A small primer experiment

[Bjornb]

So, after much back and forth texting with Sgt. Mike where we were lamenting the total lack of Small Primer rifle brass (Palma style) available for the longer cartridges (especially 30-06 that could be made into 30XCB cases), we finally hatched a plan wherein I would make up a small batch of "home cooked" SR-primed cases for a test. (It appears that much of the bench rest crowd uses small primer cartridges, so we figured we'd see what happened).

I took a chapter from Larry Gibson's tutorial on how to fit boxer primers to Berdan cases, and basically drilled out a dead primer (anvil removed, crud brushed out) with a #18 drill bit. This created a bushing that allowed for the SR primer to be seated fairly snugly in the pocket. I made up 5 cases like this:
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I then loaded a total of 10 rounds (all in .308W): 5 of the pictured cases and 5 control rounds, unaltered, with large primers.
Primers used were CCI BR2 (large) and BR4 (small) bench rest primers.
Charge was 42 grains of IMR4895, bullet used was SMK 155 Palma, Moly coated. Firearm used was my Ruger GSR scout rifle, with the 16.5" barrel.

The accuracy of the load was as expected (this is not my most accurate rifle), and the experiment showed that my homemade "bushings" leave a lot to be desired:
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Now here is the upshot of this test, and I'm sure there are guys here that can explain these numbers:
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I'd like to have someone explain to me how a change from large to small primer could increase MV by 300 fps.
The powder charges were dispensed in one sitting from my RCBS Chargemaster 1500.
The rifle barrel recently had about 100 rounds of Moly coated bullets through it with only some minimal cleaning (Ed's Red), so it was "conditioned" for Moly, if that means anything (I got a box of 500 Moly SMKs cheap and they are all being shot in this rifle).

[btroj]

How do the bushings work in decapping? Does the bushing remain in the primer pocket or does it get pushed out with the spent primer?

The velocity difference seems way too big to simply be due to primer change. Something else is happening, I just can't see a small primer/large primer change making that much difference. Then again, I could be wrong?

[Bjornb]

The bushings were partially scorched and came out with the spent primers.
I'm at a loss when it comes to the velocity; Sgt. Mike's theory is as good as any to me.

[Motor]

Bjornb,
I have the Lapua LR cases...... now to get a small quanity of the Lapua Palma (SR) and try again to validate what you seen today. I have a theory but:
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sorry that one never gets old ha ha.

Serious side My "thoughts" are that the power level of the large rifle primer is "blowing" the bullet and powder further forward than the Small Rifle primer increasing the volume area with the small rifle primer not launching the bullet and powder further which now has the ability to burn more effeciantly in a smaller volume area. Proof ? nah it's a thought right now.

Now to round up brass ship to you and you do some cast experiments and see what happens.
(or the moly bullets your choice it's valid either way)

the other interesting note is that the SR primer loads was almost equaling a 22" barrel MV from a 16.5" barrel image if you had been shooting a 22" or longer

The theroy high lighted in red can be tested fairly eaisly. Seat all the bullets so there is minamal or no bullet jump.

[btroj]

In a rifle with the bullet close to the throat the bullet isn't gonna move far from the primer blast. This is a serious problem in revolvers where the bullet isn't retarded by anything in the cylinder throats but a rifle is built very different.

Isn't this part of what changing seatig depth does?

Has as anyone ever tried seeing how hard the bullet is wedged into the throat from primer blast alone? It would be tough to quantify but might be interesting to see what happens.

[petroid]

Do you know which chrono reading was from the leaking primer? If so, what is the ES if you remove it? I realize the sample size would be smaller but if it was the lowest number, it would make a difference. More testing, please!

[Bjornb]

Do you know which chrono reading was from the leaking primer? If so, what is the ES if you remove it? I realize the sample size would be smaller but if it was the lowest number, it would make a difference. More testing, please!

It was the first shot, with the second highest MV.
And no, I'm not running another test until I feel secure that I won't get this kind of gas leakage. It's my brand new rifle fer crissake!:mrgreen:

[btroj]

Any visible erosion on the bolt face? That would scare me if it happened frequently enough. I also don't like the idea of high pressure has comng back towards my face.

[fastfire]

In a rifle with the bullet close to the throat the bullet isn't gonna move far from the primer blast. This is a serious problem in revolvers where the bullet isn't retarded by anything in the cylinder throats but a rifle is built very different.

Isn't this part of what changing seatig depth does?

((Has as anyone ever tried seeing how hard the bullet is wedged into the throat from primer blast alone? It would be tough to quantify but might be interesting to see what happens.))

The bullet don't go anywhere, it doesn't leave the case in a .308 Win.
Don't ask me how I know that.

[303Guy]

The bullet don't go anywhere, it doesn't leave the case in a .308 Win.
Don't ask me how I know that.The bullet might not leave the case when there is no powder in said case. Try substituting the powder with something like grits and see if the bullet moves. Neck tension is important with regard the bullet moving. I over annealed some case necks and the bullet gets shoved into the throat, clear of the neck and stays there. Doesn't seem to hurt accuracy any. That's with the identical capacity 303 Brit with 180gr bullets. I seem to recall with an un-annealed neck the bullet stays put.

Comparing large and small rifle primers, what is the difference in power between the BR2 and BR4 primers? I use Federal which are supposed to be the lowest power primers. Perhaps a test using that same load but with two different power LR primers would reveal something?

[petroid]

It was the first shot, with the second highest MV.
And no, I'm not running another test until I feel secure that I won't get this kind of gas leakage. It's my brand new rifle fer crissake!:mrgreen:

I definitely don't want you to damage your rifle or yourself! Just interested to see how this project progresses.

[tomme boy]

Get a new battery for your chronograph.

Had the same thing happen to me shooting slugs one day.

[44man]

I may have done more testing with primers in revolvers and made bushings for the .45 ACP to go to a SP primer with great results. The SP in the ACP revolver made a huge difference so we found SP brass.
It is entirely true, the wrong primer can move a bullet. Much depends on case capacity and it's ability to absorb primer pressure due to volume. It also depends on the powder and charge amount.
Seen it with staring loads of 296 in the .454 with the SR primer where the boolit and powder charge was blown into the bore without enough flame for ignition, solved with a LP mag primer. All loads ignited with a Fed 150 but the 155 was more accurate. Max loads still better.
I have no idea what a SR primer would do in a .308 case but I would think less airspace is better.
I would not trust primer cups for bushings, made mine from brass rod stock for a good fit. The leakage can ruin a bolt face.
To see the BR 4 has more velocity must mean it has more fire and less pressure and the BR 2 is moving bullets.

[Bjornb]

Get a new battery for your chronograph.

Had the same thing happen to me shooting slugs one day.

You can never completely discount this possibility, especially after the fact. However, with the 2 (small) groups of ammo chrono'ing as closely together as they did (for each individual group), and since the Chrony worked fine and gave plausible readings for about another hour, it's not very likely. And the battery was on its 2nd range trip.

[Cap'n Morgan]

303 Guy's suggestion of substituting the powder with some sort of filler makes good sense. The 'larger volume' theory from LR primers is the obvious answer and a test with a powder substitute would most likely confirm it.

[Doc Highwall]

I have a Remington 40X chambered in 7.62 NATO with a strain gage attached that I have fired both the Lapua cases with large rifle primers and Lapua Palma cases with the small primer and I did not notice any large changes like that.

Is there a large difference in case weight between the two 5 case samples that could account for the difference in velocity?

[Bjornb]

I have a Remington 40X chambered in 7.62 NATO with a strain gage attached that I have fired both the Lapua cases with large rifle primers and Lapua Palma cases with the small primer and I did not notice any large changes like that.

Is there a large difference in case weight between the two 5 case samples that could account for the difference in velocity?

They were all regular LC cases, but not individually weighed.

[Doc Highwall]

Weight of the cases as you know affect the internal case volume with heaver cases having less volume then lighter cases. I would be curious as to what they weigh.

[Jackpine]

I am reading with great interest and scratching my head over your results. Was the chrono work done back to back or was there a significant time difference between with the chrono shut off. Or did one set of ammo set in the sun or was anything else not consistent between the two shootings?

Back in the 80's I was into handgun metallic silhouette shooting and acquired on of the first xp100 in 7BR that first came out. (I got it because a friend and fellow silhouette shooter was a firearms distributor and knew I was a load tinkerer and owned a chronograph-not a common tool in those days--and I furnished him with my data.) There was no preformed 7BR brass yet. I remember forming it from full sized 308 small primer cases. They were actually head stamped 22BR. This was before the wonders of internet discussions, so the circle of conversation was much smaller. I shot with folks that formed 7BR brass from conventional 308 cases. I never chronographed any of that ammo but comparing sight adjustments between 50 and 200 meters led us to believe that there was no significant velocity difference of common loadings with only the primer being the difference. The advantage of the small primer was obviously touted as better accuracy with the short, stubby, efficient case and the speculated wisdom was that there would be no advantage with full sized cases. If memory serves correctly, the gun was amazing to me as it shot just about anything that you stuffed in it quite well. Totally stock, it would group the best loads at around 3/4 of an inch and the worst loadings were 1 1/2, at 100 yards.

Here is a link to someone who tried 308 with small and large primers with identical loadings. There are no chrono results, but his comparison was shooting at 1000 yards, with no perceptible difference in elevation, which would certainly show up at that distance.
http://riflemansjournal.blogspot.com/2010/01/cartridges-lapua-small-primer-308.html

I hope you will be able to do more extensive experimentation and share results.

Jackpine

[tomme boy]

Still say the chrono was messed up. Either the or the way the light was hitting the sensors was throwing it off.

Like I said. Mine read about 500 fps higher. Yet half an hour later I tested a load with a proven load and it was right on. Everyone told me to change the battery and I never had another problem. I chalked it up as the battery. But doing a little reading it seems it happens to all chrono's just like it happened to yours and mine sometimes. Figure out a better way to secure them primers and run the test again. I and others would sure like to see what happens. Even if it is just one round over the chrono it will tell you. Just shoot other rounds first to see if it is reading right and then shoot the one round right away.

[Bjornb]

Tom that's exactly what I plan to do. I'll run the test again and report back. The results were too strange.

[dtknowles]

Tom that's exactly what I plan to do. I'll run the test again and report back. The results were too strange.

Locktite those puppies or something.

Tim

[Bjornb]

Good read sarge. And there's German Salazar again.

[Eutectic]

I saw some conversation on another forum talking about Elmer Keith's forward ignition experiments.

We got a box of modified cases from Elmer before he died. I still have them.... They were for his .285 O.K.H. wildcat. O.K. H. were three fellows that worked on this experiment. O'neil, Keith, and Hopkins I believe they were. Either O'Neil or Hopkins was a crackerjack machinist as well. He modified the cases... new '06 for the .285 O.K.H.

The tube appears to be 1/8" brass hi-pressure tubing. Each case is drilled and tapped in the flash hole and the forward ignition tube is screwed into the flash hole from the inside! Intricate little job. The forward ignition tube extends forward about 1 1/16" above the floor of the case, or about mid powder charge. I studied a couple with various lighting and was able to determine there are no radial holes through the sides of the tube. I couldn't get a photo, and I won't sacrifice a case! They are still new even if they are 60 or 70 years old. Most people have never seen these forward ignition cases of Elmer's much less a box of them still unfired.!!

A few pictures are attached and I will ponder how to photo the tube someway...

Eutectic
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[popper]

Think the 105mm cases had the primer tube also. Don't remember the primer bruiisance test results , I do wonder if the results would be the same using dacron? Load is close to full load of powder, SRP should give better burn due to more concentrated & more localized ignition source.

[runfiverun]

Every time I sub out lp for lr primers in a cast load I have to raise the load
if the load is low enough I get velocity variations with a colder primer
your horizontal stringing is telling you the story
its umm ignition related
go head say it

[bstone5]

The primer tube is in a lot of cannon brass.

Many years back ash trays were mad from 105 and 5 inch she'll cases.

I still have 105 ash tray I made while in the Marines.

[MBTcustom]

Has as anyone ever tried seeing how hard the bullet is wedged into the throat from primer blast alone? It would be tough to quantify but might be interesting to see what happens.

I have.
The answer is "not much". It really depends on the internal volume of the case, but in a 30-06, you'll barely get the boolit engraved, unlike a 45ACP which will push the boolit about 4" down the barrel (depending on the primer) or the 44Mag which will lodge the boolit in the throat, like the 357 will do in turn.
With a cartridge like the 308, I was intentionally trying to stick a boot in the barrel just past the throat, (wrote it up in my sandbox test thread) in order to observe what the boolit goes through when it is engraved. (I designed the 30XCB boolit off those tests BTW).
The point is, that the primer alone was not enough to get the rear of the boolit into the barrel. Pretty much just jammed the boolit into the rifling really hard. I had to put in 1-1.5 grains of Bullseye pistol powder to get it totally engraved.

[popper]

As BR & SP 45 have found, SP do give a better controlled ignition/burn. I'll repeat my ? for a non-commpressed load with dacron.