Ka-boooom and stumped

[mstarling]

The loading range given by Sierra for a 150 gr bullet is 13.8-17.0 gr of H110/W296. If you really had 14.4 in the case you should not have seen the secondary explosion effect.

I'd still like to know how you explain the expanded head which shows the radius of the barrel mouth well away from the extraction groove in the case.

[Rikky Lee]

That head expansion is my concern too. Not sure how you can get that much expansion inside the chamber, Feel like sectioning the brass to see if there is anything on that side?

The only thing I have seen similar results from old brass annealed on a BBQ to red hot and then used. Not the best idea - resulted in severe brass flow and a chipped bolt head ... and a new chamber ... and stock fixing ...

[pomofo]

I second the idea that the bolt unlocked while pressure was still high. What position is your gas port in? I have a carbine-length system and I was getting blown primers and enlarged primer pockets from loads that Quickload was estimating at 38000PSI, well within the loading parameters. I suspected that the H110 was too fast for the gas system and causing the bolt to unlock too early. I switched to Acc. 1680 and have had no problems since. Your loads with M80 ball would be generating around 35000PSI, I don't know how long the tracers are but I would guess pressure probably at around 40-42000PSI.

[alpine44]

Thanks for all the ideas and suggestions so far.

Here is some more info:

The rifle has a carbine length gas tube.
The bolt was closed (lugs locked behind extension) after the event.
The BC was pushed slighly backwards but not enough to unlock the bolt.
There is actually enough space available between barrel and bolt for the "homemade Weatherby belt" due to the generous radius on the barrel end (see picture).



Blown primer, belt on case head, and extrusion of case into ejector hole are clear signs of EXCESSIVE pressure (much more than "blue pill" proof loads).

Where did this pressure come from?

[cav_scout_tj]

Clearly too much pressure.

The usual suspects:
Too much powder-- maybe, powder is so fine, it could have flowed alittle around the measure? Try the measure, messing with it while throwing charges, weight what you get, see if you can get more weight. The case only holds so much, so double charge not possible.

Wrong type of powder-- would not be the first guy.

Bullet setback-- When hitting the feed ramp, the bullet can get pushed all the way into the case. Depends on case neck tension, brass and bullets.

Secondary Explosive effect-- not well known, but the H110 powder has ominous warnings attached to it. Temperature appears to be a factor, what was the outside temp, and temp of the rounds? Cold==bad.

Over on THR I found this

Slow single-base powders or heavily retarded double base ball powders need pressure, not just ignition heat, to burn well. They generally burn well in a full case with no air space (other than inter-powder space). In cold conditions, with weak primers, and/or low pressures (light loads) it is quite possible to get a "bad burn" (large percentage of propellant never ignites.) This manifests itself in erratic velocites, hangfires and so-called "bloopers".

Occasional detonations of light loads of slow powders was well debated in the 50's and 60's and is now an accepted fact of interior ballistics. They are easier to understand given the facts above, and the fact that most energetic materials have two burn rates, a deflagration rate and a detonation rate. Propellants are distinguished from explosives (by the knowledgable, anyway) as energetic material being consumed at its deflagration rate. Explosives are self-consuming at their detonation rate.

Most energetic materials can be consumed at either rate, given certain circumstances. Dynamite lit with a match will produce a blazing bonfire, but no explosion. Expose it to the concussion of a blasting cap and it will detonate. Light it with a match in an small, unvented closed chamber (called a "bomb") and it will transition from deflagration to detonation once the pressures get high enough (assuming it is not already completely consumed.)

Detonations of small charges of slow powder work the same way. They deflagrate (relatively) slowly. Often pressure waves move the powder around and pack it against the base of the bullet. Single base extruded grains often break and are crushed by the slowly rising pressure. Now propellant is burning front-to-back, where it was burning top-to-bottom and/or back-to-front.

Pressure waves then bounce back and forth along the case, sometimes meeting each other in opposing directions and producing peak pressures nearly twice the expected, given the volume of gas. This impact or concussion sometimes pushes the burn rate of the propellant into its detonation rate.

Modern reloading manuals warn against reducing loads of W296 and H110 in magnum pistol, and H4350/IRM4350 and slower in magnum rifle, for this reason.

[DonT]

I would agree with other posters. With the bulge all the way around the base of the cartridge one of two things my have happened... Please note that I am not questioning but just making an observation.

One the gun was not in battery when discharged. As one poster stated it is not easy to fire an AR out of battery but it can happen.

Second, the gun came out of battery before that pressure dropped significantly in the barrel (a timing issue). This "could" happen if too light a buffer was used or a lighter than normal buffer spring was used. Also hammer springs delay the bolt unlocking, if this is lighter than stock it could contribute to the problem... What happens is the lighter springs fail to delay the bolt movement until the pressure subsides to a safe level. The bolt starts to move prematurely and starts to extract the round before the pressure has adequately dropped in the barrel, as this happens a portion of the case is left unsupported while pressures are still near peak levels, the extractor is working the brass abnormally hard since the case has not fully retracted from the chamber walls so it is putting undo stress on the case, all this comes together in a catastophic case failure. When this happens it has all the signs of the bolt not being fully closed when the round was fired.

Since you seem pretty confident the gun was in battery when fired I would opt for door #2.

Glad you were not hurt and the gun did not suffer more severe damage.

Just one old guys take on the issue......

DonT

[mstarling]

I have heard of folks (but not seen it) having case head problems in rifles set up for subsonic ammo when they shoot supersonic ammo. Usually this involves a three position gas block left set at the wrong port size.

(I have shot some ammunition built with 14.5 and 15.5 gr of W296 (supposedly the same as H110), 147 gr FMJBT projectiles using LC 97 brass as a start. Was fireforming cases with the gas port CLOSED). My barrel is a 16.5" tp555 Douglas SS unit.

No problem with 14.5 gr. 15.5 gr was getting a "little" warm and required that the brass be very gently tapped out with a rod.

If you are ABSOLUTELY certain (that is you actually checked and clearly remember doing it) that the bolt was fully closed before the trigger was pullet, I wonder is your gas port is just too big for use with this load ... and the pressure started opening the chamber before the bullet left the barrel.

[alpine44]

The most important thing to keep in mind is that the bolt was closed after the event.

The BC was pushed back to the point where the cam pin would start rotating the bolt, but the bolt could not rotate because of the brass wedged between barrel and bolt. The bulge (belt) on the case will fit between the barrel and the bolt due to the chamfer radius at the breach end of the barrel.

Also, an out of battery discharge will not force brass into the ejector hole the way this event did.

All signs point to the fact that the powder charge developed A WHOLE LOT more pressure than desired. The question is: Why?

I agree with the assumptions that the powder measure is a suspect. It could have trown less powder (some got stuck in the funnel) or more (powder that got stuck on the previous shell came loose).

My gut feeling is that filling the case until the long tracer bullet bottoms out would not yield these drastic results. I may verify this in 2 weeks with the tree and string method.

I lean towards explaining the dramatic results with a low charge and a subsequent detonation or Secondary Explosion Effect. An indication of a SSE may be the smoking of the front to the cartridge seen in pic#4. A simply overcharged cartridge would have sealed well at the neck and would not show any smoking.

The outside temp was 54 degrees F and the cartridge was probably around 60 deg F.

[HUNTER2]

Sometimes it is small things that make us remember what we are dealing with. I'll bet there has been more than one to double check their loads after reading this. Sounds like you are on the right track. Thanks for sharing....

[alpine44]

There are a lot of dumb things we all have done that are not worth sharing.

This however was one of those "gotchas" that simply leave you scratching your head. When I started loading the .300 Whisper/Fireball I thought that it can be relatively safely loaded in a progressive press whithout checking each case individually.

I also have been using W296/H110 extensively in my S&W629 .44Magnum with excellent results and through that I am well aware of the low charge issue with these powders.

I guess we all have to make sure, by checking each charge, that we do not unintentionally enter the danger zone with these powders. Using magnum primers (e.g. CCI450) and a factory crimp would probably not hurt either to guard against ill effects with low charges.

In about 2 week I will be back home with my toys and will report how much H110 could fit in a case behind a properly seated M25 tracer, without this overcharge becoming noticeable during bullet seating. If the amount is not too outrageous I may prove, using tree and string, that an overcharge may blow a primer but not cause the brass mutilation seen in the photos. This would be the proof of my assumption that the event discussed in this thread was caused by an unintended undercharge.

If a whole lot more than 15-16gr of H110 will fit in the case without becoming noticeable, then we are unfortunately left with flipping a coin to decide whether overcharge or undercharge caused the Ka-boom.