More snippets about SMLE barrels

Here's something interesting about early SMLE (No1 Mk111) barrels.

The rifling is tapered. (Or at least, supposed to be.)

Reference is: A. I. D. 483F, to accompany Specification S. A. 307F, 2 Sheets.

Section view of rifling shows:

Dia. of Bore .303 to .305 Rej(ect)
Depth of Rifling .0055 (at 14" from muzzle) to .007 at muzzle

Longitudinal section drawing shows-

At the muzzle:

Dia of Bore .303 Acc. .305 Rej.
Dia of Rifling .3155 Acc. .3205 Rej.

At 11" from the breech end:

Dia. of rifling .3125 Acc. to .3175 Rej

What were they thinking?

Later (circa 1938) drawings effectively specify a parallel bore and groove, with high and low tolerances, however the maximum groove diameter is still far in excess of nominal bullet diameter.

I do not have details of the rifling cutter setup. What I do know is that a lot of effort went into lapping the bore to a mirror surface before rifling. This often produced barrels that are larger in finished bore diameter at the muzzle than in the middle. By current barrel making philosophy, this would be regarded as "not a good thing".

Looking through a few "new" (as in never fitted, just out of the wrapper) late production Australian SMLE barrels, it is very clear that the bores were lapped to a fabulous finish, while the grooves show very obvious striations from "fuzzy" cutter edges.

Flat based Mk7 projectiles will be hammered into the rifling grooves at launch. The problem is that as the bore pressure drops closer to the muzzle, it is likely that the bullet will cease to be forced into the grooves. If the rifling tapers outwards towards the muzzle, gas blow-by will occur. If the taper is bad enough, there will be a lot of very interesting turbulence acting on the base of the bullet at departure. If you are silly enough to shoot Mk8Z or other boat tails in such a barrel, it will get even more disconcerting.

If these barrels were rifled as a large OD blank the final profile turning may have opened up the bore by relieving stress. This would have resulted in opening up the muzzle more than the breech end.

Current custom barrel manufacturing practice is to mark the large end of the blank so it is used for the chamber. The blanks are normally made longer than necessary so the tapered rifling (last 1.5 to 2 inches), if any, at the muzzle is cut off. Many so called custom barrel these days are not lapped at all.
The result is the lands have rough reamer marks on top and the grooves are smooth from being ironed by the rifling button.
I have several never fitted Mauser barrels and apparently they lapped after rifling. They show almost no tool marks in the bore and what lines you see are lapping lines that run the length of the bore. There are no reaming marks on top of the lands.

If you slug your barrels with a lead plug at each end and measure the bore and groove diameters you will often get a surprise. It is also good to push a lead slug all the way through the bore. Some times you may find a constriction. If you are a cast bullet shooter it is a rude surprise find a "trumpet" shaped bore. They are usually considered difficult to get to shoot well.

Tapered rifling in the SMLE is described elsewhere, including Skennerton's The Lee Enfield Story. That type of rifling was adopted with the SMLE Mk 1 in 1902, but as early as 1904 a commission found that it resulted in reduced accuracy. In typical British fashion, the specifications were never changed and tapered rifling remained in production until 1917 when the change to parallel rifling was made as a wartime expedient.

What were they thinking? Well, the original idea was to increase muzzle velocity to make up for the shorter barrel of the SMLE. Apparently that goal was met, but at a loss of accuracy and, naturally, increased trouble and cost in production.

The rifling was cut to a depth of .00575" from the chamber to 14 inches from the muzzle, at which point it was deepened to .0065". So it really wasn't tapered in the sense of a constant rate change, but rather a sort of "sloped step" at 14" from the muzzle.

Jim

If I remember correctly, the No8 rifle decreased the depth of rifling towards the chamber so as to do something fractionally marginal with the rifling grooves on the bullet. I had to learn it parrot fashion once but didn't understand the reason then either

L.R. Tippins, in his book, noted the tapered rifling and criticised it heavily, as he did most features of the Short rifle.

Quote:

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Originally Posted by Mk VII

L.R. Tippins, in his book, noted the tapered rifling and criticised it heavily, as he did most features of the Short rifle.

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I'm not familiar with Tippins book James; can you supply more details of it please?

Modern Rifle Shooting In Peace, War and Sport. At least 4 editions, the latest dated 1913.