schnuersi said:

graylion said: So as an overreach replacement for .50, how about a long recoil 20mm? Which one? x82, x102 or x128? In what role? What is the intended target? Who is to use it? What .50 loading? Mobility? Just saying it should have a longer effective range than .50 is not really helpfull. Depending on loading and bullet the .338 Lapua can reach longer effective ranges than common .50 loads. There have been other long range cartidges specifially designed to overreach .50. No need to go bigger.

As a more destructive successor in the same role, also with more range utilising the aiming capabilities of RWS. 

graylion said:

As a more destructive successor in the same role

That is still pretty soft since the .50 is used in several roles.

Since its supposed to be more destructive I assume its for anti vehicle work. Basically any 20 mm outperforms the .50 cal in this regard. The question is what is the desired target spectrum and required range.
The 20 x 139 APDS used by the German Army can effectively engage an BMP over the frontal arc behond 1000 m. It penetrates ~50 mm RHA at this distance. If you want to engage Soviet era vintage AFVs in the 1000-2000 m range band this is the smalles gun that delivers this capability.

graylion said:

also with more range utilising the aiming capabilities of RWS.

Again any 20 mm will do.

nincomp said:

Do you see any advantage to higher-pressure variants of these cartridges?  Not necessarily 80,000psi, but more than the 52,000 psi used by the ARC to protect an AR15's bolt.  BTW, Hornady publishes "bolt gun" loads at 62,000psi for the 6mm ARC, but it is difficult to gage the performance increase vs the "gas gun" loads since since they used a 24" (610mm) barrel for "bolt" data and 18" (457mm) for the "gas gun".

Well, if you increase the pressure, you can also reduce the case diameter.

Compacted loads, as used for LSAT spiral 3 for example, or even aggregated loads used for caseless rounds, are also an interesting path to follow. As a sidenote, the "reduced volume" claimed by Textron for their telescoped rounds (versus conventional rounds) was a direct consequence of the use of compacted powder loads.

The FAMAS MSD powder load was 2.4 g and the cartridge volume was smaller than the 5.56 x 45 mm case. Using an aggregated load of common NC powder allows a loading density of ~1.5 g/cm^3 (instead of <1 g/cm^3), so suddenly you can put your typical .22 Nosler charge into a .222 Rem case.

When you write about "compacted powder loads", are they make with common powders literally pressed tighter in the case, or are they something else?

FWIW, it seems that Hornady compacts regular powders more than most to make its SuperPerformance ammunition.  I read in the 6.8 SPC forum that some "standard"  Hornady factory ammo is compacted so much that the cases bulge slightly.  One barrel maker recently noted that his chamber reamers  have been modified to make extraction of these rounds more reliable.

schnuersi said:

The 20 x 139 APDS used by the German Army can effectively engage an BMP over the frontal arc behond 1000 m. It penetrates ~50 mm RHA at this distance. If you want to engage Soviet era vintage AFVs in the 1000-2000 m range band this is the smalles gun that delivers this capability.

I was thinking defeating STANAG 4

Yes, you've seen exactly why I like it. 

Now for a couple things you may not know.

There's evolutions of this basic design in sizes up to 35mm fire out of battery very low recoil autocannons.

Like you said, it's very independent of operating principle and caliber.

It also matches the way we use mounted and dismounted guns currently extremely well. (By this I mean that by separating out the housing into it's own thing that the gun isn't dependent on and doing the same with feed system and other components  you essentially have a Lego gun you can adapt to anything while all still using the same basic internals etc)

There's also some other things about how the basic concept works that make it very amenable to being reconfigured drastically very quickly in the field with minimal tools, knowledge, or spares.

Basically if you did this right you could turn a helicopter or rws gun into a saw with a couple quick module swaps and vice versa.

There's also the manufacturability factor.

You could turn these things out in the thousands very easily and at a fraction of the cost of other guns.

graylion said:

I was thinking defeating STANAG 4

20x139 will do that easy. All 20 mm cartidges down to 20x102 with an APDS shot will do that. 20x82 most likely not. Especially since there is no APDS avaiable.

"Compacted powder load" are regular powder loaded in 3-4 partial steps, with a tool compressing the load after each partial charge.

https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2012/armaments/Wednesday13627drummond.pdf

So, instead of putting 1,8 g into the case, you drop 0,6 g, compress the load, add 0,6 g, compress it, then the final 0,6 g and finish compression. It seems that you can reach ~1,2 g/cm^3 with ball powder.

"Aggregated load" is regular powder with an energetic binder (2%), hot pressed into a mold, and you can achieve around 1,5 g/cm^3 this way.

A mix of 50/50 Polyvinyl Nitrate and DiNitroPolystyrene, diluted in Cyclohehanone and Acetone, is a good binder. You spray the grains with the mixture, allow the solvent to evaporate, then compress the powder (10 MPa) in a mold at 125°C.

schnuersi said:

graylion said: I was thinking defeating STANAG 4 20x139 will do that easy. All 20 mm cartidges down to 20x102 with an APDS shot will do that. 20x82 most likely not. Especially since there is no APDS avaiable.

Thing is that you can get light RWS for 30x113, but not for 20x102. Hence 30x113 may be easier. Less clever maybe, I'll agree. So you can get a 30x113 on your APC. I have found API rounds for 30x113, but no idea whether they'll defeat STANAG 4.

If we want 20x102 on an RWS, we may need a long recoil gun. And I don't see the point. 20x102 has an E0 of ~50kJ, 30x113 85 kJ. But I get that there is no APDS available at the moment from what Uncle Google tells me. Would there be any point to developing it? And any idea of the armour penetration of API?

graylion said:

Thing is that you can get light RWS for 30x113, but not for 20x102.

^^ ... sure you can. But 20x102 is not exactly common for AFV use.
Here is one for 20x139: https://www.kmweg.de/systeme-produkte/bewaffnungssysteme/fernbedienbare-waffenstationen/flw-200plus/

graylion said:

If we want 20x102 on an RWS, we may need a long recoil gun.

Why would that be the case? The recoil impulse of the 30x113 is significantly higher than even that of the 20x139.

graylion said:

And I don't see the point. 20x102 has an E0 of ~50kJ, 30x113 85 kJ

The energy density is more important for penetration than pure energy. The surface area of a 20 mm round is less than half compared to 30 mm. But the KE of a 30x113 is not twice that of a 20x102 wich is the least powerfull 20 mm AC cartidge (save of the 20x82). Energy density is also important to retain KE over range. Somthing a 20 mm APDS is very good at. The 30x113 full bore shells not so much.
The API you found most likely is an old design and was used for the DEFA or ADEN cannon. These AP shots have very low penetration for a 30 mm cartidge. For the M230 usually only HEDP is used and the penetration is not exactly stellar. It will hardly be able to penetrate STANAG K4 unless at optimum impact angles. The 30x113 was not developed with armor penetration in mind. Its a high payload shell for use from aircraft. The 30x173 was developed for ground to ground and ground to air use. Its penetration is very good. But the recoil is high and the guns are on the large and heavy side.

I just mentioned the 20x102 mm because its a pretty common 20 mm load and the least powerfull. Personally I think 20x139 is a better choice.