Pressure check question

[crimsoncomet]

Hey guys,
I had some time to go out and shoot my remington 700 LTR in 308 this weekend. Had a blast. It's been way to long since I have been to the bench. This was a load development session. I found a sweet load. I was working up using varget and 150 TTSX that I plan on hunting with. Best load i found was 46.5gr of varget in Lapua brass, with a 210m primer. It was running over the chrony at a 5 shot average of 2742 fps with a SD of 8.54. Not to bad from a 20" tube. You could cover the group with a nickel, minus one flyer i decided to pull. I always seem to do that. I worked all the up to 48gr of varget. I checked all the brass for extracter marks and they looked good. One thing i forgot to do is check for bolt lift. How impotant is this? Should i shoot a factory round to see how that feels and then my load? I know that i will get mixed oppinions. I plan on loading the best two or three loads and heading back to the bench. Anyway, I am very very happy with the load, and did not expect to yield that velocity from the short barrel.

 

[918v]

The bolt ift is the first thing you should be paying attention to. It should feel smooth until you you lift it 3/4 of the way up, then a "gulp". This is the point where you either feel a "gulp" or it gets sticky. I work up my loads to the point of stickyness, then back off, always wanting to have the "gulp". It kinda feels like two steel balls camming against one another as opposed to the sticky feeling which feels like a glass rod breaking.

Another way to get the feel of the "gulp" is by dry firing and lifting the bolt to recock the striker. You know what that feels like?

 

[crimsoncomet]

Thanks for your reply. I will make sure i pay attention when i get back to the range.

 

[rcmodel]

I think hard bolt lift can be over-relied upon sometimes.
Perhaps it is meaningful in a factory gun. Perhaps not.

But a custom rifle with a glass smooth chamber, and lapped bolt lugs, and trued bolt face will blow up before hard bolt-lift is noticable.

I think a much better indication of high pressure is loose primer pockets, and brass extrusion into the ejecter cut or plunger hole in the bolt face.

When you start seeing shiny spots on the case heads matching the ejector cut or hole in the bolt face, it's past time to back off.

rc

 

[MEHavey]

46.5gr Varget under a (Barnes)150 TTSX is a right stiff load for a 60,000psi 308Win according to multiple sources:

- QL says it produces 70,000psi/2,867fps at 70 degrF coming out of 20" barrel when loaded in a 54.8cc Lapua Case. If I do the velocity ratio-cubed estimate, it's 61,500psi. If I adjust the effective powder down to meet the velocity you chrono'd, it still produces 60,500psi. (At this point, lot-lot variation in powder could matter a lot.* So could temperature.)

- It's almost 2gr over Sierra 5th's max.
- It's almost a full grain over Hornady 6th's max

Then again...
- It is just under Hodgdon's max of 47gr. (Ditto Speer's 14th)
- It's 2gr under Nosler 6th's max.

Look for brass brightness at the ejector slot first and foremost, and primer pockets next. Don't even think about changing powder lots without starting over, and watch out for hot days....

(But as Patrick Swayze says (again), "...opinions vary....")


* No pun intended.

 

[crimsoncomet]

Thanks for the replies fellas. I saw no brass extrusion on any of the rounds fired. I thought the speed was a bit high coming from a 20" barrel, but i have seen stranger things. I did see some brass extrusion awhile back, while shooting some 165 gamekings and varget. I have seen it and know what it looks like. I wonder how accurate QL is? Barnes data says start at 43gr and max is 48gr. That doesnt mean every rifle will shoot like that, but in a standard remington factory action it didnt seem to give me any trouble. I will take another look at my brass when i get home, but I already looked at it several times. I know most people say you should lose around 50fps per inch of barrel. But that is an averge. I have heard of guns losing a lot more and not losing that much at all.

 

[crimsoncomet]

One thing i did see, was it looks as if the primer as some flow back around the firing pin. But, this seems to also show up with factory loads as well.

 

[918v]

Thats because the firing pin hole is too big.

 

[crimsoncomet]

Thanks 918v. Here are some pictures of the primers. Everybody has me re-thinking what knowledge i have gained over the past couple years of loading.

Win. factory


46.5gr of varget


The 48 gr load is a bit more distinct than the 46.5gr load. BTW barnes data ranges from 43.5gr to 48gr. I started at 44.5gr and see this on every round. The factory round is definately less distint.

 

[918v]

Mine does that too.

 

[45crittergitter]

All of the "field" methods of checking for safe pressure (brass condition, bolt lift, primers, etc.) are unreliable at best. Certainly some indications are correct, but usually by the time they're noticeable or measurable, you're already too hot. Or maybe not. You can get primer signs and ejector shaving with low pressure loads if you have excessive headspace. Good load data is your first tool. Chronograph results are a good indication. Beyond that you would have to use some type of pressure or strain measuring equipment. DO NOT believe your loads are safe based solely on brass/primer appearance and bolt lift/extraction.

 

[918v]

Speak for yourself. My bolt lift feel method is far more reliable than any load data developed for some test barrel.

 

[brickeyee]

The 48 gr load is a bit more distinct than the 46.5gr load. BTW barnes data ranges from 43.5gr to 48gr. I started at 44.5gr and see this on every round. The factory round is definately less distint.

You have a slightly oversize firing pin hole.

Even less than high pressure loads can show some cratering where the primer is unsupported from the larger than required hole.

Notice that the curve at the edge of the primer has not been flattened and appears smooth.

 

[rfwobbly]

Notice that the curve at the edge of the primer has not been flattened and appears smooth.

This is one of Walkalong's photos that's so good I had to lift it. The primer on the far right is what Brickeyee is talking about.

 

[brickeyee]

Excellent picture.

The problem with 'reading' primers, or even case head expansion is that there are still uncontrolled variables that affect the result.
Case head expansion depends on brass hardness, but every time a case is fired and then sized the working hardens the brass some more.

On a relative scale pressure is high if primers are flat.

Pressure is high is you have more than a few tens of thousandth of case head expansion.

Neither reliably says HOW high.

Soft primers can flatten at acceptable pressure.

Even using strain gauges and piezoelectric gear calibration is a problem.

Commercial labs often have 'standard loads' that they use to try and reduce error, but it is still NOT a high accuracy process.

That is why even SAAMI uses things like 'peak average' pressure over N shots in a 'standard' chamber, with limits on how much variation is allowed between the individual shots.

 

[Clark]

My Quickload program and my jug of Varget are in agreement on velocity with no finagling.

Loose primers at 77kpsi and no lose primer at 72kpsi.

But I am a grain and a half off on your velocities.
Are you at 2.8" OAL and not touching the lands?

 

[ranger335v]

Extruded head brass into the ejector hole/slot is a certain sign of excess pressure on any one firing; STOP and back the charge off!

'Sticky' extraction comes from a stretched chamber and case, in which the steel chamber springs back more than the case so the case actually gets 'grabbed' by the chamber. BACK OFF!

Swollen, or blown/leaky, primer pockets can come from a single firing; BACK WAY OFF! Loads that develop loose primer pockets as they are reused are also too hot, back off.

From what I read on the web, primers are about the only thing a lot of "modern" reloaders pay any attention to; that's a serious mistake! "Flat" primers usually come from a loose chamber fit (excess "headspace"); adjust the FL sizer to just bump the case shoulder and most primer flattening will go away. Cratered primers are, as described above, usually due to an oversized pin hole OR a weak firing pin spring. Pierced primers usually occur due to a too sharp or a too long firing pin. A blanked primer can also occur when the pin extends too far (about 60 thou of protrusion is more than sufficent). Primers with defective cups (too thin, or with soft spots) may blow out of pin holes on the shoulder radius and pit the bolt face. Of course high pressure will contribute to any of these primer problems but they are not, of themselves, certain indicators of excess pressure.

 

[amlevin]

46.5gr Varget under a (Barnes)150 TTSX is a right stiff load for a 60,000psi 308Win according to multiple sources:

It may be stiff for 60,000PSI but SAAMI specs for .308 Winchester is 62,000 PSI.

The primers still show a radius at their edge even though there is some flow in the firing pin hole. The hole in the bolt is too large.

I would think your approaching the max however and when you start seeing speeds of 2800, you're there.

 

[denton]

Conventional pressure signs don't usually kick in until about 70,000 PSI. According to Ackley, primers start falling out at around 80,0000 PSI. So, if you're not seeing sticky bolt lift, brass extrusion, etc., you are probably under 70,000. That's a far cry from what SAAMI thinks is safe, but it's your gun, and your eyes and fingers.

For a long time, I thought you could estimate pressure from primers. I no longer think that is true. I did a test where I shot progressively hotter loads, then popped out the primers. Under a microscope, I arranged the primers from roundest to flattest. Then I checked that against the charge. I could find no correlation.

 

[45crittergitter]

Take your primers, bolts, brass, etc. and compare them to the pressure readings derived from your strain gauge correctly mounted and calibrated on your rifle and see how close your indicators are. Good luck with that.

 

[918v]

So what's your solution?

 

[Clark]

2, 2011,
918v
Speak for yourself. My bolt lift feel method is far more reliable than any load data developed for some test barrel.

This is the next level for me in internet style. Posts like Karate chops.