Home Forums Hunting & Shooting Ask The Gunwriters squib loads and enlarging flash holes

Tejano, Good question, I too wondered, and doing some reading in years past...apparently the theory is that the .063" flash hole (avg give or take a few thou) was developed to increase the duration of the jet of flame issuing into and enveloping the powder charge to ensure the entire powder column was exposed to the primer's flame rather than just the rear end of the column. Supposedly if just the rear of the full size charge is ignited, powder will compact forward at the base of the bullet, which is traveling forward en mass...which is increasing combustion chamber volume, which is dropping pressure, which drops heat...in a downward spiral which leaves us with a bunch of unburned powder going downbore and the bullet at low velocity. The flash hole orfice is a restriction and with light charges the case does not grip the chamber walls as do full charges, the firing pin drives the case forward, the primer pistons back out of the case to the bolt face. So, increasing the size of the orfice reduces the primer pressure, the primers explosive charge vents thru the enlarged orfice with dramatically lower pressure and the primer does not set back out of it's pocket. The large sudden flame is completely adequate to ignite small charges of more volatile pistol powders or black powder. So, to your question..I don't think the powder charge pistons the primer back...it's caused by the primer detonation itself in concert with the standard orfice. I may have misinterpreted what I have read, I am still shaking my head at "brisance".
A few years ago I conducted some testing in a darkened shop used an un barreled action firing off various primers and just visually observing the flame. At the conclusion of my highly scientific tests...I now buy whatever primers are on sale. And in very accurate rifles, use what the chronograph tells me to use. grin.

flintlocke,

For over a century one of the supposed tests of primers involves photographing the flame length--which continues with digital photography.

But as a guy who worked for one of the major U.S. primer factories pointed out to me several years ago, when I was working on an article about primers, is that length of the flame--or any other differences in appearance--have nothing to do with the flame's duration or temperature, both of which have major effects on powder ignition.

Well, that was my conclusion as well, hence, my selection of primer in any given load is: what's on sale, and how low the S.D. In that order. But,,,I have had some low S.D. loads that had the amazing ability to avoid tight groups. The more I learn about my avocation....the less I know. And I knew so much when in my 20's.

I know the feeling!

Thanks flintlocke & MD, good explanation it cut through my hazy understanding. I should understand primers backing out. As a kid I shot a thankfully empty shot shell with my pellet gun. The primer ejected with enough force the anvil went pretty deep in my leg, stayed there for awhile too as I didn't want to tell my dad about my primer experiment.

Thanks flintlocke,

That helps me wrap my head around why the primer back-out is reduced.

I admit I am still struggling to understand how/if this reduces shoulder set-back. If the blow of the firing pin is responsible for forcing the case forward in the chamber, and this force is responsible for moving the shoulder back, I can't picture how reducing the primer pressure will help mitigate this. It seems to me it would just help to disguise the fact that shoulder set-back is taking place by not allowing the primer to back-out into the excess head-space.

Thanks all for the discussion so far,

Frosty

I need to explore this more. Are some of the powders we've been using for reduced loads better than others? I've been using trailboss in reduced loads but now the cases are becoming unusable. Does unique, 5744, or blue dot provide more pressure in these sorts of loads to preserve dimensions?

Unique, over a hundred years in use, was designed for reduced loads, other than a little dirty, it just works. Lots of reports of wierd stuff with Trailboss. Although champion shooter Venturino has selected 5744 as his "go to" powder, people a lot smarter and experienced than myself have been turning away from it. I dislike it because invariably, in large capacity black powder designed cases an examination of the bore after a shot will reveal a lot of unburned powder. That doesn't hurt anything says the experts, but I am uncomfortable with it in my ancient heirloom guns.
Anyway, I'm just flapping my jaw not knowing your case capacity, bullet weight class, bore size, desired velocities. Old (!) Lyman manuals are the best, tremendous range of useable powders that were actually tested in real guns, you can still get them on Ebay. Lyman's latest Cast Bullet Handbook , in my opinion is a big fat zero. If I could only have one book? Lyman #45. Just one man's unqualified opinion.

I use Bullseye, Unique, Red Dot for low vel loads. No issues of this nature for me. I do take the precaution of keeping .30-06 brass segregated for such use but not rimmed brass.

I know a lot of fellas use Trail Boss but I never warmed up to it. Poorest bang for the buck for reduced loads in medium to large capacity cases, plus mediocre accuracy in my experience. It shines for its originally intended usage- light loads with a relatively full case of powder in BP pistol cowboy action loads.

Lyman 45 manual is my all time favorite also.

I gave 5744 a whirl but came back to 4759 for most of my .30 caliber medium velocity cast bullet shooting. Thank God I have a goodly supply of that powdered gold. I'll never forgive them for discontinuing it. It cost me a small fortune to stock up after word of that went out.

Listen to Gnoahhh, I do.

I've been using trailboss to shoot cast 286s at 1350 for grouse shooting. Velocity was sufficient and accuracy has been one hole at fifty yards. My son's rifle hates trailboss but is very accurate with 5744. Based on what's started happening with cases using trailboss I intend to experiment with unique. Ideally there should be enough pressure to keep the shoulders right yet little enough that case life should be just about forever.

Makes sense.

This is accurate and a good explanation. Another way to say the same thing is that the priming compound is in a small chamber made up of the primer pocket and the primer cup; other than that little hole, the pressure of the primer explosion has no where to vent, and tries to push the primer cup and primer pocket apart.





The firing pin blow doesn't push shoulders back. The force of the firing pin blow might move the case forward to the limit of the head space, but it is small in comparison to the primer ignition force. It's the primer force itself that is capable of moving the shoulder back by pushing on the area around the flash hole; enlarging the flash hole reduces the primer force against the case by reducing the area it has to push on.

I hope that helps.

Thanks Yondering, that makes sense.

So is there an optimal diameter to enlarge flash-holes for these low-pressure loads? Obviously there has to be enough brass in the pocket for the "feet" of the anvil to rest on.

Frosty

You're not asking me, but, you will see people opening them up to .120"...don't do it, too much of a good thing is a bad thing. I think that number came from the days of the Speer plastic indoor bullets propelled by the primer only. The die hard .45-70 shooters pretty much agree on .096". Again, I don't think it should even be done to begin with unless there is a demonstrated need. My first "go to" with case shoulder setback would be faster powders and or larger charges and or powder positioners (fillers). 50 years of subsonic cast bullets and I have never needed to open up flash holes. .308, .30-06, .358, 8x57. None of the rimmed case family ever needs it unless igniting black powder.

I don't have any particular info to claim anything different than what flintlocke said.

I can point out though - 9mm (i.e. mid-30K psi loads) Federal non-toxic primed brass has flash holes about .100" dia., and it doesn't seem to cause any issues using it with normal primers in mild or normal loads. I've had no issues going that big with other cartridges using small primer brass. Haven't tried any bigger with large primers.

I think the flash hole size you can get away with will depend on how mild your reduced load is. At some point I have to think that brass is no longer good for more powerful loads, but haven't found that point myself.


flintlocke - you've been loading subsonic cast bullets for a lot longer than I have, but I do it as well for suppressed shooting. I've definitely encountered some loads where case capacity is too large and the bullet too light to make up for it with fast powders; even using Clays, Red Dot, Bullseye, etc the load gets very position dependent and shoulder setback has been an issue. 100gr cast bullets in the 35 Whelen or Remington, 90gr XTP (I know, not a cast bullet but same issue) in 308, 30/06, etc, stuff like that. A cotton wad holding the powder against the primer makes the load consistent, but I don't like to risk any sort of filler, even cotton ball, in the suppressor. Do you have any fixes for that sort of load, beyond just using heavier bullets? That's what I generally do, but the lighter bullets can make for quieter loads.

I think I have let my mouth run a ways past my brain here. I love this hobby, and the entire subject so I get a little carried away. Truth, 99% of my experience is with the cartridges of the World Wars, and other than a few thousandths here or there, and a couple cc's case capacity...they are amazingly the same. My experience with powder positioners has actually been limited to the .32-40, .38-55, .43 Spanish, .44-77 and the .45-70. These latter cartridges have a high internal volume and benefit greatly as to accuracy and velocity uniformity with powder positioners.
OK, that said, in my opinion, you are doing exactly the right thing...you are using the proven safe powders for your application, if you are not happy with the results, you are upping bullet weight, perfectly good logic. You have shoulder setback with the components of choice? That is the time to start opening flash holes. Now adding a suppressor to the equation...you are leaving me in the dust, I have never fired a suppressed arm in my life.
As a footnote, the Skagit River Valley, to be honest, is soggy. Where I live and the range setup in my yard is a tinderbox six months of the year. Like you, I prefer cotton powder positioners, but have had some fire danger issues, rarely, but it does happen. You can see cotton filaments drifting in the air after a shot when the light is right, but perhaps I was using too much. So, I have gone to dacron or polyester which just disappears somehow and miraculously does not leave any plastic deposits in the barrel or throat. I wonder if the synthetics could be used with the suppressor without getting a deposit?

Here's a possible. Borax, the 22 mule team stuff sold in the laundry section, was used as a fire retardant. Maybe still is. Soaking your cotton in a solution of that stuff may solve your burning cotton problem. Experiment. Borax is an alkali and I can't imagine it would cause any problems. Borax - wiki

Huh. That's interesting.

As for fillers, that's another rabbit hole to explore. I tried to like them years ago but have shelved them since then. They work, for sure, but the little bit of improved performance for me don't justify the inherent risks in using them.

A filler has to occupy all the air space in a case right up to the base of the bullet. Failure to do so can very easily result in a ringed chamber. (A small tuft of filler tamped down on the small powder charge to hold it tight against the flash hole becomes a secondary projectile inside the case. When it impinges against the bullet base a weird shock wave- for lack of remembering the scientific description of it- is generated which can very well create a radial dent in the chamber wall at that exact spot- a ring in other words. Same principle that causes a ring in the barrel of a .22 if a bullet gets stuck in it and another bullet is fired against it in an effort to eject it.)

Putting a small tuft behind the bullet only does no good because the small powder charge is still "free to roam around the cabin". It's gotta be a big tuft to completely fill the unoccupied space in the case. A wee bit trickier than one might think. Make it big enough to engage the powder charge and bunch up well into the neck, and let the bullet compress it firmly when seated.

Inert fillers are best. Stuff that burns, aside from the environmental fire hazard mentioned above, can (will) leave a shiny buildup of melted material in the barrel that'll have you hating life when you try to remove it, given enough shots between cleanings. I learned that lesson when using polyester fillers, so went back to using traditional kapok, and then gave it up altogether. (Hint: excellent source for kapok is old life preservers.)