Flash Hole
- Thread starter blvedere
- Start date
You'll lose the benefits that SR primed cases accrue.
bobm
Silver $$ Contributor
If you are talking about .308 Win. Palma cases with small rifle primer pockets, definite con. SR primer cups are about .020-.022 thick. LR cups are .027. By enlarging the flash hole diameter you would be allowing more gas pressure volume against a thinner cup. Pierced primers by firing pin is a possibility.
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What are the benefits ?
bobm
Silver $$ Contributor
Dusty Stevens
Shiner
Dont mess with em
There's a reason that some prefer the smaller flash holes . Don't enlarge them don't chamfer , this too increases volume to the cup and reduces the length of the dia, again increasing the volume and possibly the pressure to the primer cup .
If you want better primer ignition with long powder columns, check out primer tubes .
Not as easy as reaming primer holes ! Primer tubes may not be the correct term , it's been along time but were used on larger artillery shells and tried on smaller rifle cases for accuracy .
If you want better primer ignition with long powder columns, check out primer tubes .
Not as easy as reaming primer holes ! Primer tubes may not be the correct term , it's been along time but were used on larger artillery shells and tried on smaller rifle cases for accuracy .
I've seen the Winclean flashholes, too. The idea that they would increase primer pressure was, indeed, "common knowlege" once upon a time, but may be faulty physics. The time required for gas to get back through the hole and build pressure in the primer would be greater with a small enough hole, but under exposure to constant pressure, the final value just depends on the diameter of the primer as piston. The question, then, is whether the pressure settling time increase caused by a smaller flashhole is great enough to let the chamber get past its peak pressure before the pressure equalizes? I haven't tried calculating it, but my gut says the primer pocket volume is so small, there is no way the pressure drop lasts that long.
Clearly, Winchester found its less vigorous "green" primers needed a bigger flashhole. I don't think it will hurt anything, but will it help? Uniforming the flashholes with a flashhole deburring tool helps ignition consistency with harder-to-light ball powders. When I was using Accurate 2520, I had the accuracy of 168 grain Sierra MatchKings in my M1A at 100 yards improve from 1.25" to 0.75" from deburring the flashholes. For 4895 and Varget (stick powders), it made no difference at all that I could measure. They both shot at close to the 0.75" value with the right charge. The gun jsut liked 2520 best, with the proviso that the flashholes had been prepped. I have no idea what enlarging them would have done?
A thing to consider is that benchrest shooters go to some trouble to get cases with small primer pockets and then locate the weakest lot of primersthey can find. They deburr their flashholes for consistency, but don't want a big flame, finding that primers introduce more velocity variance than powdercharge erros do, and that leaving the pressure curve to the powder works best. I would not be enlarging flashholes for this reason.
Clearly, Winchester found its less vigorous "green" primers needed a bigger flashhole. I don't think it will hurt anything, but will it help? Uniforming the flashholes with a flashhole deburring tool helps ignition consistency with harder-to-light ball powders. When I was using Accurate 2520, I had the accuracy of 168 grain Sierra MatchKings in my M1A at 100 yards improve from 1.25" to 0.75" from deburring the flashholes. For 4895 and Varget (stick powders), it made no difference at all that I could measure. They both shot at close to the 0.75" value with the right charge. The gun jsut liked 2520 best, with the proviso that the flashholes had been prepped. I have no idea what enlarging them would have done?
A thing to consider is that benchrest shooters go to some trouble to get cases with small primer pockets and then locate the weakest lot of primersthey can find. They deburr their flashholes for consistency, but don't want a big flame, finding that primers introduce more velocity variance than powdercharge erros do, and that leaving the pressure curve to the powder works best. I would not be enlarging flashholes for this reason.
We now have option #3 as Starline's SRP brass variants use the large flash-hole if I understand things right. IMO, this is a waste of time .......... but practical experience should show up what works and what doesn't in due course.
With SRP / small flash-hole variants of some relatively common hunting cartridges (243 Win and 260 Rem) now available alongside the 'traditional' LRP / 2mm flash-hole from Peterson (and others?) expect some confusion amongst both retailers and shooters - decap pins stuck in cases; potential loss of MV and zero or even hang/misfire in very cold hunting conditions etc.
We've only recently starting receiving Peterson brass in the UK - similar prices to Lapua here but you get a nice ammo box and even expanded foam lid liner thrown in - and when I ordered 50 260 Rem recently by phone I checked they were the LRP variety. (The rifle blanks SR primers like nobody's business at low pressures and is useless with SRP brass.) The shop assistant was sure I was some sort of nut I'm convinced - the possibility of an SRP variant was new to him and from his voice it was apparent he was pretty sceptical about this whole issue.
Incidentally, on the question of charges / MVs in 308, a recent test load I used needed a much larger increase with Palma SRP brass - 1.4gn to return MVs to those of LRP, both types in same capacity Lapua brass, with the 167gn Lapua Scenar and Viht N150.
http://www.targetshooter.co.uk/?p=2621
With SRP / small flash-hole variants of some relatively common hunting cartridges (243 Win and 260 Rem) now available alongside the 'traditional' LRP / 2mm flash-hole from Peterson (and others?) expect some confusion amongst both retailers and shooters - decap pins stuck in cases; potential loss of MV and zero or even hang/misfire in very cold hunting conditions etc.
We've only recently starting receiving Peterson brass in the UK - similar prices to Lapua here but you get a nice ammo box and even expanded foam lid liner thrown in - and when I ordered 50 260 Rem recently by phone I checked they were the LRP variety. (The rifle blanks SR primers like nobody's business at low pressures and is useless with SRP brass.) The shop assistant was sure I was some sort of nut I'm convinced - the possibility of an SRP variant was new to him and from his voice it was apparent he was pretty sceptical about this whole issue.
Incidentally, on the question of charges / MVs in 308, a recent test load I used needed a much larger increase with Palma SRP brass - 1.4gn to return MVs to those of LRP, both types in same capacity Lapua brass, with the 167gn Lapua Scenar and Viht N150.
http://www.targetshooter.co.uk/?p=2621
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avidflyer
Gold $$ Contributor
If I remember correctly, didn’t you do some experimenting with uniforming the small primer flash holes to .0625 when Lapua first came out years ago (308) Didnt want to say enlarging just uniforming and slightly opening the 1.5mm hole to 1.58 especially for the decapping pen with no ill effect?
I've never run side by side tests, but have always uniformed both large and small flash-holes on quality brass with the appropriate Sinclair tool which IIRC opens the small variety to 0.061" (1.55mm), or using a Lyman debur / uniformer on those with ragged punched internal 'frazes'. Results on thus treated Lapua and Norma brass have been excellent.
Even with Lapua cases, you can usually feel small differences in diameter according to how much metal the uniformer removes. Some cases produce hardly any resistance so a quick single turn and it's done (of the Sinclair variety that indexes off the primer pocket and only touches the sides of the flash-hole, not the entrance or exit). At the other extreme, there are usually one or two in a box that must be smaller than the norm as they need three or four turns of the tool and produce a little more in the way of brass dust.
Over the early years of the BRs and PPCs, the benchrest experimenters did a lot of work on this. They consistently reported that 70 thou' is the tipping point. Enlarge the flash-hole to there or beyond and ES and groups increase rapidly.
I decided that this tedious, but one-off chore was worthwhile some time back even with very high quality brass in the days when I shot a lot of 223 Rem in F/TR. One box of Lapua 223 Match (2mm / 0.079" flash-holes) had three examples in the 100 ct box that were markedly smaller than the norm - so much so that the Sinclair tool tip had trouble entering the flash-holes. Even the best of manufacturers produces the occasional aberration. With some US brass, such as some lots of recent Winchester for instance, off-centre holes, oval varieties, markedly oversize seems almost the norm, and that's without the punched-through brass spikes that need removing.
Bryan Litz looks at this issue in Modern Advancements in Long Range Shooting Vol 2 and finds that removing such junk with a uniforming / deburring tool makes major improvements to ES/SD, much more so than playing with neck tensions and/or annealing. He found no benefit though in cutting a venturi on the flash-hole as some people like to do.
Even with Lapua cases, you can usually feel small differences in diameter according to how much metal the uniformer removes. Some cases produce hardly any resistance so a quick single turn and it's done (of the Sinclair variety that indexes off the primer pocket and only touches the sides of the flash-hole, not the entrance or exit). At the other extreme, there are usually one or two in a box that must be smaller than the norm as they need three or four turns of the tool and produce a little more in the way of brass dust.
Over the early years of the BRs and PPCs, the benchrest experimenters did a lot of work on this. They consistently reported that 70 thou' is the tipping point. Enlarge the flash-hole to there or beyond and ES and groups increase rapidly.
I decided that this tedious, but one-off chore was worthwhile some time back even with very high quality brass in the days when I shot a lot of 223 Rem in F/TR. One box of Lapua 223 Match (2mm / 0.079" flash-holes) had three examples in the 100 ct box that were markedly smaller than the norm - so much so that the Sinclair tool tip had trouble entering the flash-holes. Even the best of manufacturers produces the occasional aberration. With some US brass, such as some lots of recent Winchester for instance, off-centre holes, oval varieties, markedly oversize seems almost the norm, and that's without the punched-through brass spikes that need removing.
Bryan Litz looks at this issue in Modern Advancements in Long Range Shooting Vol 2 and finds that removing such junk with a uniforming / deburring tool makes major improvements to ES/SD, much more so than playing with neck tensions and/or annealing. He found no benefit though in cutting a venturi on the flash-hole as some people like to do.
avidflyer
Gold $$ Contributor
I remembered something you had wrote about the process.... All of the small flash hole tools I have seen from sinclair (not that that means anything) are .0625 seems it doesnt matter as long as you are below the .070 mark thanks
How do you define a venturi on a flash hole ?
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