If your actual measured velocity for 44.5gr was 2827.7fps average and 44gr was 2797.8fps (velocities predicted by GRT), then the simulation calibration is completed and matches real world (ideally within 10fps). You can use the simulated pressure values as reference compared to max SAAMI or CIP pressures with the new calibrated values. If not, more inputs need to be modified.

 

[Sdupontjr]

Oh man. Thanks a lot for the info. I'll try that out.

 

[Sdupontjr]

Also I noticed that the load ratio was 102.7% in GRT. 45.3 sloshed around in my loads seated where I was. Wonder if that has something to do with it?