could one improve on the 338 lapua, in terms of hit probability.

for example a 338 case launching a saboted 6mm at 4000fps. would this be practical , what would the drrawbacks be for a modern sniper.

my other question is, some rifles like the IWS2000 and HK MEN 9mm, used a recoiling barrel. how did they expect to be accurate at 2000m?

Speed is a rapidly diminishing commodity,BC  like diamonds is forever, a 4000 FPS round would be down to 3000 at 100m or so after that the poor BC of a 6mm bullet would suck big time plus you still need to 'kill' a human sized target at extended range , for that you need mass, Improving 338 LM has been going in two directions.

300 and 338 Norma push the shoulder back to better utilize higher BC bullets and .375Swiss P ups the ante by going higher in caliber 

300Norma builds on new 30cal bullets that are already approaching 240grains more than so matching the BC  .338bullets, problem like always is the MAGsize OAL  , you could single feed much more potent .338 bullets but not in current actions.

High BC solids have been around long time and on paper offer huge gains, but its all in the razor-sharp tips that extend OAL vell past mag length

well apparently the swiss used a sabot round and it improved hit probabilyt. Surely if you scaled it up to lapua-energies, with a very heavy 6mm/ .257 bullet, it would match the range of a 338, with far better probability?

and what about the recoilling barrels on those weapons...would they need a special optic or laser FCS to achieve accuracy?

The group size was bigger but increased hit probability relied partly on the flat trajectory minimizing errors in range estimation.  That is less of an issue with modern rangefinders.  Remember, it is being compared to older 7.62x51 designs.  It would not compare as favorably against modern sniper systems.

There is more detailed info on the Swiss ammo ballistics somewhere on this site.  It was discussed several years ago, so a good enough search may find it.

Here is some info from another forum, ironically, with most of the info coming from Tony Williams and JPeelen who are regulars here:

https://forum.cartridgecollectors.org/t/m948-slap-ballistic-information/27474

regarding my 6mm at 4000fps, if it was a 120grain with extreme VLD shape, how would the BC compare to a 338lapua military loading?

and what weight do UK snipers use for 338 bullets

There is a huge gap in BCs 

Berger

6mm 115g VLD bullet

if you had a .257 bullet with a .330 G7, would it suffer much at 1.5km, compared to 338?  surely the difference would only be apparent once you reach 2 miles or so

Its probably still would be hard pressed to compete ,but most importantly .338 brings energy on target and bullet lenght enables decent AP cores to be used . Pushing anything not saboted at speeds past 3500fps seems to be kind of a fools errand as barrels get toast extremely fast , we are not talking 1000 rds but 600rounds like in some small bore magnums.  

.25 cal bullets with high BC are kinda new and at present are being tried out quite successfully in necked down 6GT and 6.5x47L parent casses.

As ballisticians say cartridges and chamberings follow bullet design, not the other way around, so we will see if 25 gains traction , chamberings for it will apear

A friend of mine that has a PHD in fluid dynamics and occasionally does ELR says that from his knowledge of fluid dynamics, the right caliber for the range will be stumbled on by trial and error, he says things do not scale well at all  and each medium and speed range has own optimum, so he says let's imagine we shot 338, 375,408,416,50 cal at ELR  with enough data points one caliber or bullet will pop out of the order it might need to be completely new caliber which he assumes is more likely than not but given enough trial and error one caliber will prove supreme because its closest to optimum 

All these are shot from .308 sized magazines and actions.