gatnerd said:

Thats certainly the ideal...

Perhaps I am missing something.  How is it "ideal" to design a cartridge in complete disregard for weapon characteristics like barrel length, feeding reliability, magazine capacity, etc?

gatnerd said:

The question is whether we will actually see that occur.

I would submit that we sorta have seen it happen with the 6.8 TVCM, which was designed without regard for what seems likely to be the acceptable barrel lengths of NGSW-R and AR.

gatnerd said:

Theres been talk of a desire for a 'GPC' type intermediary cartridge for 15+ years, yet theres yet to be any manufacturer step up to the plate and create an intermediary sized weapon.

Nearly all of the talk about an intermediate-length GPC has been in internet forums like this one. 

There's been little military interest in such a cartridge, and no real demand in the civilian market.

gatnerd said:

Perhaps the LWRC SIX8 as an exception. 

No.  The SIX8 receiver length is the same as that of 5.56 receivers.

That's because cartridge OAL of 6.8 SPC is the same as 5.56 NATO.

gatnerd said:

Do you think it would be backwards compatible with current .50 bmg weapons?

That will depends on the weapon manufacturing year, probably.

For example, ours M2HB from WWII are in perfect shape and could fire a regular diet of M33/M8 ammo (working at 280-290 MPa) all day long, but if you feed them with some Short-Range Training Ammo (SRTA) running at 350-380 MPa, you will break the bolt locking lug after around 4,000 rounds, even if the STANAG allows for a MAP + 3 SD < 445 MPa.

Current production M2HB are OK with those SRTA ammo.

gatnerd said:

In terms of down range performance, do you see the value of the .61 primarily in increasing penetration, or increasing incendiary payload?

Payload ! Just like the 20 mm MG151 versus the 15 mm version, or the 20x102 vs. the 14.5x114.

If you want to increase the penetration while keeping the .50 "Form Factor", then the .460 Steyr or the .416 Barrett are the way to go (similar core length, but higher MV and better BC, so much more impact velocity).

EmericD said:

Payload ! Just like the 20 mm MG151 versus the 15 mm version, or the 20x102 vs. the 14.5x114. If you want to increase the penetration while keeping the .50 "Form Factor", then the .460 Steyr or the .416 Barrett are the way to go (similar core length, but higher MV and better BC, so much more impact velocity).

In terms of ground use, what would be the primary goal/effect of increasing the payload? Would it be for improving the ability to set cars/technicals on fire over current .50? And is the current .50 lacking in that regard? Or would the payload offer some other benefit, such as improving the ability to shatter/smash through walls and concrete? 

In terms of concrete penetration, is the rule similar to steel and ceramic,  where core length is paramount? 

Great info on SRTA/pressure on bolt life.

One bureaucratic snafu on the .61. Being over .60, that would make it a light cannon, rather than a machine gun by US military definition.

Other than that, it would be awesome!

gatnerd said:

Would it be for improving the ability to set cars/technicals on fire over current .50? And is the current .50 lacking in that regard? Or would the payload offer some other benefit, such as improving the ability to shatter/smash through walls and concrete? 

Today, most APCs are already immune to 14.5x114 mm AP rounds on the frontal and lateral arc, so I don't think trying to increase the AP capability of a .50" is the way to go.

The .50'' SLAP is already existing, fielded and delivering 14.5 mm AP performance, but only the USMC is using it, and in limited numbers.

The .50" MP, on the other hand, is used by nearly every western armies, and gives decent AP capability + 1 g of HE payload and fragments. Those fragments are not going to penetrate any armor but are great to defeat cameras, sensors, antennas, RWS... located on every current APC, and also to effectively defeat "technicals" or suicide bomb cars.

The current way to "upgrade" the M2HB is to replace it with a 40 mm "High Velocity" grenade launcher, so a truly "high velocity" round that retain the external ballistics of the .50 BMG, but with a bigger payload (the .61" vs. .51" is a ~20% increase of the diameter and ~70% increase of the volume) may be the way to go (or the diameter should be increased to ~18 mm to carry enough HE to justify the upgrade, like the experimental 18x81 mm Tarantula).

The problem with the 14.5 mm / 20x102 mm / 30x113 mm upgrade is that those 3 rounds are using twice the propellant load of the .50" BMG, you simply can't expect to replace a M2HB with those without any side-effect.

for a 50 replacement, what are you thoughts on a higher powered round which is designed around a very potent Apfsds round. (all of it's ammo options are telescoped including API)

lets say the muzzle energy was 20k Ft Lbs....would the dart be much use against vehicles in realistic scenarios?

Why not just do this?

Yes, a full auto version of the IWS. 

Seriously though, wouldnt an APFSDS with 20k ft lbs be more destructive than an API round?

I came across this truly superb article on the effectiveness of a 20mm AMR. 

Worth checking out. Theres penetration fragmentation data, long range ballistics, recoil impulse data, surveys of current off the shelf systems, and potential design features of a future AMR rifle:

https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2009/infantrysmallarms/thursdaysessionxi8503.pdf

Theres 48 pages of great data in there, but the conclusion is: