A Theory offered by CINDERS

[Bruce_in_Oz]

Several thoughts.

"Good old Cordite" burns faster than AR-2206(H) and WAY faster than AR-2208.

The solid-based SMKs being boat-tailed, do not "set up" the same way that the traditional, open-based Mk7 bullets do.

.303 barrels can run out to some horrendous GROOVE diameters, eg. .320" and STILL be in spec.

Furthermore, the FACTORY practice of hand-lapping a strange taper in the barrels lead to BRAND NEW barrels having belled muzzles.

Thus, a boat-tailed, SOLID-based bullet is going to require a SERIOUS boot up the backside, if it is to be "upset" to fully seal the distance between the .312" bullet starting diameter and the potential .320" groove diameter.

Without a complete and CONCENTRIC gas seal, the bullet will be off to a bad start as it gets going. If it is still "off centre" and POSSIBLY NOT perfectly aligned with the bore, when it comes time to exit the muzzle, it will get really weird, moreso if the crowning is not "perfect".

With a less-than-perfectly aligned bullet, as the boat-tail starts to exit the muzzle, the gas bleed will be uneven, causing more "bullet tilt" and thus, precession spiraling, in turn causing randomly enlarged groups.

In the real world, all bullets fired from rifled barrels, precess to some extent. Even good old Mk7 ball does it. Talk to the "old guys" about bullets "going to sleep" (becoming more predictably stable) as they go down range. In Mk7, the design of the bullet, i.e. seriously base-heavy, adds to this phenomenon. FBHPs, like the new "Mk7 equivalent" FBHP from the Bullet Factory, will also do the same.

Interesting little, sort-of related video [URL="https://www.youtube.com/watch?v=7kB__9CteSg"]here.

Hornady have an interesting prospect, here: https://www.hornady.com/bullets/rifle/303-cal-312-150-gr-interlock-sp#!/

Note NOT a 174gn bullet.

One further "interesting" issue is the state of the throat / leade.

ALL .303 rifles made for service ammo have LONG throats, because they had to be able to handle ANY service ammo of that calibre. MAYBE late production N04 and No5 barrels were made with shorter throats, but ALL service SMLEs had long ones, originally designed to handle Mk6 and earlier ammo. SAME overall bullet length, give or take a couple of thou, BUT radically different profiles and thus bearing surfaces.

A hard-worked throat (LOTSA Cordite) will be a somewhat longer than when it left the factory. This unwanted free-bore WILL alter the pressure curve and also PEAK pressure AND muzzle velocity like.

Keep us posted.