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Steel AP & caliber choice   Ammunition <20mm

Started 18-Sep by hobbes154; 1464 views.
hobbes154

From: hobbes154

18-Sep

It is often stated that a requirement for steel cored AP favours larger calibers than would be dictated by external ballistics alone. See e.g. the .270 British or various posts in the Design Space thread.

Is this purely because steel is less dense than lead, so you can't obtain the same sectional densities with steel AP in smaller calibers? E.g. in this pic of .30-06 ammo M2 AP (far right) is much longer than M2 ball (second from right). 

I ask because .30 caliber bullets are often criticised for relatively poor sectional density, which I assume would be equally bad for ballistics and penetration, but that might only apply to ball ammo.

(Just thinking of pure AP, not API where volume obviously matters separately from SD.)

QuintusO

From: QuintusO

18-Sep

This is a very complex topic that involves many factors, but basically yes, if your projectiles are less dense then typically they need to be larger. Sectional density scales with caliber linearly, so if your absolute mass density is lower then you need a larger caliber and heavier projectile to reach equivalent sectional density for a given projectile volume.

Another advantage the larger calibers probably had is they tended to be a bit less streamlined which probably helped their presentation against armor. But take that as speculation, not fact.

hobbes154

From: hobbes154

18-Sep

Ta.

Edit: so it's still basically linear with caliber, like with larger guns - there isn't some special threshold around .30 for some reason?

Edit 2: seems funny the Brits would have cared so much about .27 vs .25 then, but I guess that's what hard performance thresholds (10mm @ 100yds or whatever) do.

  • Edited 18 September 2020 23:02  by  hobbes154
QuintusO

From: QuintusO

18-Sep

hobbes154 said:

there isn't some special threshold around .30 for some reason?

Depends on era. .30 cal in 1920, for example, is pretty much the intersection between reasonable size and recoil and enough projectile volume for AP and tracer projectiles with the technology of the time.

It was a legitimate sweet spot for 50 years.
 

hobbes154 said:

Edit 2: seems funny the Brits would have cared so much about .27 vs .25 then, but I guess that's what hard performance thresholds (10mm @ 100yds or whatever) do.

The great irony is that the Ideal Caliber Report is an excellent piece of work. The Brits subsequently threw it out, and went with .280, which was really not that good of an idea, all things consider (it had inferior performance to .30 T65, with almost no attendant advantages).

QuintusO

From: QuintusO

18-Sep

Something worth remembering about 7.62 NATO specifically is that they had to entice western nations to adopt it as both a rifle and machine gun round. That's a lot of requirements ground to cover, which implies a fairly capable round. It's actually pretty remarkable it ended up as small as it was, rather than say 7.5x54 French sized.

autogun

From: autogun

19-Sep

QuintusO said:

Something worth remembering about 7.62 NATO specifically is that they had to entice western nations to adopt it as both a rifle and machine gun round. That's a lot of requirements ground to cover, which implies a fairly capable round. It's actually pretty remarkable it ended up as small as it was, rather than say 7.5x54 French sized.

The French did not have access to the new powders used in the 7.62x51, and were initially unable to replicate the performance of the US loading.

autogun

From: autogun

19-Sep

QuintusO said:

The great irony is that the Ideal Caliber Report is an excellent piece of work. The Brits subsequently threw it out, and went with .280, which was really not that good of an idea, all things consider (it had inferior performance to .30 T65, with almost no attendant advantages).

Yes. I have yet to see any official documents which might shed some light on exactly how (and who, and when) the Report was binned.

nincomp

From: nincomp

19-Sep

Where can I find a copy of the "Ideal Caliber Report".  My google-fu is weak tonight.

autogun

From: autogun

19-Sep

nincomp said:

Where can I find a copy of the "Ideal Caliber Report".  My google-fu is weak tonight.

There's a detailed summary starting on page 15 of THIS.

gatnerd

From: gatnerd

19-Sep

Emeric had mentioned that penetrator length was a huge factor in actual armor penetration.

From memory, I believe he quoted 32mm penetrator length = 20mm tungsten penetrator in terms of AP performance. 

So in that regard, a wider caliber allows you to have a overall longer projectile while retaining bullet stability.

What role that played in historical calibers, I have no idea. But for a future AP load designed to penetrate Level IV ceramics with steel penetrators, that could be a factor.

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