a source for multiple thickness sheets of cork?

[Centurion]

I was raised as an experimental sort, so am not shy of trying something different. The intention of bedding the handguard was to avoid two problems: any weakness that could cause splitting; and to use an absorbant material that would help reduce barrel vibration in the weak barrel of the original No. 1 Mk 111. If the handguards are completely free-floating then they will be relatively weak and there will be nothing to stop barrel vibration in the bedded foreend, whatever rigid method is used. Cork is fine because it is like sponge, to a certain extent. However, I found that I got one inch five shot groups from my 1917 No. 1 by fibreglass-bedding the barrel and using the silicone application in the handguard. That's good enough for me!

[Bruce_in_Oz]

And another thing:

ALL of the traditional "recipes" are contingent on the use of Mk7 ball ammo. if you start fooling around with ANYTHING else, it all goes out the window, exactly as it does when you fool around with loads in ANY rifle.

Note that there were changes to the SMLE bedding and sights with the move from Mk6 to Mk7 ammo. No4s were BUILT AROUND Mk7 ammo, not the other way around.

The bedding acts to damp out SOME excursions but not ALL. The key is consistency: if the barrel wiggles the same way for every shot, the grouping, (bullet aerodynamics permitting) will be consistent. It may not be where "instinct" may suggest, but without a consistent start point, the sight graduations would be mere decorations.

Furthermore, barrels do not just vibrate in the vertical plane. The very fact that the bullet is being bashed through a spiral inside the barrel is a clue to the interesting possibilities. The barrel twists like a torsion bar during bullet travel. This is happening at the same time that various parts of the barrel are deflecting in different radial directions. High-speed photography and a good set of strain-gauges can provide further enlightenment.

The trick is to have the last two inches of barrel pointing in EXACTLY the same direction, (whatever that may be), EVERY time. Once that is achieved, you can redesign the sights to make them useful. Hanging a spike bayonet on the end of a No4 barrel is a proven way of getting a different set of trajectory figures for a given ammo and sight setting.

Remember that there is a lateral offset on SMLE frontsight bases. The "ears" of the nosecap are similarly offset, to accommodate for the REAL WORLD lateral shenanigans of the bullet in flight.

If you change the bullet or its propellant quantity / burn rate / pressure curve, ALL of the designed-in performance gets bent out of shape.

[Peter Laidler]

Never a truer word spoken Bruce in Oz. The voice of common sense........ To really understand the dynamics of a barrel which is really a simple harmonic tuning fork, you need a degree in music

[villiers]

Why are BOTH my No 5s more accurate at 300 metres and a bit more accurate at 100 than at 50?

[Patrick Chadwick]

Patrick, before even hazarding a guess, are you quite sure that your rifle, under identical conditions using identical ammo, preferably on the same day on the same range, actually produces groups with a smaller MOA spread at 300 that at 100, and at 100 smaller than at 50? If so, please give figures. I have heard this sort of thing before, and up to now have had to file it in the "shooting the breeze" category. Proper measurement and documentation would, of course, be a very different matter.

[Centurion]

Well-said Peter and Patrick. I have developed several loads using available propellants and bullets which I am content with. Mind you, I am in my late 50s and limit myself to fair weather 300yd shooting for the vast majority of the time. You can't compare this kind of shooting to that which military loads were designed to accomplish.

[Bruce_in_Oz]

Villiers wrote: Why are BOTH my No 5s more accurate at 300 metres and a bit more accurate at 100 than at 50?

Bullets are not perfect, nor are barrels.

A couple of microns deviation in the centre of mass of a bullet, or, indeed its aerodynamic centre, will set the bullet precessing. It both wobbles AND travels in a helical path around the nominal trajectory. Thus, at shorter ranges, the groups may be relatively smaller but more erratically patterned.

Air resistance has almost zero effect on spin rate, as opposed to linear speed. At any given linear speed, the faster the spin, the more stable the bullet. (Up to a point).

The spin tends to keep the bullet pointing in the direction of the last two inches of barrel at the point of departure. Gravity makes the bullet fall towards the centre of the planet and air resistance starts stripping off speed from the moment the bullet exits the muzzle.

Speaking of muzzles, there is another factor at play, and it ties in with discussions of bullet form. When flat-based bullets, like the Mk.7, exit the muzzle, the transition of the tail is very abrupt; one millisecond it's there, the next, it's not.

Boat-tails have a more gradual egress, relatively speaking. After the main parallel section has passed, the taper of the boat-tail allows gas to start bleeding around the bullet while it is still in transit past the muzzle. ANY imperfection in the bullet or the muzzle will have the effect of applying a lateral force to the bullet, and this force will make the tail of the bullet move laterally. It wobbles, in other words. The boat-tail provides significantly less drag at transonic and subsonic speeds, but, leaving the muzzle, almost all of the drag is from the high pressure zone (shock cone) that is formed at the FRONT of all supersonic bodies.

The No5, being shorter, has a lower muzzle velocity. Consequently, it has a lower spin rate. On top of this, the balance of a Mk7 bullet is towards the rear. Adding the lightweight filler inside the nose moves that balance point even further to the rear than if the core were all lead. Not only that, but, because of its length, Mr. Greenhill's formula dictates that it must be spun at the same speed as the equally long, but somewhat heavier, and slower, Mk6 round-nosed bullet. Thus the Mk7 bullet required the same 1:10" twist. The No5 retained this twist, BUT launched the bullet at a lower velocity, and thus, spin rate (revs. per second).

So, from a No5, the bullet will have a somewhat different balance of gyroscopic and aerodynamic forces to juggle until some equilibrium is achieved.

Turbulence from propellant gases churning around in the flash hider may not be too helpful, either, but is probably a lesser concern. There was a very good reason that the US Army Marksmanship Training Unit used to (and probably still do) open out the flash-hiders on their Match M-14s with tapered reamers.