Cast bullet loads

[Dougan]

Sorry if this turns out to be a daft question...

I've read a couple of times that a standard load for a .303 (or similar) indoor round, is approx 4 gn of any fast powder (e.g. N310) - I'm using a 'low-load' of 3.9 gn of N320 in a .357 case, and am wary of going too low because of what I've been told about 'flash-over' (though I've never witnessed it or heard of it happening)...

...so with so little powder in a .303 case, is there not a risk of a 'flash-over'?...or is that how it works; i.e. a quick bust of pressure that's fine in a case of that volume...?

[dromia]

Flash over or Secondary Explosion Effect (SEE) is a know condition that happens with low loading density of medium to slow rifle powders.

Basically with low loading density of slower powders where the powder lies along the length of the case in the chamber below the level of the flash hole, when fired the primer flash can go over the powder causing an incomplete burn that drives the bullet a short ways into the barrel and stops, the powder in the case finally combusts and pressure builds however the bullet has now become an obstruction in the barrel and and the pressure spikes sometimes with catastrophic results.

That is why for reduced cast boolit loads we always use fast pistol powders which are easily ignited and are not position sensitive and do not cause SEE. The only thing you need to be careful about is using fast pistol powders in smaller rifle case where less that 4 gns are needed to give the reduced velocity there you are entering a world of very small loads and unpredictable powder behaviour.

Any powders burning rate is dependant on sufficient loading density to create sufficient pressure quickly for the powder to burn efficiently this is usually guaranteed by the flash igniting most of the powder at time of ignition because it moves through the powder column. Reducing loading density can alter the burning rate of powders and published data no longer applies, for example reduced rifle loads using fast pistol powders give far lower velocities than predicted by Quick Load for instance and for that program to work with them the BA of the powder needs adjusting the low density conditions.

For rifle powders my rule of thumb is never go below 60% case fill.

Reduced rifle loads with fast pistol powders should never be used with condom bullets because of the higher start pressure and greater barrel friction.

Stick with published data, in this case Mattern and Harris, and you will be on well trodden territory.

For a more scientific explanation of SEE (and how powder works and builds pressure) I would recommend the publication Firearms Pressure Factors by Wolf Publishing.

This is a series of articles from the Handloader magazine mostly written by Lloyd E Brownell Ph D in response to an article in Handloader written by Col G.O.Ashley which said that SEE was fiction.

Good, responsible, safe handloaders stick to published data, those who go beyond published data with out effective pressure testing equipment are just trusting to luck and are unsafe in my book. As handloaders the human error is always a risk to our known handloads and accidents await us all, no need to exacerbate that risk by making it up on the vain hope that looking at primers and not having to pound the bolt open means that the load is safe.

[dromia]

BTW your 357 load should be fine, 320 is a fast pistol powder easily ignited. The potential for SEE conditions is far more likely in bottleneck rather than straight wall cases.

[Dougan]

Thanks for an excellent and comprehensive answer Adam :cheers:

I'll be getting round to this at some point; and will definitely be sticking to the published data...and probably a few more questions (although the info on here is building into quite a little library).

A quick search for 'Firearms Pressure Factors' only shows the CD version, so I'll keep an eye out for a hard copy.

[dromia]

http://www.amazon.com/gp/offer-listing/ ... ition=used

[Dougan]

Cheers Adam, but $50 is a bit steep - looks like there's been the odd one on eblag (though not in the same condition)...so I'll keep my eye out...

[dromia]

Aye that is what happens when things go out of print.

Those bloody CDs are no replacement for a book, I hate reading stuff of any length on screen so I end up printing stuff out, so much for a paperless environment these computers were supposed to bring in.

Those ebooks they are making now seem only able to handle there own formats. :cool2:

I have a lot of stuff in PDF and text that I would like to read on an ebook but no its all about you buying an ebook so that you can only read their stuff at their price, theiving

[Ovenpaa]

My Kindle thingy reads Pdf's without a problem and I have quite a few reference articles on it as well as books all in this format.

[dromia]

How do you load them on?

[Ovenpaa]

Just copy them across via my PC or email them to myself and select save. Then use the file explorer to find them and move them if needed.