5R rifling

Butch Lambert said:
I've read some of what you gave as reference and actually have a piece of a barrel of about 2" inside diameter made that way. I'm looking for the tests that show the barrels get more accurate after 3500 rounds.
Butch
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That was proven by arsenal accuracy test barrels long ago, specifically .30-06 and .308 Match grade ammunition testing, by Frankfort Arsenal IIRC.
Thats always been rather commonly accepted, and I believe Whelen and Hatcher also took note of this.

Now these were military barrels, not custom finely lapped bores pretty much pre broken in like some target barrels of today, so the same process may not work out that way.
The Military bores improved with use for the first 3500 rounds leveled out for a few thousand more, then began to degrade in long range performance.
Accuracy standards of the Pre WW2 era were such that barrels were generally swapped at 4,500-5,000 rnds before degradation of long range performance set in, but Whelen considered 18,000 rnds to be the usable bore life for combat uses.

Hotter loads of course burn out throats more quickly.
 
Butch Lambert said:
So I guess this has nothing to do with competition barrels.
Butch
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It may depending on the barrel. The Accuracy barrels used for testing ammunition by lot were most likely cut rifled, not button rifled, or hammer forged, whether the last two would react the same is hard to say.

In general the effect is that mechanical erosion of lands is gradual, resulting in a tapering of the bore, the bullet is presented with an ever tightening opening so sizing of bullet to bore is ongoing creating a much better seal against blowby.

The Rifles used in long range match of the era were most often either military barrels or barrels made by the same methods.
The cartridges tested by Frankfort were in fact intended for competition, at ranges up to 1000 yards or more.

Civilian shooters recognized the benefits of this effect when they had barrels with eroded throats shortened at the shank and rechambered, the set back barrel usually being more accurate than when new.

Excessively hot double base powders which caused rapid throat erosion could destroy accuracy within 400-500 rounds, so rifles used with those propellants seldom got any benefit from the taper effect.

The point is that rifling patterns which duplicate the natural wear in pattern as manufactured would benefit with a higher level of accuracy from the start, no breakin required.
The very slight radius to edges of lands and at the angle at bottom of groove, are a part of the wearing in effect. If coupled with a taper lapped bore then you might have a superior barrel for best long range accuracy and consistency in velocities. Add to that a modern Stainless alloy that resists thermal gas erosion and it would have a much longer and more consistent accuracy life. Bullets would also suffer less from blowby gases, something thats always been a factor with boat tail bullets since they have little or no upset.

One thing about long range accuracy, any inconsistency of velocity is greatly magnified. A rifle capable of excellent groups at 100 yards might land in maggie's drawers half the time at longer ranges.

Some rifles were known for the ability to self compensate for velocity variations, the SMLE for example, and most have some compensation factor, but the less variation the better in any case.

PS
If you were to use a very hot load which results in rapid throat erosion then a barrel with the qualities I mention would allow better accuracy during its shorter than usual useful life. No breaking in period to speak of.
 
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In my limited experience with match barrels, throats get tighter, not looser with fire-cracking erosion. The damaged metal stands proud from the original dimension of the barrel and causes a reverse choke effect that is damaging to bullets and accuracy. You can feel this constriction quite plainly with a tight patch. Useful long range match accuracy can be partially if not largely restored through the application of an aggressive abrasive in the throat. Such an abrasive is silicon carbide, commonly sold as Clover valve grinding compound. Short stroking a throat with a tight fitting brush wrapped with steel wool and loaded with Clover sounds like ghastly treatment of a target barrel but when the accuracy drops off from bullets getting swaged down in the now tight throat and then rattleing up the rest of the bore not much risk is involved and it's a lot cheaper than cutting the chamber off and re-chambering. I think 240 grit is about right but some prefer even coarser, like 180. Keep in mind that as you stroke the abrasive breaks down and gets finer. If that doesn't help then you can still shorten the barrel at the shank and rechamber to eliminate the damaged throat.

I've never seen any barrel get more accurate for 3000 rounds and flatten out at peak performance for several thousand more. Even long lived 308's are on a clear decent by 3000 rounds. OK, there is the 30 BR to consider.

I'm not an arsenal and haven't tested many thousands of "match barrels " back in the day, but this is what I see happening today. The idea of best accuracy at many thousands of rounds because of erosion creating a choked barrel is nuts. A barrel that gives the kind of accurate life being claimed here will be nitrided to preserve bore dimension, not "worn into accuracy".

A not quite as old a gunner.

Greg
 
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The amount of constriction you speak of is a product of temperature and products of combustion on the propellant used. The extent of its effect on the bullet is a product of the acceleration curve of the propellant bullet combination and ability of the bullet to conform under G forces of continued acceleration beyond the constricted zone.

A similar situation would be the difference between the MkVII and the MkVIIZ.
A bore subjected to thermal/gas erosion by Cordite could not stabilize a bullet from the MkVIIZ very well at all, but a bore subjected to ten thousand rounds or more of MkVIIZ , with its lower flame temperature and lower molecular weight of products of combustion, could then be used with MkVII and accuracy while not great would be well within limits.

If the rifle were used only with lower temperature 3031, which was originally formulated specifically for the .303 cartridge, and the bore was on the tight side of the rather generous tolerances of the day, an accuracy life of 6,000-12,000 rounds could be expected.
If only MkVII with its higher temperature propellant were used accuracy dropped off from the first five hundred rounds or so and by 1,500 rounds the telescoped SMLE rifle would not be accurate enough to be depended on for long range work. Snipers were forbidden use of the telescoped SMLE in training, only standard iron sighted service rifles were used. The scoped rifle could then be fired only enough to sight it in, all other firing had to be at live targets in the field.

The Sniper rifle barrels using R5 rifling, coupled with alloys highly resistent to thermal/gas erosion, have an expected service life of 10,000 rounds with the present ammunition. Its likely that if they reverted to the single base powders once commonly in use for Matchgrade 7.62 ammunition that the accuracy life would be much longer. The rifle was originally rated as having an accuracy life of 15,000 rounds, but that was downgraded later on.

As someone mentioned earlier the military barrels were suited to open base FMJ bullets, in recent years the enclosed base open point bullet has become the standard.
The effect of the propellant gas oposing inertia during the acceleration cycle may be less for an enclosed base compared to an open base.

Short stroking a throat with a tight fitting brush wrapped with steel wool and loaded with Clover sounds like ghastly treatment of a target barrel
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Sounds to me like you are creating your own taper bore effect.

I did much the same with an old two groove .303 barrel years ago.
The bore was lightly pitted and scored by steel cleaning rods, accuracy was mediocre at best.
I polished out the bore from the breech using a tightly molded leather patch secured to an undersized jag. The edges of the patch were molded as wings to engage the two groove rifling.
At first I simply secured a strip of 1000 grit silicon carbide paper to the body of the molded patch.
After a few passes I went to ever finer polishing papers and crocus, finishing up with white ultra fine polising compound.
The abrasives only contacted the center of the very broad lands. Only when I progressed to the fine polishing compound did I allow it to reach the edges and the grooves.
Appearance of the bore from the breech shows slight rounding of the edges of the lands, that radius highly polished.
Using a lead slug pushed in from breech to muzzle the gentle taper can be felt. The muzzle is perhaps .001 smaller than the origin of rifling.
After this the rifle has proven capable of consistent sub MOA performance with my best groups under 1/4 MOA.
So by my limited experiance I'd have to conclude that a gentle taper does improve accuracy.
 
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