unexpectedly difficult case extraction

[TonyH]

Two weeks ago I returned from Canada with a number of unfired rounds in 20Tac. This is my standard load using 23.7gr of N130, Nosler 40grNBT seated around 10 thou off, CCI BR4 primer, Dakota (Lapua) brass twice-fired, mildly neck turned, neck sized using a Redding .229 bushing for a minimally tight enough hold.

In very warm conditions (mostly 30 - 34-ish degrees C) in Ontario, at only a moderate height above sea level (perhaps 1000-1500 feet tops) my cases were extracting OK just as they did at home in a distinctly cooler Devon, even though I suspect the temperature was boosting my chamber pressure a bit.

Last night I went out with the rifle for the first time since, and bagged four bunnies at a modest 255 - 275 yards. Each time, the bolt handle was a bugger to lift! I had to rap the handle sharply upwards... My primers are intact and don't look noticeably more flattened than I'm accustomed to.

I'm a bit concerned at this: I never build excessively hot loads, and this load chronos at comfortably under 4000fps with no signs (previously) of undue pressure. I see no reason why, in the significantly less warm conditions of Devon, my standard load should suddenly decide to behave as if it's a bit too hot...

These twice-fired handloads haven't yet been shoulder bumped, though I was planning to do just that with all my ex-Canada cases, with the aid of a comparator on its way from Davy; but I'm still mystified as to why last night's cases should have been tough to extract when i did not experience any difficulty in Canada.

Any suggestions welcome...

Thanks, Tony

[DaveT]

Two weeks ago I returned from Canada with a number of unfired rounds in 20Tac. This is my standard load using 23.7gr of N130, Nosler 40grNBT seated around 10 thou off, CCI BR4 primer, Dakota (Lapua) brass twice-fired, mildly neck turned, neck sized using a Redding .229 bushing for a minimally tight enough hold.

In very warm conditions (mostly 30 - 34-ish degrees C) in Ontario, at only a moderate height above sea level (perhaps 1000-1500 feet tops) my cases were extracting OK just as they did at home in a distinctly cooler Devon, even though I suspect the temperature was boosting my chamber pressure a bit.

Last night I went out with the rifle for the first time since, and bagged four bunnies at a modest 255 - 275 yards. Each time, the bolt handle was a bugger to lift! I had to rap the handle sharply upwards... My primers are intact and don't look noticeably more flattened than I'm accustomed to.

I'm a bit concerned at this: I never build excessively hot loads, and this load chronos at comfortably under 4000fps with no signs (previously) of undue pressure. I see no reason why, in the significantly less warm conditions of Devon, my standard load should suddenly decide to behave as if it's a bit too hot...

These twice-fired handloads haven't yet been shoulder bumped, though I was planning to do just that with all my ex-Canada cases, with the aid of a comparator on its way from Davy; but I'm still mystified as to why last night's cases should have been tough to extract when i did not experience any difficulty in Canada.

Any suggestions welcome...

Thanks, Tony

 

 

I have had the same false pressure indications when I had been neck sizing only for around 6 - 8 times on 338 Lap Mag Lapua brass... a quick run through a Redding body die sorted it out.... I think the brass had simply expanded to the extent that it had nowhere else to go and could not shrink back enough for 'normal' extraction.

[TonyH]

I have had the same false pressure indications when I had been neck sizing only for around 6 - 8 times on 338 Lap Mag Lapua brass... a quick run through a Redding body die sorted it out.... I think the brass had simply expanded to the extent that it had nowhere else to go and could not shrink back enough for 'normal' extraction.

Dave, thanks, what you suggest is in line with my own best guess: my load is not excessively hot, but those cases need a bit of sizing after three firings. What puzzles me is that I barely had any instances of sticky extraction in Canada, putting quite a lot of rounds downrange; yet the other night, four rounds in succession proved particularly tough to extract in conditions where if anything pressures should have been slightly lower than they had been in Canada...

I'll bump the shoulders back a fraction, as you suggest, and see what happens.

Regards, Tony

[DaveT]

Dave, thanks, what you suggest is in line with my own best guess: my load is not excessively hot, but those cases need a bit of sizing after three firings. What puzzles me is that I barely had any instances of sticky extraction in Canada, putting quite a lot of rounds downrange; yet the other night, four rounds in succession proved particularly tough to extract in conditions where if anything pressures should have been slightly lower than they had been in Canada...

I'll bump the shoulders back a fraction, as you suggest, and see what happens.

Regards, Tony

 

 

Tony... this is reaching WAY into guesswork but I wonder if the much higher Canadian temperatures were enough to cause a minute (but enough) expansion in the chamber metal sufficient to let you get away with it?

[Andrew]

Could that all the travel, and the bumping and jarring associated with it, have caused some of your powder granules to break down? This would change the burn rate a bit and probably alter pressures.~Andrew

[fizzbangwhallop]

My Blaser was a bit stiff to close the bolt on the other week and that was down to a bit of grunge in the chamber. That combined with ammo stored in a leather pouch which had left them a bit dirty... not enough shooting recently to replenish the cases

 

atb

Fizz

[TonyH]

Thanks to DaveT, Andrew and Fizz for your interesting suggestions. I'd never heard of the powder breakdown idea - anyone experienced this? My ammo travels in plastic 50-boxes, padded inside to stop rattling, and the boxes are wrapped in foam rubber inside another container which itself is padded by clothes etc in my checked baggage... I'd guess that flying, it receives less of a pounding than when I drive to the range. And after Canada my rifle was thoroughly cleaned, so no chance of a dirty chamber boosting pressure or making cases sticky. But these are creative suggestions! I'm expecting a shoulder-bump comparator today, made by Davy, so after processing and loading my thrice-fired cases I'll report back on how they behave...

Regards, Tony

[Andrew]

Thanks to DaveT, Andrew and Fizz for your interesting suggestions. I'd never heard of the powder breakdown idea - anyone experienced this? My ammo travels in plastic 50-boxes, padded inside to stop rattling, and the boxes are wrapped in foam rubber inside another container which itself is padded by clothes etc in my checked baggage... I'd guess that flying, it receives less of a pounding than when I drive to the range. And after Canada my rifle was thoroughly cleaned, so no chance of a dirty chamber boosting pressure or making cases sticky. But these are creative suggestions! I'm expecting a shoulder-bump comparator today, made by Davy, so after processing and loading my thrice-fired cases I'll report back on how they behave...

Regards, Tony

 

Tony: The reason I mentioned this possible"breakdown" is because I have seen it before at least once. Some ammo that had ridden around in the back of a Jeep for two months caused unexpectedly high pressures. When we pulled the bullets we found that there was, along with the powder, a fine dust that we assumed was coating abraded from the powder granules. I would suspect that this phenomenon would be very dependent on loading density and the kind of powder.~Andrew

[achosenman]

TonyH, there have been documented cases of excessive pressures due to powder breakup.

 

The instance I remember was someone putting a live round in a tumbler by mistake and then treating it as normal, once he discovered the error.

I imagine a round bouncing around in the back of a jeep would be akin to a tumbler. However, you describe a very well thought out handling arrangement so on balance I wouldn’t have thought your high pressures would be caused by this.

 

 

ATB

[Andrew]

TonyH, there have been documented cases of excessive pressures due to powder breakup.

 

The instance I remember was someone putting a live round in a tumbler by mistake and then treating it as normal, once he discovered the error.

I imagine a round bouncing around in the back of a jeep would be akin to a tumbler. However, you describe a very well thought out handling arrangement so on balance I wouldn’t have thought your high pressures would be caused by this.

 

 

ATB

 

I'm thinking you can package the cartridges as securely as possible but unless the powder/load density is high, the powder grains themselves are free to bounce around inside the case. In our case, the ammo from the Jeep (which didn't see a lot of rough terrain, but did vibrate like an SOB) had a fine dust come out along with the powder grains. Most of the grains were intact but the fine dust that followed the powder made us think the powder had just rubbed off the coating.~Andrew

[achosenman]

I'm thinking you can package the cartridges as securely as possible but unless the powder/load density is high, the powder grains themselves are free to bounce around inside the case. In our case, the ammo from the Jeep (which didn't see a lot of rough terrain, but did vibrate like an SOB) had a fine dust come out along with the powder grains. Most of the grains were intact but the fine dust that followed the powder made us think the powder had just rubbed off the coating.~Andrew

 

Andrew you have a point and I agree the loading density is a big factor in this.

 

The reason I suspect it is something else is that when I ran QL using the data Tony supplied, the pressure was big and bold and in red at 59263psi. The predicted velocity is 3630fps from a 24 inch barrel.

 

I made an assumption on a case capacity of 30.5gn since this is what my Lapua 223 brass comes out at, and I gave a COAL of 2.185 since he mentions seating close to the lands. However 2.245 is the longest I've seen, but this still gives rise to 57315psi.

 

That load if my assumptions are on the money, is a very hot load for my taste. I am led to believe modern high quality actions can allegedly withstand repeated exposure to around 60-65000psi without damage, but I would think the accuracy would be iffy at best and case life somewhat reduced.

 

ATB

[Andrew]

Andrew you have a point and I agree the loading density is a big factor in this.

 

The reason I suspect it is something else is that when I ran QL using the data Tony supplied, the pressure was big and bold and in red at 59263psi. The predicted velocity is 3630fps from a 24 inch barrel.

 

I made an assumption on a case capacity of 30.5gn since this is what my Lapua 223 brass comes out at, and I gave a COAL of 2.185 since he mentions seating close to the lands. However 2.245 is the longest I've seen, but this still gives rise to 57315psi.

 

That load if my assumptions are on the money, is a very hot load for my taste. I am led to believe modern high quality actions can allegedly withstand repeated exposure to around 60-65000psi without damage, but I would think the accuracy would be iffy at best and case life somewhat reduced.

 

ATB

 

You are most certainly right about the pressure and accuracy. What caught my attention was the fact that the ammo shot well in the UK and then seemed to have no problems in Ontario, but then exhibited pressure when back in the UK. For me, the only thing I could think of was the powder degradation. It seemed to fit the circumstance. I know that the ammo we rescued from the Jeep went right back to normal when we reloaded the cases with fresh powder.~Andrew

[achosenman]

You are most certainly right about the pressure and accuracy. What caught my attention was the fact that the ammo shot well in the UK and then seemed to have no problems in Ontario, but then exhibited pressure when back in the UK. For me, the only thing I could think of was the powder degradation. It seemed to fit the circumstance. I know that the ammo we rescued from the Jeep went right back to normal when we reloaded the cases with fresh powder.~Andrew

 

Hi Andrew I take your point. Perhaps it’s a combination of a hot load and powder degradation due to the abrasive effects of travel…enough to push it over the edge. That would fit the facts as we understand them.

 

My reasoning for being a bit sceptical was that military forces “travel” a lot and don’t suffer similar problems. As we both know I’m sure, that’s never as comfortable a mode of travel as taking a commercial flight.

 

ATB

[achosenman]

This got me thinking about a thread I read on Long-range hunting .com a while back.

 

Here is the link to the thread. Scroll down a few posts to find what Kirby Allen (aka Fiftydriver) has posted on the subject of repeatedly loading "hot loads"

 

It might be worth a look if anyone is inclined.

 

ATB

[TonyH]

Andrew and achosenman, thanks for your continued interesting contributions, which provide food for thought. I'm certainly prepared to consider the "powder breakdown" possibility - though I'd point out that I always develop loads that more or less kiss the base of the bullet, so not much powder movement subsequently. I am also prepared to consider that my load is a bit hot, even though I have always done my best to avoid excessively hot loads and my load of 23.7gr of N130 was arrived at quite early in load development, not displaying any signs of excessive pressure... I might drop the load by a fraction to be on the safe side.

That link to discussion of chamber pressure in Rem 700 actions for .338 Lapua is very interesting - and I note what the author says about case size and pressure transmitted to the receiver and the bolt head. What he says about the limited extent to which a .223 case can transmit dangerous pressure is encouraging.

However, I've just processed all my ex-Canada cases, bumping back the shoulders to put them into a "once fired" condition, and trimming to standard length. I understand the industry standard max length for 20Tac is 1.76" - none of my 3X fired cases had stretched that far, 1.757" at most and the majority a little less after starting from new at something over 1.75. These cases chamber readily.

We'll see what happens when I fire some of them in the field.

Regards, Tony