Primers vs pressures , how much of a difference

M

Metal god

New member
I had some 308 LC-12-LR cases that had been sitting around for a while . I was saving them for a new load that may come along . Last year I found a pretty good deal on 168gr Hornady BTHP bullets . I thought what the hay this is a good time to pull out the LC-12-LR brass . I had also just bought some Federal GMM primers .

I go ahead and work up a load doing the OCW method . I find I get great results doing so with the final load being

168gr Hornady BTHP
43gr IMR-4895
LC-12-LR , H2o = 55.2gr average
Coal 2.850
Max possible COAL 2.855

That load is really good but then I thought I should try to run a new load development . This time I'll use standard LC-14 brass and Win LR primers . I have many more of each . So I go ahead and work up a load from scratch with the new brass and primers but still use the same bullet and powder .

On this load I did a ladder test as apposed to the OCW method . Now doing the ladder test I was getting sticky bolts at 42.2gr and real brass flow at 43.0gr . Big time pressure signs 1.5gr less then I tested to with the other load . I tested to 43.4gr with no issues in the OCW test but could not get past 41.8gr with the new primers and brass with out sticky bolt lifts .

At that point I found that interesting but did not feel it to be odd . I just figured the brass volume must be lower along with the primers putting out much more flash/energy .

How ever last night I decided to check the volume difference of both cases and found there to be very little difference with the LC-14 cases having slightly more volume ( LR = 55.2gr the LC-14 = 55.4gr ). I don't know but can't imagine that .2gr being a big factor which leads me to conclude the Win LR primer alone is the reason for the extra pressure .

I'm just having a hard time wrapping my head around the idea that just changing the primers caused such a big difference as far as pressure signs showing up .

FWIW the loads were the same , OK not exact . The LC-14 cases had the shoulders bumped back .001 more the the LR cases but other then that they were the same . Same COAL , Bushing , flash holes de-burred , case pep as a whole was the same .

Is it really possible the primer alone is pushing me past max pressures at well below all manuals max loads ? Or am I over looking something ?
 
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You no doubt have the Lyman 49th or 50th. In either check the table on page 57 showing pressure difference between five primers used with identical .308 loads. The Winchester primer produced the next to highest pressure, only exceeded by the CCI 34. The primer could account for your pressure increase.
 
The current military primer mix is the FA 956 mix,

PATR 2700 Encyclopedia of Explosives Vol 8 gives the composition

FA 956

Lead Styphanate 37.7 +/- 5%
Tetracene 4.0 +/- 1%
Barium Nitrate 32.0 +/- 5%
Antimony Sulfide 15.0 +/- 2%
Aluminum Powder 7.0 +/- 1%
PETN 5.0 +/- 1%
Gum Arabic 0.2%

There are plenty of primer compositions for there are many applications for primers other than small arms. This is a list of military priming mixtures, FA 70 is the old corrosive primer, I was able to identify PA101 as a fuse primer composition. All of the compositions to the right of FA90 are more sensitive than rifle primer compositions, so these are probably used on a variety of explosive or propellant devices.




So given that primers are a mix of materials, all of which vary by percentage in the mix, and the purity of the starting chemicals also varies, and that primer mixes are proprietary to the manufacturer, it is a wonder that switching primer brands does not cause more problems.
 
I've always read that Winchester LR primers were among the hottest, approaching magnum primers. I primarily use them, but have also used CCI and have only noted a very slight increase in speed. Less than 10fps, and no noticeable difference in pressure.

You are well under max for IMR4895, right on the edge for H4895. In my experience a load right at max might be effected enough by different primers to push it over the edge. But you say you're getting pressure signs at only .4 gr over a starting load.

Without chronograph numbers I don't trust any traditional pressure signs. I think the primers MIGHT be PART of the issue, but you have something else going on here. Any guess is just a stab in the dark without actual muzzle velocity.
 
My notes say the 43gr LC-LR load has an average velocity of 2717fps with slight sticky bolt at top . I remember that now and changed the COAL from 2.855 to 2.850 to back it off the lands a tad . 2.855 is right up touching the lands . I went back to the range to test the new COAL but only got 10 rounds down range before my scope died . No chrono data was recorded at that time .

The LC-14 41.6gr load I don't have an average velocity because the scope died before I could get to it . I do have the ladder test numbers though . They were very erratic and after testing I concluded that was do to the bullets just touching the lands . I had similar results as far as erratic velocities with the other load as well with that same 2.855 COAL

40.6gr = 2549
41.0gr = 2613
41.4gr = 2497 :confused:
Those three shots hit the 300 yard target with very little difference in vertical stinging . There was a total of 1/2" between the three . At 300yds that's not to bad .

41.8gr = 2609
42.2gr = 2734
42.6gr = 2717 this is where the sticky bolt got much more pronounced and I started to see extractor marks on the case heads
43.0gr = 2597 :confused: but still had sticky bolt
43.4gr = 2682 :confused: big time sticky bolt
43.8gr = 2779 had to really force the bolt open

The last three charges had clearly defined extractor and ejector marks on the case heads .

I wish I had better data for you guys on this load . All of those velocities are only one shot each from the ladder test . I got similar erratic numbers with the other load at the same 2.855 COAL during load development . I tested COAL's from 2.815 to 2.855 . only the 2.855 had erratic velocities in both test . During the seating depth test of the OCW method 2.845 and 2.855 both shot real well but the 2.855 had those velocity issues so I went to 2.850 to see if splitting the difference would help . unfortunately my scope broke before I could complete the final testing . I did send it back and got a new one free ( got to love Vortex )

Either way I've shot the 43gr charge with the Fed primers with out issue enough to know the load with the Winchester primers is definitely different .

I will add that I'm glad I keep pretty good notes because my memory told me there was a 2gr difference in the loads . How ever my notes have it just over 1gr really being the difference . I corrected those numbers in the OP .
 
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Sounds like you have multiple variables in play. I've seen data for 223 Remington showing the equivalent difference to that of about 5% increase in powder charge in terms of pressure (about 11% peak increase). German Salazar's Rifleman's Journal had some large primer pressure and velocity difference data, but he's closed that off to any who are not invited, so we can't link to it any longer to let you see the numbers.

Another board member said in 2012 ATK switched Lake City over to commercial type conformal pressure test equipment to improve compatibility with commercial ammunition. I know Federal makes the Mk.316 m.0 brass, but don't know about the M118 LR brass. That may have been old Lake City, while the new brass may have been post-modernization LC. I would take an automatic centerpunch and see if it indents any more easily and deeply than the 2012 brass. It could conceivably be softer.

With kind permission from Jim Ristow at RSI, the plot reproduced below showed that moving the bullet back from the lands by 0.030" dropped pressure 20%, though it's not your chambering.

RSI6PPCthroatjam2_zps7abe8a9a.gif


This older data is, at least, 30 cal. It has less drop (just 10%) and it takes moving a quarter inch back from the lands to get that. The reason is that it's a round nose bullet, so the sides taper very slowly and open the gas bypass annulus with the throat much more gradually with seating depth than happens with a pointed nose bullet. It's the gas bypass change that causes the pressure change.

seatingdepthvpressure_zps326eb859.gif


What's important about that second image to your situation is the shape of the curve. Note that the first five thousandths would not do much to pressure on either curve. The difference would still be inside normal shot-to-shot variation. I've measured that much variation in bullet ogive-to-case-shoulder distances on loaded rimless bottleneck ammo before, which means the actual bullet position in the throat portion of the chamber is varying that much. So I consider it to be inside the noise of bullet-to-bullet variation. Some measurement of that variation is eye opening. Even on Sierra Match bullets I've measured an extreme spread in bullet base-to-ogive contact point of 0.008".

In the 1995 Precision Shooting Reloading Guide, the late Dan Hackett described a 40X in 220 Swift that he could not get shooting under about half an inch (5-shot groups) and that typically was more like three quarters of an inch. Then one day when he was loading for it he switched bullets to a Nosler 50 grain bullet and in so doing adjusted his micrometer seating die to compensate for its base-to-ogive length being shorter than the previous bullet he was loading. The problem was, he turned the micrometer thimble the wrong way and ended up with the bullets way back at 0.050" off the lands. He had 20 rounds loaded before he noticed the error. He considered pulling them and reseating, but decided just to shoot the 20 up as practice rounds. To his amazement, the long jump bullets produced two quarter inch groups and two true bugholes in the ones.

Bottom line here is that each gun's preferred bullet jump has to be tested. I wouldn't assumed jammed or a few thousandths off the lands will be best. It could be a good bit further back, just depending what your gun likes.
 
Thanks condor for pointing out pg 57 in the Lyman book . It does show for what ever load they used a 1600psi difference in the two primers I'm using . That having the Win LR primers creating more pressure then the Fed match primers . Now that we know the WLR primers are hotter . can we assume they are the lone cause of the higher pressures .

I guess if the 43gr charge with Fed primers are right on the edge of max pressure . I can see the WLR primers pushing it way over the top . I do have the bullets pretty close to the lands so there is no real blow by as far as gasses escaping around the bullet before hitting the lands . This is generally going to spike the pressure anyways right ?

I just had another thought . If the ignition of the WLR primers push the bullet into the lands before the powder can really get going . Maybe that's the issue . If that is happening that cartridge/load would act more like the bullet started out jammed into the lands rather then being .005 off .

hmm , Interesting :cool:
 
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Again, look at the second plot. Contrary to popular mythology, making contact with the lands does not cause a sudden big jump in pressure. It's a steady change as you back away from that contact point. Even if the change is more rapid than for the round nose bullet that data came from, it will still be the case a change as small as 0.005" is unlikely get you outside the span of normal shot-to-shot variation in peak pressure. Make bigger steps if you want to see clear change.
 
Thanks UN . I did do a fare amount of seating depth test and found the closer I got with this bullet the better they grouped

These are my 300yd targets after I found that 43gr was my OCW using the LR brass
fpaxM7.jpg


As you can see I moved closer to the lands in .010 increments resulting in the groups consistently getting smaller as I went . That last bottom right target is a 2" group or 2/3 moa at 300yds

This target was shot at a later date with the bullet right at the lands . 300yds and again 1/2 moa

JXcHP9.jpg


I've tested that load from 2.815 to 2.855 . With 2.855 by fare being the best .

Now what I may have done was incorrectly assumed the new load would also prefer to be close to the lands . That ladder test was done at 2.845 or .010 off the lands . As you can see the other load of 43gr has no problem shooting well at that same COAL with no pressure issues .

Contrary to popular mythology, making contact with the lands does not cause a sudden big jump in pressure. It's a steady change as you back away from that contact point.
Click to expand...

My issue is not with the seating depth as far as I know . I will how ever go ahead and load some up at 2.805 and 2.795 and see how those do . I really don't want to go to much below book COAL .

My issue is , why is the WLR primer rounds showing pressure signs at 42.2gr but the Fed primer loads don't show pressure signs at 43gr with all other things being equal except the primers .
 
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Here is the seating depth test target for the WLR primer load after I concluded 41gr was the best charge after the ladder test .

As you can see the groups shrunk as I get closer to the lands . Center target is 2.825 , bottom left is 2.855 . All are sub moa
yVU3Ti.jpg


I think the fact this load is 41gr and the other is 43gr is where I came up with the loads being two grains apart pressure wise .
 
Perhaps I misunderstood. I thought you had also changed the case lot at the same time as the primer. My thought, given the changes made at Lake City in this time frame, is that you might also have softer brass, which would stick at a lower pressure.

There are two consequences to changing seating depth. One is that the peak pressure and barrel time change. If, for example, you seat the bullet significantly deeper and then re-fire the Audette ladder you can find your load is no longer quite in the middle of the flattest spot of the ladder's vertical POI curve. This means, with a different seating depth you may need to retune the charge weight. For that reason, a better seating depth may remain disguised until you've rerun your OCW round robin again.

One way of dealing with that is with the help of a chronograph. As you change seating depth, watch for velocity change. Adjust the charge weight to get the same velocity you had before the change and that will be close to the same barrel time. It won't be exact because a circumstance in which a greater charge produces the same velocity in the same gun will have a slightly lower peak pressure and slightly higher muzzle pressure. That increases barrel time. A velocity match just means the average pressure behind the bullet was the same, but whether that average comes from lower peak and higher muzzle pressure (longer barrel time) or a higher peak and lower muzzle pressure (shorter barrel time) doesn't change the final velocity.

This all assumes, as the Audette theory does and as Chris Long's OBT theory does, that matching barrel time is critical. However, Varmint Al's FEA analysis suggests it isn't necessarily that simple and that much of the muzzle deflection that moves the bullet up an Audette ladder is actually due to the effects of steel deformation under pressure and the magnitude of the recoil moment. Nonetheless, I would take it under advisement that a change in seating depth may be moving your Audette tuning around. If the load is a true OCW load it will move a lot less than a ticklish load does. Nonetheless, if you are seeking maximum accuracy, refiring the Auddette ladder after the adjustment may reveal a charge tweak is needed.

I should add that you may have found your best seating depth in that gun already. I'm just wanted to be sure you don't miss the possibility another good one exists. A lot of guns seem to have two seating depth sweet spots, and occasionally you can find three.

Also, it's important to note you and the gun are already shooting very well and that squeezing out further improvement may take a lot more additional effort than getting it this far did. To me it's just fun to try, but there are certainly no flies on how it's shooting now.
 
Good info to think about there UN . I had similar thoughts about the pressure changing as seating depth changed while doing the OCW test . Dan never talks about that so I stuck to his directions . The 43gr load in theory should have an .8gr cushion to work with +/- .4gr . The idea of maybe dropping my charge a tad to compensate for the extra pressure I get from moving the bullet closer to the lands is interesting . I'll try that as well as trying some shorter COAL's .

While I was running the tests I had two different threads going on two different forums . I was posting the results after each range trip . Neither were TFL :o . I had many guys telling me to try several different things that were outside the directions Dan puts forth on his web page . I tried to stay true to the original method as much as I could . I did how ever change the original start distance to 200yds rather then 100yards . It just seemed like it would be easier to see the definition of the groups better at 200yds .

I did change the brass as well and read what you posted about that , sorry forgot to address it in my post above . That again is an interesting thought and one I had not considered . Do you have a home brew test I can do to check there hardness ?
 
I did change the brass as well
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Is there a weight difference between the 2 brass? The chamber has a volume, running from bolt face to where bullet seals the bore, put more mass/matter in it, and pressure goes up.
 
No the H2o volume is pretty much the same only .2gr difference . The actual brass weight was around the same as well 185gr +/- a few tenths .
 
Metal:
If we are still on the issue of the pressure increase, have you looked at the table on page 57 of the Lyman manual showing higher pressures created by the Winchester primer with .308 loads? That could provide at least a partial answer.
 
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Condor

I did look at that and posted a response in post # 7 . The primers I'm using are in that data and show a 1600psi difference with the load they used . It was helpful in so much as knowing that they at least have some impact on the issue .

UN I went back and looked at the second pressure chart and although I understand the whole of the chart . I don't understand how it applies here . It seems to show pressures as you move the bullet off the lands but it is already at or over max pressures throughout the chart . It also seems to show HUGE jumps rather then the .030 to .040 I'm working with . Are you saying there is very little pressure difference in the bullet being at the lands or .040 off the lands . I've always believe there would be a big difference in that example .

Again, look at the second plot. Contrary to popular mythology, making contact with the lands does not cause a sudden big jump in pressure. It's a steady change as you back away from that contact point. Even if the change is more rapid than for the round nose bullet that data came from, it will still be the case a change as small as 0.005" is unlikely get you outside the span of normal shot-to-shot variation in peak pressure. Make bigger steps if you want to see clear change.
Click to expand...

I get the .005 difference not being much . My thinking is the Win primers are acting like a mag primer and dislodging the bullet before the powder really gets going . Remember I don't crimp and use bushing dies so my bullet hold is lighter then a standard load might be . My thinking is it's doing it at all seating depths and jamming the bullet into the lands faster then the Fed match primers . So on the whole it does not matter what the seating depth is with the more powerful primers . The bullet is being launched into the lands quicker regardless of seating depth .
 
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Oh, I must have overlooked your post 7. Interesting as to how much difference primers can make. Never considered that. I always use the Winchesters along with the CCI 34s and standard CCI 200 large rifle. Where do you do your shooting in the San Diego area? San Diego along with Phoenix are second homes. Otherwise up the coast a ways at Ventura.
 
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Most of my shooting is at South bay rod & gun club http://www.sbrgc.org/ . Pala just up north a ways is good but very limited as to when there is open shooting . There is some BLM land around but it can get a little scary at some spots with other shooters around .
 
MG,

That chart was done in a lab in the 1960's and they weren't working to SAAMI standards. The reason you have to move back 0.25" in the second chart where you only had to move back 0.030" in the first chart to see a big difference is the first chart is a pointed bullet and the second chart is round nose. The round nose has an extremely gradual taper from the bullet shoulder to the start of the radius of the round tip, so it has to be moved back pretty far to open the gap between the bore and the sides of the bullet much. That gap is what the bypass gas flows through between the time the neck expands away from the sides of the bullet and the bullet moves forward to obturate (plug up) the bore. The gas bypassing has the same effect as reducing start pressure.

A pointed bullet, by comparison, opens the gap much more with much less seating depth change because of the greater angle along the sides of the ogive. Assuming you are using a Spitzer nose bullet, you're going to see something closer to the first graph in terms of the seating depth and pressure change relationship. The reason I put the second graph in was just to show that the increase in pressure isn't a sudden jump that occurs when your bullet is seated to contact the lands, as some folks think, and that your 0.005" gap probably won't reduce the pressure much from land contact pressure as compared to an 0.020-0.030" increase in seating depth will.
 
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