When Hebler and Krnka experimented with tubular projectiles about 130 years ago, the goal was to dramatically reduce air drag. It was believed, the inner cross section would be passed by the air undisturbed. 

They could not know the peculiarities of supersonic aerodynamics and the creation of shock waves inside the tube by its "mouth". The plan works only at very high Mach numbers. If velocity drops beyond a critical value (dependent on tube diameter and length), the tube flow becomes choked. Therefore, modern existing tubular bullets (like the one shown by Tony, or the defunct Rheinmetall LKL-projectile) are used for achieving an additional brake effect. They are training projectiles for shorter ranges, not enhanced ranges. 

Hopefully someone fit in aerodynamics on this forum can tell us the approximate Mach number at which a 5.56 mm tubular projectile would work as intended. I would not be surprised if it were useful only while it flies well above Mach 3.          

Vaguely returning back to the original subject,  could you have a bullet with striations running longitudally along its length. (From tail to nose)

To get a visual idea,  Google bullet striations and look at the 9mm rounds.  Imagine that but Much deeper. 

They could either be straight,  or angled which would add spin to bullet. 

Then you have a wad behind the bullet. 

The idea is that it gives the drag profile of a tiny caliber bullet,  but the overall bore size is closer to. 22 - thus it gives no barrel wear. 

Also,  someone has a patent for a disk shaped bullet

https://patents.google.com/patent/US20140150319A1/en

Regarding jpeelens thoughts on the tank training ammo being short ranged,   a rifles tubular ammo would only need be stable at 600m

Aerodynamics is such a fascinating subject!

If useful only above a high Mach number, tubular projos could find a new life through rail guns, whose velocities are usually very high.

Does anyone have any thoughts on my striations idea?  Assuming it's viable I think it's far superior to tubular/sabots or anything else. 

Remember the idea is to emulate the drag profile of a microcaliber,  and yet avoid the barrel wear problems

smg762 said:

This conflicts with other sources which said it could be up to 33% less accurate than normal 762. 

Well yeah, but in still air 7.62x51 can shoot .5 MOA at 1000 metres. The whole point to sabot is to improve accuracy in less permissible environments. 

smg762 said:

Remember the idea is to emulate the drag profile of a microcaliber,  and yet avoid the barrel wear problems

What barrel wear? That is unrelated to calibre. 

Well as my posts often show I'm a proponent of calibers around. 17 or so.  With very heavy-for-caliber bullets,  you get extreme range yet very low recoil and weight. 

Obviously the drawback is major barrel wear.  So the striation idea was to emulate a. 17,  but with a bore size closer to. 19 or. 22

Do you intend for the striations to impart the spin rather than rifling?

I haven't thought that far ahead.  The big 'sell' was to literally decrease the drag profile.  It's almost like a finned bullet. 

First issue is whether the reduced bearing surface is problematic. what are the implications for rifling.  And could it increase barrel wear. 

Second of course is whether a straight/angled set of cutaways would actually improve drag. 

A non spinning projo would definitely have less drag with straight grooves. But ours has to be spinning.... 

So the first thing I'm going to say is wrt the striations idea, and to that I'll simply say supersonic and even subsonic drag doesn't appear to work the way you think it does.

If you want to see what a low drag projectile looks like take a look at sears haack bodies and the work of von karman.

Next, onto barrel wear.

It turns out if you aren't speccing a midgrade steel from 70 years ago... It's just not really a problem... Like, at all.

Look at what HK and Daniel defense are doing with actual good bbl steel now and realize that even if you run peak pressures up to 70-85k psi it's very possible to make barrels with acceptable lifespans affordably without doing anything exotic.