Experiences with high overbore cartridges and powder bridging

[fmuguira]

Looking for any information or experiences on powder bridging within a cartridge and pressure spikes. We ve built 2-3 6mm-280 AI s and in testing found Retumbo to be a little quick burning ( pressure signs before we hit anticipated velocities) and then moved to R33 and US869. Our velocities were higher without pressure signs at 80 degree temperature range. (3400 FPS range with 27-30" barrels)

Fast forward and we ve seen high pressure with same R33 loading at 40 degrees. Same batch of loads. Chose R33 over 869 as thought it would be less temperature sensitive, and logic would anticipate LESS pressure in colder temps.

So in trying to figure out what's happening came upon the powder bridging notion and read an old post on this forum that I think Fiftydriver wrote saying that stick powders in small caliber overbore cases can cause powder/bridging and erratic pressures and that ball powders are less prone to such.

Have any of you experienced such and was there a viable solution? R33 is a large kernel stick powder and I can see where this makes theoretical sense. We re burning 64-65 grs in the case behind 108-110 gr bullets.

Interested in any experiences. Thx

 

[fmuguira]

High overbore... sorry I missed that when proofing.

 

[MagnumManiac]

I see this is possible, but not probable.
Powder burns within 2" of the case mouth, even if overbore.

How are you getting the powder in the case?
Is it slightly/heavily compressed?

I think it has more to do with an uneven burn caused by a powder that is too slow rather than an issue with bridging
I have difficulty with RE33 in my 338-416 Rigby Improved due to poor start pressure on occasion, sooted cases is a dead giveaway.

Cheers.

 

[fmuguira]

Powder goes into case easily, not a compressed load.. around 95-97%.

Load work up everything went smoothly, no issues. Then at later date as mentioned, same rounds left over from testing work up exhibit pressure with extractor marks on brass and snug bolt lift.

Running CCI250 s with load as it's a fairly large powder charge.

We plan to pull a few bullets, reseat a new one and see if there's any welding and fire the load as is. Also thought we might dump the powder on a couple, return it and reseat bullet to see if clumping of powder could be possible. Lastly thought we d vibrate a couple of the loads rounds and then try that. Then load new rounds with same lot of powder, same charge and shoot those. Aside from all the above testing that's where some googling found a little bit of info and guys talking of powder bridging.

All the above steps worth a try and we ll run everything with the LabRadar working to see what velocity info we might surmise.

 

[scope-eye]

Hi The mag primers are the source of your problems as far as bridging and pressure spikes are concerned, try some non mag primers if it does not fix it it will greatly reduce your problems.
It would be safe to say that almost everything I shoot is "High Overbore"

Dean

 

[MagMan]

I use H1000 and 107 SMK's in two different 6-06 rifles and have never had a pressure issue. Or suspected a bridging issue. One is cooking at 3450 and the other 3375. Primed with BR2's.

 

[Pdvdh]

Hi The mag primers are the source of your problems as far as bridging and pressure spikes are concerned, try some non mag primers if it does not fix it it will greatly reduce your problems.
It would be safe to say that almost everything I shoot is "High Overbore"

Dean

Any idea why a non magnum primer would reduce the problem with powder bridging?

 

[scope-eye]

My take is they ignition is less abrupt, harsh, ect, I guess there are a dozen words you could use, anyways the lesser intense flash allows the powder to burn smoother more evenly. I guess sorta like when you pour gas into a funnel, at just the right amount it flows evenly but when you tilt the gas container to much the funnel is overwhelmed can't keep up and overflows.

That's my story and Im stickin to it. LOL

Dean

 

[fmuguira]

Interesting.... I ll have to add a couple std primer loaded cartridges to the test!

Do U think that the Mag primer becomes less abrasive/volitile as the bore diameter increases and thus the problem disappears? My 257 Weatherby burns 70 + grs of powder in a bore that's only .013 larger and never had a problem there ...

 

[fmuguira]

Also, might the 40 degree shoulder on the AI case contribute to the problem? Or, fairly deep bullet seating due to only .050 free bore?

 

[scope-eye]

No The primers intencity stays the same the thing that happeneds is, at the slightest sign of bridging or you could call it cloging it just goes for bad to worse in that short millisecond.
It's a true bottleneck in the true sense of the word, as far as neck dia any increase in size is huge no matter how small it is considering what we are doing is borderline insanity.

Dean

 

[scope-eye]

Also, might the 40 degree shoulder on the AI case contribute to the problem? Or, fairly deep bullet seating due to only .050 free bore?

Yes both of those are huge contributing factor, look at AI case and a regular one a 25 deg neck will flow smoother imagine you were pouring water though it. And as far as the free bore is concered all that pressure from the spent powder is trying to exit through an already reduced passage, "neck" then it is met with bullet resistance from the short freebore which adds insult to injury don't forget the bullet has started to move forward before all the powder is all burnt. like I said the funnel is overwhelmed.

Dean

 

[scope-eye]

I have often loaded ammo to hot I mean for me to admit it is "to hot" it is passed stickey bolt amoung other tell tales signs. Anyways I never pull them apart all I do is seat the bullets deeper into the case, usually 50 thou 100 thou is not rare it solves the problem every time.

Dean

 

[fmuguira]

That's another potential test... we re running oal at .020 off lands; might seat a couple rounds at .070 off and see if that does anything.