QuickLoad - Start Pressure for Jammed Loads - .308

[crewchef]

Wanting to know if anyone has figured out what an accurate Start Pressure is in QuickLoad for jammed .308 load. Read the product review and saw what they were recommending for the 6BR but search didn't come up with anything for .308. Thanks.

 

[louielouie]

I still use the recommended 'jammed' pressure but have changed the initial start pressure of the bullets based on the bearing surface length of my 6.5 mm bullets. Pressure = ((1420 * bearing_surface_length) + 2853) - which is about 3630 for 123 Amax and 3756 for the Sierra 140 SBT. This is based on Wolf LR primers - I add 100psi for BR2's, 200 for GM210M's & 400 for Winchester LR's.
Think of this as a guideline to manage variables along with actual case capacity, bullet weight, powder burn rate and that nebulous 'weighting' factor. I'm usually within 10 ~ 12 fps of predicted velocities using H4350 & h4831sc. RL17 & A4350 are close but the temp variable doesn't track as well.

 

[crewchef]

Thanks louielouie appreciate the input. I was asking about the start pressure because it's mentioned in both the review here and that recent shot show video about tweaks too. Maybe the 7200 addition is ok for some rounds but was just too much for 6BR?

 

[TheSnake]

I still use the recommended 'jammed' pressure but have changed the initial start pressure of the bullets based on the bearing surface length of my 6.5 mm bullets. Pressure = ((1420 * bearing_surface_length) + 2853) - which is about 3630 for 123 Amax and 3756 for the Sierra 140 SBT. This is based on Wolf LR primers - I add 100psi for BR2's, 200 for GM210M's & 400 for Winchester LR's.
Think of this as a guideline to manage variables along with actual case capacity, bullet weight, powder burn rate and that nebulous 'weighting' factor. I'm usually within 10 ~ 12 fps of predicted velocities using H4350 & h4831sc. RL17 & A4350 are close but the temp variable doesn't track as well.

Do you have any other tips you can share? This is the first I have seen of a type of formula that will help us with QL and thanks for this.

 

[Nomad47]

I still use the recommended 'jammed' pressure but have changed the initial start pressure of the bullets based on the bearing surface length of my 6.5 mm bullets. Pressure = ((1420 * bearing_surface_length) + 2853) - which is about 3630 for 123 Amax and 3756 for the Sierra 140 SBT. This is based on Wolf LR primers - I add 100psi for BR2's, 200 for GM210M's & 400 for Winchester LR's.
Think of this as a guideline to manage variables along with actual case capacity, bullet weight, powder burn rate and that nebulous 'weighting' factor. I'm usually within 10 ~ 12 fps of predicted velocities using H4350 & h4831sc. RL17 & A4350 are close but the temp variable doesn't track as well.

Are you saying you use the formula for jumped bullets and add 7200 PSI for jammed bullets? And I'm guessing adding about 600 for 215Ms would be about right?

 

[BoydAllen]

Just out of curiosity, what do you mean when you say "jammed"?

 

[crewchef]

Bullet seated so it is in to the lands when chambered.

If you read the product review "power user tips" item number 6 it talks about recommendation for adding 7200 to the base pressure for bullets seated into the lands. It goes on to say their observations were that adding less than the recommended number worked better for the 6BR.

So the question was, is the 7200 addition too much for .308 too? I can play around with it and see if adjustments make it match up to what the chrono says. Just don't want to be making adjustments to too many fields to make things match. Or maybe more importantly making bad adjustments that I shouldn't have.

Here's the link;
http://www.accurateshooter.com/gear-reviews/test-quickload-review/

 

[BoydAllen]

Since we see differences in accuracy with different seating depths into the lands, do you think that these different depths produce different velocities?

 

[TheSnake]

Since we see differences in accuracy with different seating depths into the lands, do you think that these different depths produce different velocities?

Different seating depth make a big difference in pressure so they must make a difference in velocity. The problem I have is I adjust everything to match what I have and my velocity is always off 50-75 from my Chrono. I don't want to steal the OP thread so I am going to start another thread in the reloading section called "Using formula's in Quick Load" if we can please pick this up there.

 

[crewchef]

BoydAllen, Just getting started with QL and haven't had a chance to check it's predictions against actual fired loads but I did look at seating depth in QL today. Starting at 2.820 COAL if you seated it .020 longer/shorter QL was predicting around 8 FPS change. If I had moved far enough to get compressed powder or to put bullet into lands it would have more significant change. Actually on my to do list to chrono several rounds at different lengths to see if predictions are accurate.

TheSnake, Based on what I've read so far Burning Rate Factor and Bullet Weight seem to be the two parameters to tweak when you're trying to make QL match up with actual test results. I built one load today in QL that I had good data on from test sessions. QL predicted 2722 FPS, real chrono data was 2717, pretty darn close. Hoping it's projections on the new stuff I'm working up will turn out as well. I'll continue to treat it like any other load data and start well below and work up but nice to have an idea of what to expect. I'll be watching your other thread too. Hopefully several experienced QL users will share what they've learned.

 

[louielouie]

Sorry for my tardiness - had some surgery on my elbow since I last posted.

I'm taking the view that all things related to pressure are cumulative: longer bullets, 'hotter' primers, 'jammed' or seated into rifling. That's why I have only a few hundred psi from a short bullet to long, weak primer to hot. In my formula, the (1420 * bearing_surface) sets the slope on the x-y axis of your pressure gain relative to bullet length while the y-offset 2853 is the what you vary to initially set the shortest bullet to 3625 - the initial starting pressure. This is from the chronograph observations when I've changed single components at a time and of course might not be right for you but the methodology might be.

That nebulous 'weighting' factor for my 260 Remington is now 0.44 but I used the scaling from the http://bulletin.accurateshooter.com/2011/04/overbore-cartridges-a-working-definition/ chart to adjust that value, not just the efficiency of the design (shoulder angle, body taper, etc.) plus trying to get 'best fit' of my data.

I've got electronic 'post-it' notes all over my desktop with measured bullet lengths, max seating depths, case capacities, etc. for quick references.