Primer Blow Out with NO Over Pressure Signs?

[BuffCyclist]

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

No no, I ENJOY math, I was doing that on my lunch break so I win. :P


I knew those measurements would be too insignificant to matter, but the math was easy and I was bored. But that’s precisely WHY I am still befuddled, because I trimmed all brass to 2.005” with my Wilson trimmer (brand new and worked perfectly), chamfered/deburred. They all were measured exactly the same and unless the ogive moved, the rounds were all within 0.005” of each other (closer based on bullet tip measurement but as we’ve established, that isn’t a standard measurement). The charge weight is what I was trying to nail down.


Sure, I will admit now that the 42.8gr charge was probably a little too hot (why the bolt didn’t even get slightly stick on 42.4gr I don’t know) but the charges were all double checked after recalibrating the scale so why did the 38.0gr charge blow out the primer. At this point, I am still leaning towards the primers being on the smaller side of the tolerance and loose primer pockets (faulty brass), combine the two and there you have it.

[BuffCyclist]

Allllllrighty, my Hornady Chamber Length gage and comparator have arrived and I finally have time to play around. Here's what I found.

Hornady Case = 2.007"
Comparator Gage Length = 1.005"

Out of 10 attempts, my averaged Chamber Length is
Base to Ogive = 2.3694"

After seating a bullet to that length and measuring:
Base to Tip (168gr SMK) = 2.949"

This makes sense, because I was measuring it at 2.950-2.952" before, but now I am more confident in the value.

So, I went back and measured/recorded all values for my remaining 51 rounds.
Average Base to Ogive = 2.2195"

2.3694" - 2.2195" = 0.1499"
2.949" - 0.1499" = 2.7991"

Which means my bullets are all seated to 2.799" (averaged) which is what I was aiming for before.

Now, with all of this in mind, I need to decide if I can seat a Sierra 168gr MatchKing to a length of 2.949" - 0.005" (0.005" off the lands) for a length of 2.944" or 2.364" (base to ogive) with my new Lapua brass. I still haven't tracked down any CCI #200 primers locally so I might try this again with the WLR primers and see if I get the same issues (only changing the brass and COAL).

After doing some math (can show work if needed/interested), I have found the bearing surface of the Sierra 168gr MatchKing bullet to be 0.500" ogive-to-ogive.

Tip of bullet to base ogive = 1.078"
Base of bullet to tip ogive = 0.632"
OAL of Bullet = 1.210"

1.210" - 1.078" = Base of bullet below base ogive = 0.132"
1.210" - 0.632" = Bullet above tip ogive = 0.578"
1.210" - 0.132" - 0.578" = 0.500"

So, my target OAL length (base to ogive) is 2.364"
Hornady Tool Case Length = 2.007"
Exposed bullet bearing surface (between ogive to ogive on bullet) = 2.364" - 2.007" = 0.357"
Therefore, my contact of brass to bullet = 0.500" - 0.357" = 0.143"
And finding the percentage = 0.143/0.500 = 0.286 = 28.6% contact.

This is less than the recalled value of 33% minimum contact surface between bullet to brass. The 33% value would mean my round OAL is
(.33/2) / 0.5 = 0.165
0.165" - 0.143" = 0.022"
0.022" + 0.005" = 0.027" away from the lands
2.949" - 0.027" = 2.922" COAL with Sierra 168gr MatchKing.

Alright, so that seems EXTREMELY long. I already know it won't fit into my rifles internal hinged floorplate magazine. But, is there a reason I should NOT load my next rounds to this length of 2.922"? I need to verify that the bullet to brass contact should be a minimum of 33% of the length of the bullet's bearing surface.

It looks like the AICS magazine max COAL length is 2.880". Since I plan to add a DBM at some point that uses AICS mags, should I load these rounds to 2.875", giving me a total of 0.074" away from the lands and 0.005" shorter than magazine max COAL?

Yes. I am probably overthinking this A TON! However, I want to make sure I don't have any more problems like that other time, even if as some of you said, I was seeing over pressure signs in my 42.0+ charges, even though it isn't the conventional primer cratering as I understand it.

[C Ward]

You want full bullet dia in the whole neck of the case . If not you end up with neck tension and run out issues .

The longest you want to go is where the boat tail ends or starts , depending on your viewpoint, even with the neck shoulder junction .

Don't worry about getting close to the lands with the factory chamber it will create more issues than benefits .

[BuffCyclist]

You want full bullet dia in the whole neck of the case . If not you end up with neck tension and run out issues .

The longest you want to go is where the boat tail ends or starts , depending on your viewpoint, even with the neck shoulder junction .

Don't worry about getting close to the lands with the factory chamber it will create more issues than benefits .

Is this a hard set rule or just a rule of thumb? I've seen this in a few places, but no one ever has a source, its just a "what I was told" type of thing.

Not sure if you read the rest of this thread or not, but I had serious problems at 2.800" length with these reloads, and am trying to eliminate possible sources of high pressure. I do not want to have my bolt lock up again and do not understand the precise source of the high pressure (since by now I think we've come to the conclusion that I did have high pressure).

eta:

Not to mention, the top of the shoulder according to the SAAMI spec drawing above is 1.7118”, add .308 and you’re at 2.0198”. This is beyond SAAMI’s max length, so that rule doesn’t make sense, nor is it even feasible in this situation.

[TheBelly]

If you can't move the bullet in the case with your bare hands, then there should be enough tension.

ive also heard of the 'one diameter' rule. If you load correctly, run out isn't that difficult to compensate/correct.

[<MADDOG>]

OP: have you tried factory loads? (If I missed it in my reading, my apologies in advance)

I hate to sound simplistic; but you guys are getting into brain surgery when it may be a mole.

[BuffCyclist]

OP: have you tried factory loads? (If I missed it in my reading, my apologies in advance)

I hate to sound simplistic; but you guys are getting into brain surgery when it may be a mole.

Thanks for the reply. Yes, I shot 20rds of Federal Gold Medal Match (168gr) to zero the scope, and then another 20rds of Black Hills Armory 175gr HPBT. They all shot perfectly. But then again, so did the first 12 of my reloads (1 shot each at charges from 38.0-42.4gr IMR4064, then 42.8gr locked the bolt, the primer fell out badly, shot the 2nd 38.0gr charge and same thing happened but no primer blow out).

And no, I don't think this is brain surgery. That would be me sending the rifle off to a gunsmith to just blueprint the entire brand new rifle. I haven't done anything else with it because I have some colleagues in town from Chile and have been attached to them about 12-14hrs per day the past 2 weeks. I am also waiting on new (Lapua) brass to arrive, which I will test the reloads with that again, so in the meantime, I am measuring everything possible and thinking of every single possible cause of the high pressure on the 42.8gr load and then the 38.0gr. My gut feeling is still telling me it was the brass.

[C Ward]

Yes I've read the whole thread , Hoser all ready said this once but since it seems to have fallen on deaf ears , the pressure problem is not from OAL it is from too much powder . For the OAL to cause a pressure issue in a rifle the bullet would need to be jammed into the lands . The peak pressure with a rifle round comes when the pressure spike happens when the bullet hits the lands and it builds until it over comes the resistance and pushes it down the bore . The closer to the lands the bullet is the sooner and faster this happens , increasing peak pressure

OAL is used to tune for accuracy not pressure . The chamber on that gun will not let you get 5 - 10 thou from the lands with ammo that will be reliable so it makes no since to compromise the reliability of the ammo to load it long . The Match King bullets are very jump tolerant and distance from the lands is a non issue .

99 times out of 100 too much pressure equals too much powder , back the powder charge down . Brass is simply a container for the combustion process and the only affect it has on the pressure is the space available inside for it to happen . Also as of late Lapua brass has had less case capacity then the newer LC brass so the powder charge will need backed down even more .

[BuffCyclist]

Yes I've read the whole thread , Hoser all ready said this once but since it seems to have fallen on deaf ears , the pressure problem is not from OAL it is from too much powder . For the OAL to cause a pressure issue in a rifle the bullet would need to be jammed into the lands . The peak pressure with a rifle round comes when the pressure spike happens when the bullet hits the lands and it builds until it over comes the resistance and pushes it down the bore . The closer to the lands the bullet is the sooner and faster this happens , increasing peak pressure

OAL is used to tune for accuracy not pressure . The chamber on that gun will not let you get 5 - 10 thou from the lands with ammo that will be reliable so it makes no since to compromise the reliability of the ammo to load it long . The Match King bullets are very jump tolerant and distance from the lands is a non issue .

There are no deaf ears, and no need for sarcasm or getting aggressive. I am taking all input I can receive and examining every possible source.

Precisely, the OAL comes into play when at the lands. So, jam the bullet to the lands or get extremely close, work up my loads THERE and then when I seat deeper I will not have pressure issues due to jamming.

99 times out of 100 too much pressure equals too much powder , back the powder charge down . Brass is simply a container for the combustion process and the only affect it has on the pressure is the space available inside for it to happen . Also as of late Lapua brass has had less case capacity then the newer LC brass so the powder charge will need backed down even more .

If you firmly believe the over pressure signs I saw were from too much powder, then please, explain to me how the primer blew out on the 38.0gr charge when I can guarantee it had 38.0 +/-0.02gr of IMR-4064. That is 4.8gr LOWER than the charge where I had the primer blow out on as well, including the ejector marks on the case head and totally locked up bolt (bolt wasn't even sticky at 42.4gr and I know sticky bolts, my 22LR bolt rifle is difficult to cycle, but my Rem7005R can be operated with my pinky, which I do while shooting because its easy and doesn't disturb my sight picture as much). The 38.0gr charge primer blew out and will slip in/out of the case now. That case does NOT have the ejector imprint.

Yes, I admit that 42.8gr might be too much powder. But I can not explain logically or scientifically why the 38.0gr charge blew out the primer either. If all I had was the primer blow out on the 42.8gr load, then I would not be as hesitant or careful with altering my loads and continuing forward. As it is now, I would be lying if I said I wasn't somewhat hesitant to pull the trigger again on that rifle, even at the 38.0gr charge. Again, I am leaning towards it being a brass issue - loose primer pockets - or a primer issue - slightly undersized primer cup. Until I can eliminate these two variables, I can not rule out any other variable and while I wait for my brass to ship, I am researching in every free second I have.

I also haven't found anywhere that has said explicitly with evidence that loading 308Win to 2.944" (0.005" off my lands) will cause problems. Nearly everyone says to load to magazine length and be done with it. But precision shooters say to start at the lands and back off when WORKING UP POWDER LOADS (granted, working up powder loads IS done for accuracy, ... ). Yes, this rifle is NOT a GAP, Surgeon or other precision built rifle and has a very long chamber. But concentricity is easy to measure/compensate, as Belly said and confirmed by my research, so seating the bullet out to 2.944" with only 28.6% bullet bearing surface contact with the brass should not cause problems as long as it is concentric, or within 0.002" (as much as 0.003" out of concentric will cause accuracy issues, NOT over pressure issues).

eta: If I feel the need to model up my rifle action and perform a fluid dynamical FEA analysis on it, I will do so until I am satisfied. It simply means I wouldn't be able to shoot my rifle again until next year due to my limited time to work on this.

[20X11]

Flattened, loose primers also happen from excessive headspace (improperly adjusted dies setting the shoulder back too far). I read the whole post but haven't seen this mentioned or verified.