schnuersi said:

5.7 has less recoil, flatter trajectory and larger capacity magazines. Which makes it the better choice.

Less recoil would help, but the flatter trajectory won't make a bit of difference for most soldiers.

And we're looking at 20-rd mags for the FN 5.7 pistol versus 17-rd mags for the 9mm M17 pistol.

The hard truth is that 5.7 is just not enough better to justify the change.

Especially seeing as how pistols are "almost useless" in military combat.  

schnuersi said:

I am also curious how a saboted penetrator like the one of the 6.5 CBJ but fired from a 5.7x28 or 4,6x30 would perform.

That seems somewhat less than feasible. 

6.5 CBJ penetrator is only 4mm diameter.

stancrist said:

The hard truth is that 5.7 is just not enough better to justify the change. Especially seeing as how pistols are "almost useless" in military combat.

Not by itself I agree. But if 5.7 would be introduced as a cartidge for mass issue of PDWs it would make a lot of sense to change the pistols over as well.

stancrist said:

Less recoil would help, but the flatter trajectory won't make a bit of difference for most soldiers.

I was thinking of PDW use. At 50 or 100 m the trajectory of 5.7 or 4.6 does make a difference. Compared to 9x19 there is a noticable difference in hit propability.

stancrist said:

That seems somewhat less than feasible. 6.5 CBJ penetrator is only 4mm diameter.

The current AP loadings for 4,6 and 5,7 do not use tungsten. They are all with steel core. Since both have ME in the same region as 6,5 CBJ there most likely will be an increase in penetration.

For 4.6 it propably won't make much sense to use a 4 mm slug with sabot. A 4 mm hard core with a lightweight bullet seems more feasible.

For the 5.7 it might allready be feasible to use a saboted 4 mm slug.

stancrist said:

Only the Swedes would think an 8.4-lb rifle is actually suitable for use as a PDW or secondary weapon.

Actually no.
The German Army used to equip allmost every infantry man with a G3 rifle. The Swedish Ak 4 is a variant of it. Regardless if it was the AT gunner, grenadier, ATGM team or whatever. They all had an G3. I can tell you its not fun to carry a leichte Panzerfaust with two reloads plus rifle with ammo. Later it was Panzerfaust 3 with one reload. But these are very heavy and unwieldy. The carring sling from Panzerfaust 3 was designed by a sadist. Its hard, scratchy and very narrow with "sharp" edges.
The German Army still does this. Only the rifle changed. The G36 is a little lighter. But this is the general issue weapon for every infantry man except the machine gunner.

schnuersi said:

       stancrist said: The hard truth is that 5.7 is just not enough better to justify the change. Especially seeing as how pistols are "almost useless" in military combat.

Not by itself I agree. But if 5.7 would be introduced as a cartidge for mass issue of PDWs it would make a lot of sense to change the pistols over as well.

Of course.  It makes no sense to keep 9x19 pistols along with 5.7 (or 4.6) PDWs.

schnuersi said:

For 4.6 it propably won't make much sense to use a 4 mm slug with sabot. A 4 mm hard core with a lightweight bullet seems more feasible.

Concur.

schnuersi said:

For the 5.7 it might allready be feasible to use a saboted 4 mm slug.

I've never heard of any .22 caliber cartridges with saboted projectiles.  Have you?

schnuersi said:

       stancrist said: Only the Swedes would think an 8.4-lb rifle is actually suitable for use as a PDW or secondary weapon.

Actually no. The German Army used to equip allmost every infantry man with a G3 rifle. The Swedish Ak 4 is a variant of it. Regardless if it was the AT gunner, grenadier, ATGM team or whatever.

Okay, I stand corrected.  I'll rephrase: 

The Germans used to think an 8.4-lb rifle is suitable for use as a PDW or secondary weapon. 

The Swedes currently think an 8.4-lb rifle is suitable for use as a PDW or secondary weapon.

And the Americans think an 8.4-lb rifle will be suitable for use as a PDW or secondary weapon.

stancrist said:

I've never heard of any .22 caliber cartridges with saboted projectiles. Have you?

SPIW? Die Steyr entry into the ACR program?
But I agree saboted projectiles are a rarity for small arms. I am also not convinced it is necessary for a weapon with intended 100 m effective range.
My comment was more about technical curiosity. If it is possible and what the effect would be.

stancrist said:

The Germans used to think an 8.4-lb rifle is suitable for use as a PDW or secondary weapon.

Yes in the pre body armor era.
The planning is most likely done on the base of the complete loadout. Not so much individual pieces of kit.

stancrist said:

And the Americans think an 8.4-lb rifle will be suitable for use as a PDW or secondary weapon.

From my point of view it seems the US really is a special case. Most of these things seem to be based in the idea that a rifle is an effective and decisive weapon and the idividual rifleman can decide engagements. Issuing a powerfull and long ranged rifle just seems the logical conclusion of that line of thinking.