What is “powder bridging”?

i've encountered this term a bunch and have looked into it but do not feel 100 percent certain I understand what it is. I know it's a concern with stick powder in massively overbore cases. That's about it.

Not sure about in the case but powder in the hopper when reloading can bridge, or as you continue to use up powder as gravity drops it lower in the hopper, you get a clump of sticky powder somewhere up high in the hopper and and that bridge of sticky powder prevents gravity from feeding down to the lower portion of the hopper. I imagine a centerfire case is just a smaller hopper if you stand it upright.

I've disassembled cartridges and found sticky clumps of powder that I had to shake really hard to get all the powder to come out of the case. That's probably what they're referring to. I suppose a hard ball of stuck powder can significantly affect even burn characteristics.

You nailed it Mike

To add to the clarification here is an explanation from Kirby Allen;

Fiftydriver said:

Basically, when X amount of powder is trying to pass through a certain diameter hole, it has a limit in how fast the powder can flow through this opening.

With a very large case capacity round with a modern, sharp shoulder angle, this becomes a problem with smaller caliber cases. Now this depends on the caliber as well as the diameter of the case powder column and the total powder volume.

What happens is when a round is ignited by the primer, the blast from the primer forces the powder in the case foward until it is stopped by the shoulder and seated bullet. Then it begins to ignite.

As the pressure inside the case increases from the rear of the case forward, it increases the pressure on the powder. Powder bridging occurs when the pressure actually locks the powder granuals together because they simply can not pass though the neck of the case and pressure increases dramatically.

In the case of my Allen Mags, the 257 and 6.5mm are the only two that so far have had this problem. The simple cure is to use Ball powder. This solves the problem simply because this powder shape can flow at MUCH higher rates through even smaller neck diameters then any stick powder.

In the 270 and larger Allen Magnums, this has not been a problem at all but I still generally use the real slow burning ball powders simply because of their lower burn temps and they are much easier on the throat of the chamber as the powder is forced through.

Hope this helps some.

Kirby Allen(50)

Click to expand...

I think I read on here where "RoyinIdaho" experienced this with his .270AllenMag but I didn't search for it.

Yep that was me...

The above describes it.

Envision a super over bore cartridge such as the 270 AM, which Kirby said to use ONLY ball powder. He musta known something!

Load 'r with a little under 100 gr of a big kernel powder such as RL-50 and a 170 grain bullet. Yep, a good way to proof test your circa 1950s Rem 721 action.

Powder bridging = a plugged toilet. Only quicker!

Get that image outta your head!

Powder bridging most often occurs in cartridges that have these three things in common: 1) stick powder; 2) "overbore" cartridges (as in powder charge to bore diameter) and finally; 3) usually an improved case. Most cases are based off of military designs that have gently sloping shoulders for better feeding and extraction of a round. When a case is "improved" it usually involves moving the shoulder forward and giving the shoulder a more acute angle. This improvement allows more powder capacity in the case. The improved shoulder cuts down on the amount of stretching that occurs in the neck/shoulder area, thus improving case life. Some also feel that it also helps mitigate throat erosion. While I cannot address the idea of less throat erosion, I can opine as to the case life of improved cartridge brass. I used to own a 277 Allen Mag. It was based on the 338 Lapua case and burned over 100 grains while propelling a 195 gr. bullet at about 3,150 fps. I had more than 12 firings on nearly all my Lapua brass before any signs of thinning brass became apparent in the wall close to the case head.

The downside of this type of cartridge design is that it was prone to powder bridging. Due to the acute angle of the shoulder, along with a large amount of powder trying to get out that .277 bore, bridging became a potential nightmare. Pressures could/would spike quickly and become very dangerous. You couldn't predict when it would occur as it was not a consistent. But the potential for extreme spiking was enough to convince a shooter to not go anywhere near that cliff. I ran only US 869 ball powder in mine.

I'll be darned. Somebody besides me uses ball powder on occasion!

That's all I have used (US-869) in my .270 AM. I would like to try Retumbo or some of the slowest Vihtavuori's but I feel like it's Russian roulette.

So maybe this is a dumb question... I'd read about powder bridging before and understood it to affect the loading process. But from what I'm reading here, it also effects the ignition of the powder? Is this correct?

Does this assume that not all the powder is being burned in the case?

You are correct in that 'powder bridging' can occur both when reloading and also when firing overbore cartridges. With reloading, it is no big deal (unless you are using a progressive reloader). Just use a funnel with a larger mouth, or pour the powder a litter slower and the problem is solved. When it happens while firing, it becomes a big and dangerous deal. That's probably why there were so many responses regarding powder bridging when firing.

Powder bridging when firing still burns all the powder, but creates a huge spike in chamber pressure.

In actual use, you have a hard time finding a stick powder in sub/ 7mm bores that is suitable for the extreme overbore cartridges like the 6.5/ 270 rum or the Allen mags... Retumbo is already a bit fast burning in something like a 7rum (I was having pressure issues before I got to 90% fill).
Going more overbore and trying to use smaller kernel stick (that might actually flow under pressure down a 6.5 or 25 cal bore) just isn't going to happen (no one really makes the powder) so you either rick burn issues (possible gun damage/injury or at the least a poor performing load) from bridging or you put up with ball powder and sometimes having to adjust load/ velocity for temperature.
In my 7rum (and 7stw) I am lucky; I've never had bridging issues with a 7mm bore and 5010/ 50BMG or rl50 (rl50 DOES act happier in a 300rum pushing 225's). I'm probably on the edge of this happening though, with the 270AM having the problem.

Reviving my old thread here:

I was just thinking about it again, and shouldn't it be the case that the weatherby double radius shoulder ought to help mitigate this problem even in big fat overbore cartridges? Does the .30-378 have any issues with bridging? Or the 6.5-300 weatherby for that matter? Haven't seen anything to make me believe there's been any bridging with the .257 weatherby with 7828.

Maybe old Roy was onto something, that shoulder design allows for a steep average shoulder angle with smooth as butter feeding and, possibly, avoiding the problems that might arise with a case of identical volume and bore diameter with a non-curvy equivalently steep shoulder…

IMO it's more of an issue with neck diameter vs grain size than the shoulder angle.

Powder bridging is an expansion on packing problems - 65% (random) to 75% (ideal) is the highest packing density range you can get for spheres in a sphere, so there's an upper limit on how much powder can be present in the neck at a given time and a thus a limit on how much powder can flow through the neck at any time. As neck caliber decreases wasted space increases with powder grain cylinder size being held constant. So internal case pressure has to spike because more powder converts to gas in the case (which is then has to flow out of) rather than converting in the expanding barrel volume which is not constricted. And will in fact expand to the point that pressure drops and burning ceases before the bullet ever leaves the muzzle (in most cases, not all).

SC and SSC powders are shorter extruded cylinders, so they have a higher packing density in a cylinder (the neck), than a larger cylinder would.

Steeper shoulders angles would exacerbate the problem because the case becomes essentially a nozzle with high converging angle (inside shoulder angle), a very long throat (neck length), and incredibly small diverging angle (case mouth into barrel leade, not the entire barrel length since we're looking at powder flow and not gas flow).

It's almost like half of a nozzle problem, so let me butcher fluid dynamics for a second. The double radius Weatherby design is more akin to an ASTAR nozzle with convex then concave convergent angles and longer throat, rather than a basic De Laval nozzle with a single angle convergent design but longer throat. The complex convergent section should reduce pressure loss, which is a long way of saying IIRC it flows better, for want of a more technical term.

The practical answer is don't put H50BMG in a 6mm and expect happy results - you're more likely (not certain, but likely) to see more sudden and larger pressure spikes as caliber is reduced with large powders kernels. But a Weatherby might be slightly more efficient in managing powder bridging by giving better flow out of the case.

Powder undoubtedly burns outside the case in standard rear ignition cases. 300 Blackout is a perfect example of being able to partially burned kernels come out of the barrel. Those were kernels that stopped burning when pressure dropped because pressure vessel volume (meaning case volume plus bore behind the bullet combined) got to the point there were no more exponential pressure increases to continue the burning reaction. Pressure curves look like parabolas for a reason - pressure grows exponentially and would otherwise continue until it stopped very suddenly with the full consumption of powder, but since the pressure chamber volume is increasing at it's own exponential rate as the bullet accelerates (distance per second per second) the curves drops down on an arc instead of dropping of a cliff.