Something strange, can anyone explain?

L

Larry Elliott

Guest
The other day I was out at the range working up loads for my 6 mm BR and had something strange happen. The rifle's got a 25 7/16" (don't ask it's a long story) PacNor "polygonal" barrel, on an old Ruger 77 action that's been a .22-250, a .243 twice, and now a 6 BR. I'd just fired some loads with 35.0 gr of AA 2520, CCI 450's, and 55 gr Nosler Ballistic Tips in Lapua cases with no problems, so cleaned the barrel and went to some loads with 8208 from 32.0 to 33.5 gr with the same components except for WSR primers and had no problems. I cleaned the barrel again then went to some loads with Benchmark and but otherwise the same as before. The first round with 32.0 gr of Benchmark was fine with normal bolt lift, and normal looking primer. The next two rounds gave very stiff bolt lift with slightly higher velocities than the first round, but again normal looking primers. Since this load is only a grain above what Hodgdon lists as a starting load and below what I'd tried with 58 gr Hornady V-Max's so I was a little baffled.

Being baffled never stops me though so I tried the next charge increment, 32.5 gr and had stiff bolt lift with the first two rounds, with the third round giving normal bolt lift with an average MV of 3550 fps for the three rounds. Okay now that's strange to my feeble mind, but things got stranger. None of the primers looked hammered, and there was no cratering at all. WSR's tend to crater without too much persuasion in my experience anyway. I fired the 33.0 gr and 33.5 gr loads next with normal bolt lift on all six rounds fired, primers that looked about like those on the lighter charges, and only slight cratering on one primer with the 33.5 gr charge. Incremental velocity increases were fairly even from 32.0 to 33.5 gr.

Temps were in the low 80's and the bench was in the shade under a roof.

Does anyone have a reasonable explanation of why lower velocity/pressure loads in cases that had all been sized the same (FL sized in a Redding bushing die to give ~0.002" shoulder bump) and all chambered with no unusual effort produce stiff bolt lift while higher velocity/pressure loads don't? The necks were all turned and sized 0.263, and the chamber was cut for a no turn Lapua case, so tight necks weren't the problem either.

I'm stumped.
 
Larry,

Have any unexplained pressures on previous range visits with that Poly barrel...?

I seen some strange things from a couple Polygonal barrels, .224 and a 6mm... Touchy / tweeky to load for..
I perfer traditional rifled barrels myself.

cale
 
This is interesting. A friend is seeing the same thing with a .20 Vartag Turbo and we haven't 'cured' it yet. The Lapua brass didn't fire form well on the first firing. The case necks were turned, but you couldn't slip a bulllet in a fired case neck. The brass has been re-worked now, but we haven't had a chance to shoot it again. We will also weigh instead of dump the charges. - nhk
 
There was another thread where someone reported something similar, namely, everything fine with a 6BR until Benckmark powder (published load) was tried.

http://benchrest.com/showthread.php?75409-Pressure-Need-Answer

The responders went all over the map trying to diagnose what was wrong with Roland's rifle, just like they're doing here. My point was that since the only time a problem shows up is when you use a published Benchmark load, just don't do that.

FWIW
 
Thanks for the replies gentlemen. I'll check the case length, but believe it's okay - seeing and believing are completely different things though. I've experienced this once before with a .223 Rem that showed pressure signs with moderate loads and nothing with some loads that were pretty hot except a little primer cratering.

Strangely the hotter published loads with Benchmark were fine, although Hodgdon's max of 33.5 gr is listed as compressed in RP cases although it's nowhere near compressed in Lapua cases. I'm not seating as deeply as Hodgdon shows either.

PacNor's "polygonal" barrels aren't truly polygonal more like normal rifling with a 5R profile if I understand things correctly and my eyes are working. I haven't had any problems since a bad chambering job on this barrel was cured by cutting off the old chamber and rechambering (different smith).

I'll report back when I get the cases measured and another trip to the range made.
 
Larry Elliott said:
The necks were all turned and sized 0.263, and the chamber was cut for a no-turn Lapua case, so tight necks weren't the problem either.

I'm stumped.
Click to expand...

This is not an answer to your pressure concern but why did you turn the necks to 263 for a Lapua no-turn chamber?? Isn't a Lapua no-turn for a 6BR about 272/273?
 
I've had this happen with a PPC. But, since it didn't group at the lower charge, I kinda dismissed the whole thing. Once I got up to where it shot OK, the bolt was fine. Maybe I shoulda given it more attention.....
 
JerrySharrett said:
This is not an answer to your pressure concern but why did you turn the necks to 263 for a Lapua no-turn chamber?? Isn't a Lapua no-turn for a 6BR about 272/273?
Click to expand...

The previous chamber needed a 263 sized neck, and it offended my delicate sensibilities to dump 300 Lapua cases with thinner necks. With my eyesight the way it is it doesn't make much if any difference in accuracy, just shorter case life likely.
 
OK- Some powder, bullet, case, and chamber/barrel setups will give these
kind of problems due to airspace in the case with lighter loads.
As soon as loads were heavier and airspace less or none, everything ok.
Airspace with some combos can lead to small SEE incidents.
Secondary Explosive Effects. Can also be major problem if loads
to small with wrong powders. Some of these incidents have been seen
on pressure traces, with a second pressure spike, but narrower than
the main peak pressure curve at the beginning of ignition.


In all of my big bore 12ga FH and my wildcat work I absolutely
do not load with airspace, due to huge amounts of powder used.
Might be a bomb if it happened in our big cases....ED
 
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Larry Elliott said:
The previous chamber needed a 263 sized neck, and it offended my delicate sensibilities to dump 300 Lapua cases with thinner necks. With my eyesight the way it is it doesn't make much if any difference in accuracy, just shorter case life likely.
Click to expand...

Thats about what the commercial maximum chamber/minimum cartridge gap is anyway.

It will be interesting to see if better-than-commercial (Lapua) brass lasts longer with this much movement. Sizing just the part of the neck with a bushing die will still probably help accuracy..
 
Jerry, if I can get half inch groups from a live varmint rifle with the way my eyes are I'm happier than a fox in a chicken coop. I've tried just sizing part of a neck for accuracy, but found that the necks crack right where the little shoulder from the sizing on the neck is. I've got to go measure the length of those cases.
 
I checked the case lengths on all 50 fired cases in the box and they were almost all just slightly over 1.560", so they're now all trimmed. That makes me feel a little better, but even after FL sizing they were only maybe 0.005" long which isn't enough to cause a problem. All the "problem" rounds chambered easily too.
 
Larry,

This perplexed me for some time because this was a fairly normal occurance with V133 in a PPC. What a lot of people will find is when they got above 29.0 grns or so the bolt lift would be tighter and then around 29.5-6, the bolt lift would go back to normal. How this was explained to me, was there are two things that can occur that causes this. The first one is mentioned above in hubel458 post. The second seems to be the most common and occurs with cases with less body taper.

There is a pressure range where the case will smack against the boltface harder because the pressure is just not quite there to slam the brass case against the chamber wall enough to grip it (so to speak) as it does with a slightly higher pressure. I don't know if I explained that well enough but it is fairly repeatable. It is really noticeable is polished chambers that have less wall grip during firing.

Hovis
 
Check for a "carbon ring" buildup in the forward neck area. You can find it easily by buffing a loaded bullet with your favorite "neck wiper", chamber the round and look for burnishing on the bullet that doesn't resemble land marks.
 
Wilbur I don't think I've fired enough rounds with the new chamber to have built up much of a carbon ring, but I'll give it a check anyway.

I tend to think that Hovis is closer to the truth, whatever it is, and is in line with a theory I have on the cause. The primers on all the rounds fired were equally flat, and it occurred to me that the primer was being blown back before the case had expanded enough to stick to the case walls then the case finally expands and is pushed back against the primer flattening it. The primer is then jammed back all the way into the pocket, but increases friction enough to give sticky bolt lift.
 
Larry Elliott said:
I tend to think that Hovis is closer to the truth, whatever it is, and is in line with a theory I have on the cause. The primers on all the rounds fired were equally flat, and it occurred to me that the primer was being blown back before the case had expanded enough to stick to the case walls then the case finally expands and is pushed back against the primer flattening it. The primer is then jammed back all the way into the pocket, but increases friction enough to give sticky bolt lift.
Click to expand...
This make little sense to me. In fact, the whole "29-grain is a problem" notion makes no sense to me, as back in the late 1990s, I shot using around 28.3-grains of 133. What some call the lower window. No issues.

If your dies are set up correctly, head clearance (headspace) is .001 to .002. The primer isn't going to back out, at least, not more than .001 to .002 -- the total clearance. I've seen primers that baked out & were then "reseated" by firing, when firing factory brass without making a false shoulder to get a crush fit when fireforming. Not only is the primer "flat," with a stiff load, it looks a bit like a solder splatter. What you get with an overly tinned soldering iron when you shake off the excess. Even here, after forming & proper sizing, subsequent primers look normal.

So may esoteric bordering on mystical answers. How about something simple like the loading data for the current lot of Benchmark is wrong?
 
Charles,

It has nothing to do with a lite charge. I've shot V133 from 27.8 to 31 grns. The only way I know to describe it is a inbetween. Enough pressure to slam the case back but not enough to grip the wall well. Even with minimal sizing this can happen because it is still sized. It has to do with chamber wall to brass grip. Another occurance of this is with brass that has hardened. You can size it and the bolt closes fine but it seems them inbetween loads will still cause the click on opening and hard bolt life. A lot of people think that it is the web but if it was, the round wouldn't chamber normally. The load causes more bolt thrust because it don't grip.

Anyone who doubt this, I have an experiment you can run. Take an old barrel and shoot some brass in it, working up through the windows. Pay attention and write down how the bolt feels on opening. Then polish the chamber well without changing the deminsions, then load and shoot and write down how the bolt feels. You will notice a range where the bolt will open harder but as you continue up, it will ease up again, then get harder toward the top of the loads.

How many times have we seen some post on here and tell how they are afraid to go past 29.4-29.6 with V133 because they already feel harder bolt lift. It's a combo between brass, chamber walls and load that causes it.

I had this explained to me several years ago when I was discussing with them that I was afraid to go up in powder because of the bolt lift. I don't remember who was all standing there but Skip was the one telling me..."go up".

Hovis
 
Could it be that your cases need to be annealed and are gripping the bullets differently?

gt40
 
HovisKM said:
Charles,

It has nothing to do with a lite charge. I've shot V133 from 27.8 to 31 grns. The only way I know to describe it is a inbetween. Enough pressure to slam the case back but not enough to grip the wall well. Even with minimal sizing this can happen because it is still sized. It has to do with chamber wall to brass grip. . . . Hovis
Click to expand...
Well, I don't agree, but it would be easy to test if someone wanted. Take one of the "no problem" cases and lube it. Now the chamber walls won't grasp it, either. You should be able to recreate the same situation with that lubed case.

The reason I don't buy it -- but admittedly with no test -- is because moving .001 isn't going to "slam" anything. Still, I've already been wrong 5 times today that I know of (each cigarette), so another error is always possible . . .
 
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