stancrist said:

Yeah, but the M4 is really just an upgraded version of the Vietnam era XM177 series

Sure, but while the rifles are extremely similar externally, its also a case where nearly every part has been changed/upgraded between them:

-swap 11.5" barrel for 14.5"; replace flash/muffler with flash hider

-different upper receiver

-different forend

-different pistol grip and stock 

-different gas system length 

-different buffer weight 

-ambidextrous safety

-change from 20 to 30rd mag's as standard 

-multiple changes in 5.56 ammunition (from M193 to M855 / MK318 SOST / Mk262 77gr / M855A1 EPR) 

....

"In the preceding 60-year period, the Army fielded four different rifles and calibers (.30-03, .30-06, .30 Carbine, 7.62 NATO."

Sure, but these are all cases where these rifles/cartridges really weren't that great, and where small arms (really all arms and mechanical objects) were rapidly advancing.

But once you go from steel & wood to aluminum & polymer, and go from 24g+ cartridges to 12g cartridges, the options for progress then start to decrease. 

I feel like rifle designs are following a similar trend to aircraft and ships, where the primary upgrades are no longer raw mechanical performance, but rather the technology being bolted onto the machines.

...

The other aspect of rifles, highlighted by the ACR report and seen throughout various combat campaigns, is that even if you make the rifle twice as accurate, it's much harder to make the soldier twice as accurate, especially under combat stress. 

And in close range fights, like what we're seeing in Ukraine, rifle optimization seems to matter very little at all - those trench clearing videos we see from both sides could have been accomplished nearly as well with STG44's as they could have with any of the latest rifle and optics setups. 

gatnerd said:

       stancrist said: Yeah, but the M4 is really just an upgraded version of the Vietnam era XM177 series

Sure, but while the rifles are extremely similar externally, its also a case where nearly every part has been changed/upgraded between them:

???  That's what I said.  The M4 is an upgraded XM177.

gatnerd said:

-change from 20 to 30rd mag's as standard 

-multiple changes in 5.56 ammunition (from M193 to M855 / MK318 SOST / Mk262 77gr / M855A1 EPR)

Neither of those are changes to the rifle.

The XM177 does not require any alterations to use 30-rd mags, and the reason those mags were not standard until after Vietnam is because it took time to fully develop them.

There have been no changes in the standard caliber, which has been solely 5.56x45mm.  And there were only two changes in the standard Ball loading:  M193 > M855 > M855A1.

stancrist said:

Yeah, but the M4 is really just an upgraded version of the Vietnam era XM177 series.

That's not really what Colt, the Army and the Court had in mind when FNMI challenged Colt sole-source M4 production contract and the Court validated Colt as sole source supplier up to 30 june 2009...

I don't understand.  What does the FNMI court case have to do with my comment?

stancrist said:

I don't understand. What does the FNMI court case have to do with my comment?

The US Court validated that "the M4 was not a part of the familly of M16 weapons, but was actually part of a new familly of M4 weapons".

Colt and the Army formalized this recognition via so-called "M4 Addendum" to the 1967 licence.

The Black Rifle Vol II, p86, Collector Grade Publications.

The question could be, at which point an AR-15 "derivative" is no longer an AR-15?

is there much benefit in using 77grains ammo in 11-13 inch barrels?  or does it just create more bullet drpo

whats the point of diminishing returns, where a 77grain would actually reduce the range?

IDK.

EmericD said:

The US Court validated that "the M4 was not a part of the familly of M16 weapons, but was actually part of a new familly of M4 weapons".

ROFL.

stancrist said:

ROFL.

Their words, not mine!

I agree that this ruling seems strange, but more recently another court validated that 3 draining holes in a tube extension was a valid patent... so, from an IP point of view, a HK416 doesn't belong to the M16/M4 familly.

Hence my question, what is making an AR-15?

Direct Impingement doesn't seem to be a factor, because you can find "piston-driven" AR-15.

The bolt carrier retracting into a tube extension (acting as a stock) seems more positive, but some guys are relocating the recoil springs on the upper receiver to enable a folding stock, so that's not critical to the design...

The bolt carrier shape and multi-lug bolt?

EmericD said:

The bolt carrier shape and multi-lug bolt?

I don't think so

Look at the Johnson M1941 rifle bolt group: https://smallarmsreview.com/m1941-johnson-rifle-ar-15-progenitor/