Primer Blow Out with NO Over Pressure Signs?

[BuffCyclist]

That engagement area rule won't exist in numbers relating to OAL, or at least it shouldn't. An OAL of 2.800 can have different amounts of engagement area based on trim length of brass. My trimmer trims from the neck. Therefore there'd be a different ratio of engagement area if a case is trimmed to 2.005 rather than the SAAMI spec of 2.015.

Here's a question: how would you measure (down to .001) the engagement area of a bullet that is loaded into a piece of brass? You can do it, but you'd have to use some arithmetic and critical thinking skills.

Yes, that's a challenge, and please show your work.

Yes, but if you are told the max length base to ogive for the 168gr SMK, it should show you the brass trim length. If it gives 2.015 and you use 2.005, then you will need to seat 0.010 deeper to get the same engagement between the bullet and brass/neck. What I'm trying to talk about is that if you trim all of your brass to the same length and seat the bullet to the same depth, you have the same contact area between the bullet and neck, so a pressure difference should not come into play there. However, if you use mixed brass and seat to the same OAL (or ogive length), you can have different pressure based on how much contact you have between brass/bullet. More contact logically equates to more pressure (more friction to overcome to unseat the bullet). But as my chief engineer says, logic is the art of being wrong with confidence.

To calculate the engagement of the bullet, you need to use two ogive comparators and measure the ogive on the tip of the bullet to the ogive on the base of the same bullet (one comparator on bullet base, one comparator on bullet tip) and that gives you the bearing surface of the bullet. But then, you'll need to know the minimum engagement of that bullet. If I recall, I read somewhere that you need at least 1/3 of that bearing surface to contact the brass. 1/3 is a rough value and it would be better if there were a definitive value, unless 1/3 actually applies to all bullets regardless of their length and is simply 33% of that length.

In short, I'm trying to rule out ALL possible variations that cause pressure spikes.

Ahh, I see what you're saying now. You can't easily measure the amount of neck that is contacting the bullet. But, using the SAAMI geometry of the 308win case, you can get what it should be assuming you use the same length brass as the SAAMI specs. Then again, you can determine the length of the bullet's bearing surface (ogive to ogive) and then measure the distance from the ogive of the bullet to the lip of the case, subtract that value and that gives you the amount of bullet seated in the case. If you seat deeply, part of that will be below the neck and not contacting the brass, but that number could also be found quite easily (i think).

[TheBelly]

Now add in different frictional forces by the malleability of the brass at the neck.

[BuffCyclist]

Work:



Size/trim your brass to 2.015".
The engagement area = 2.015" - 1.7118 = 0.3032" ~= 0.303"

If you trim your brass to 2.005" your new engagement area is
engagement = 2.005" - 1.7118 = 0.2932" ~= 0.293"
0.2932 / 0.3032" = 0.9670 = 96.7%

So if you trim your brass to 2.005" then you have 3.3% LESS engagement and would need to modify your bullet seat depth accordingly to maintain the same contact and avoid pressure spikes from this.

This assumes your neck for each piece of brass is located at 1.7118", so you would need to measure each piece of brass and determine that.

[TheBelly]

So, how about case capacity: SAAMI spec vs. trimmed to 2.005 and that ratio as compared to the amount of volume that is taken up by the powder?

[BuffCyclist]

To determine how much the case capacity DECREASES when trimming to 2.005” instead of 2.015”, is quite simple:

Case capacity of top neck (2.015”) = pi*h*r^2 = pi*(2.015-1.7118)*((.3091/2)^2) = .02275 in^3
Case capacity of top neck (2.005”) = pi*h*r^2 = pi*(2.005-1.7118)*((.3091/2)^2) = .02200 in^3

This is assuming the top of the case
Difference = 0.00075 in^3 = 0.01229 cc

IMR-4064 VMD = .0745
VMD*charge = vol(cc)
1 cc = 0.0610237 in^3

Difference in Volume to Charge of IMR-4064 = 0.01229 cc / 0.0745 = 0.16496 gr
308 Win Published Case Capacity (2.015”) = 3.6 cm^3 = 0.21968 in^3

Ratio of case volume between 2.005" and 2.015" = (0.21968 in^3 - 0.00075 in^3) / 0.21968 in^3 = 0.99658 = 99.66%

Reduction in case volume by 0.34%

So, trimming to 2.005” vs 2.015” decreases the case volume by 0.00075 in^3. If you pack powder to the top of the case lip, using IMR-4064, the difference in powder charge is 0.16gr (assuming the VMD chart that Lee publishes is correct).

However, I argue that this is not accurate, since you will not be able to fill powder that high. Most powder charges (at least that I charge) stop at the base of the neck, so the amount of powder will not change. I do this to avoid compressed charges.


And if you had me work through this to show that by trimming to 2.005”, I had a pressure difference due to more powder in less space, then add the 0.66gr to 42.8gr and you get 42.96gr, which is BELOW IMR’s published data for max powder still by 2.14gr.


Oh, and Belly, I showed my work even though you didn’t tell me to.

eta: Added case capacity as published on the 308 win wiki page.
eta2: Found a mistake, used case mouth diameter instead of case mouth radius, really changes the answer.

[Hoser]

Wow. All this time I have been trimming to min length or maybe a little less, sticking a bullet in 10-15 thou off the lands and calling it good.

My seat dies do not care how long my brass is. They reference the bottom of the case.

I prefer compressed charges. Lower ES and SDs normally results from it.

[BuffCyclist]

Wow. All this time I have been trimming to min length or maybe a little less, sticking a bullet in 10-15 thou off the lands and calling it good.

My seat dies do not care how long my brass is. They reference the bottom of the case.

I prefer compressed charges. Lower ES and SDs normally results from it.

Are you surprised at how little of a change you get when trimming to 2.005" instead of 2.015" or surprised at the depth Belly is making me analyze the situation?

And for the record, this math is quite simple too so this level of analyzing isn't difficult. If I made a mistake, I retract my previous statement

[BuffCyclist]

Work:

I just noticed this, comparing the above spec drawing and the measurements I obtained for my LC LR brass, the dimension of the groove for the extractor is 0.4091" and mine were measuring 0.404". Which indicates there is less material in the groove for the extractor on the LC LR brass (since the primer pocket has to be the same), so a near max pressure load will deform the LC LR brass EASIER (in that location) than if that dimension were to spec and was 0.4091". I will measure my Lapua brass when it arrives and see what it measures.

Not indicating this is the source of the problem as a difference in diameter of 0.0025" is not very much to be noticeably stronger, it is a discrepancy.

[TheBelly]

Are you surprised at how little of a change you get when trimming to 2.005" instead of 2.015" or surprised at the depth Belly is making me analyze the situation?

And for the record, this math is quite simple too so this level of analyzing isn't difficult. If I made a mistake, I retract my previous statement

hoser cutting in kinda ruined the fun.

I was waiting to see how much further you were going to play along until you realized that those changes don't matter that much. I poked you with a stick by using math to hook you in.

Bottom line: be consistent and try to make things the same. Certain measurements matter, others don't need to be worried about as much. The measurements I worry about are:

trim to length
distance from the lands
charge weight

[Hoser]

hoser cutting in kinda ruined the fun.

Thats cause Im a dick.