smg762 said:

one of my ideas for a cartridge involves a modified 5.7mm.   The case is increased to 37mm long - so its very long and thin

I had a similar idea of a lengthened 5.7 cartridge, as a maximum capacity, minimum recoil assault rifle. 

However, my idea was a 5.7 lengthened to 42mm, with a VLD lead free 0.204, projectile weight of ~50gr. 

Goal was to hopefully hit 2750fps+ from a 14.5-16" barrel. This would allow for comparable fragmentation range to current M855A1, but with an increase in magazine capacity, an increase in FA controllability, and a reduction in ammo weight. 

50gr 0.192 G7 @ 2750fps = 1700fps EPR frag range @ 445m, Supersonic to 754m

~40rd will fit in a 30rd 5.56 length magazine

Recoil comfortably below 5.56 or 5.45

Unfortunately I've yet to be able to coax my friend with Quickload to draft the cartridge and run the numbers for what it would actually be capable of.

You are simply replicating the "pencil" type of long thin cases from the late 19th century, which became notorious for being ballistically inefficient. I other words, low muzzle energy per gram of propellant due to geometry against the laws of thermodynamics.   

If you only studied more closely the history of small arms and small arms ammunition, you could avoid proposing scheme after scheme that has been tried decades (if not a century) ago and turned out to have drawbacks that make it impractical.

You're basically trying to reinvent the knoxx HEC which was a military development project to essentially make a straight wall very high pressure 5.56 that got equivalent or better performance to existing 5.56.

Knoxx was fairly close to succeeding, except for the occasional round that would have a wild "pressure excursion event" (300k psi plus near as they could tell) and blow up their mann test actions lol.

Could you do something like that these days?

I tend to think that if you couldn't you could get extremely close.

Also, for the billionth time foot pounds of energy is a trash metric for any sort of performance determination, stop using it.

TBF, some older ideas might actually work now with modern materials and manufacturing techniques. 

Although I think Emeric's neckless case experiment is more likely to work than the one in the OP.

ok ii've modified it. Total length is 67mm.   Scratch built case, with base diameter 8.3mm.  (wider than 5.7)

bullet is a 6.5mm FABRL, which is only 34mm long.  Case preassure 75k. 

could 1800 ft lbs be reached?

could you hollow out the base of the bullet to add more powder?

Also, could you squeeze in a .30caliber sabot, if the neck was extremely thin....  i feel this would be the boost needed to reach 1800ft lbs

Emerics neckless brass case stuff is pretty exciting.

I do very much agree on older concepts becoming viable too. I think that our next round of material improvements is likely already possible and just waiting for someone to dust off an older concept that didn't work previously and update it.

On this note, Caleb crye's 5.56 or 300 bo feed through pistol grip concept is probably one of those things that just needs cleaned up and to have already existing innovation from outside the gun world applied to it to make it viable.

Why 6.5 fabrl?

Also when you scaled up bullet diameter you made sure to equally scale up projectile length with it right?

Otherwise your "6.5 fabrl" won't have the same performance characteristics your idea seems to need.

Gatnerd, do you mean QuickDesign or QuickLoad, because I have QuickLoad, send me the cartridge specs and I'll run them 

in terms of handling high pressures, aren't big game cartridges often straight walled?

would such a cartridge extract realiably?

Er, no. The straight-wall cases are for lower pressure rounds, generally. Those Nitro Express cartridges were big to keep pressures low enough for break-open rifles.