71 Mauser recovered bullet surpise

[twildman]

Well, I was going to title this thread 'Obturation, schmobturation", but that would be too general...

Yesterday I was fortunate enough to recover a bullet from the berm that had been fired in my 71 Mauser. Here are the numbers: Bore .432, groove .452, bullet diameter .446 (bigger will not chamber), alloy hardness 14.3, RCBS 44-370-FN mould. I was delighted to find that this reclaimed scrap alloy shoots marvelously (sub-2-MOA) and leaves no lead in the barrel, so I had been assuming that it must be obturating with my load (20 grains 2400).

So...I cleaned and miked the bullet. The groove portion was virtually as-sized diameter, with no evidence of gas cutting!!! I know it truly did not fill the grooves, as there were no longitudinal striations as on the portion engraved by the lands.

Explanation? I have none, really, except speculating that all that greasy kid stuff in those big lube grooves may help, and that the .438 nose section gives good guidance...Well, every rule needs exceptions to test in, and I guess this is just one of those.

[leadman]

I too have a 43 Mauser, using the Lyman 8 BHN .446" bullet in a barrel with .446" lands, .457" grooves. Excellent accuracy with 21 grs of 2400.

Have not recoved a bullet to measure yet but it is the future plans.

I am lubing in a .450" sizer die so the bullet is not touched and the lube is on the driving bands as well as in the lube grooves.

[twildman]

Leadman; I saw in an earlier thread that the front band of that Lyman design is a nice push fit in your gun. I really suspect that a bullet with good initial guidance and plenty of lube in deep grooves will give good accuracy regardless of obturation. I think I will test that hypothesis by trying Liquid Alox on a few of the same bullets and seeing how they perform; that was a disaster in my Trapdoor when I tried it a few years back.

BTW How is your case life in your 71? I am fortunate in not having to do ANY sizing of cases.

[Multigunner]

Whats the deal on these chambers not allowing a full diameter bullet?

Was the original bullet a heel base or hollow base style intended to expand to fit the bore?

PS
Bullet obturation is an urban myth, I know this is true because I saw it on the internet.

[twildman]

Some owners of M71s claim to be able to use groove diameter bullets, but I am not one of them. To my understanding from literature and discussions, the intent of the designer was to have an undersized, soft paper patched bullet; as to base design, I do not know. My particular gun loads very hard with my bullets as-cast at .448, and still a little snugly with them sized to .446.

Since my Trapdoor only shoots well with hollow base bullets, I doubt that obturation is a myth, but all I am saying is that my tight-chambered old relic Mod. 71 is putting undersized bullets of hard reclaimed metal into nice tight groups with no leading; I don't know how this is working, but I am enjoying it.
terry

[Buckshot]

...............With BPCR the undersized slug was used so it would chamber in a BP fouled throat. Upon ignition the slug was supposed to upset into the grooves. With the advent of smokless powder and the acceptance of the 1888 Commission, the older M1871 and M71/84's were religated to backwater colonial duties. It was said that they later experimented with smokless loadings in them also, but accuracy (obviously lack of) was an issue. I suppose at the loadings tested with the available early smokless powders there wasn't sufficient initial force to upset the slug into the grooves. The more gradual pressure rise overcame the slug's inertia and started it moving before sufficient pressure could build to cause it to upset.

As was proven by the British long range muzzle loader match shooters of the late 19th century, upset takes place only at ignition and nowhere else in the boolit's trip up the barrel.

..............Buckshot

[Boz330]

Buckshot is absolutely correct. In the modern Gibbs replicas the PP boolit is a snug fit in the bore but the weight of the ramrod alone is enough to seat the boolit so there is absolutely no rifling engagement until ignition and these rifles will shoot groups that will make any HP rifle proud out to 1000yds.

Bob

[Multigunner]

Yeah I was being sarcastic. I know obturation is a necessary factory in many rifles.

The Cordite propellants provided the initial bump up to allow the usually undersized bullets of the .303 to fill out the grooves of the very often oversized bores of the Enfields. Even more important was the filling out of the quickly eroded throat, the high temperature of cordite quickly ruining a bore for less energetic powders.

Earlier German Needle gun paper cartridges used a wooden or compressed paper sabot and an egg or lozenge shaped bullet, the sabot sealing the bore.

From what i've read on BP charges when ignited at the base of the case powder at the top of the charge is propelled still unburnt against the base of the bullet acting as a temporary seal, long enough to get past the throat. Over the charge cardwads were often used with large bore , large capacity cases, these also acted as a seal and protected the bullet base from mutilation by powder granules driven against it at high velocities.

A lead bullet .25-20 cartridge I examined has a hemispherical indented base, most likely to promote sealing.

When the SMLE MKI barrels were reverse taper lapped (to reduce friction and increase velocity when using the MkVI bullets) for the last fourteen inches of bullet travel, if done right, accuracy was still okay despite windage of several thousandths, if lapped too deep shots went wild.
MkI Cordite at 58% Nitro content delivered full thrust within the first nine inches of travel.
So a tight fit to the bore is less important once past the zone of acceleration G Forces and maximum pressures.

Some continued acceleration G force for about 1/3 of bullet travel is likely with modern rifle propellants, faster burning propellants, often used for reduced Cast loads especially in BP era cartridges, provide max pressures earlier in the bullet travel and less G force to conform the bullet to low spots in grooves or eroded areas. Mixed fouling and lube from previous shots may act to seal the rapidly diminishing gas pressure behind the bullet reducing blowby effects.

When the .303 cartridges were tested with varying amounts of excessive lube on cases and in chamber necks they found that Mineral Jelly pushed into the throat and leade formed a temporary seal that reduced blowby before the bullet bumped up increasing velocity slightly.
Undersized cast boolits with thick lubes may also end up with lube forming a temporary sealant effect at that critical moment. Chamber pressure would tend to push lube from the grooves around the body of the bullet between the bullet and surface of the throat, and into the grooves.

A number of very old Milsurp bores I've cleaned had the grooves filled almost even with the lands, the material was much like old hardened baked on laquer. The slick FMJ bullets could not scrape this out from shot to shot so bullets never actually touched the bottom of the grooves after the first few shots.
With modern solvents we generally clean our bores down to the bottom of the grooves after each range session, but riflemen in the field had neither the materials nor spare time to religiously clean after every battle. So long as the lands were visible they felt it was good to go.


PS
Cordite MkI used at the turn of the century had a nuch higher Nitroglycerin content than Cordite MD used for the MkVII cartridge.
Cordite MD has a longer less abrupt power curve, so G force upset can continue a bit further down bore, about 14 inches compared to 9 inches. Most rifle type Single base powders have less initial bump up but a much longer smoother acceleration phase. Burning time being adjusted by various means, retardents, coatings, or granule size etc.
Modern Double base powders, with only a fraction of the nitro content of cordites, ( also HiVel, and other early DB propellants) and addition of burning rate moderators, have extended burning times the pressure curve extending further than many single base powders.
Some double base powders now have nitro content of five percent or less, and the nitro is not considered an energetic component, it is used only as a plasticizer and to give better shelf life. These powders may be labeled as single base if nitro content is only 5%.

Initial bump up is more important than the continued G Force effect, otherwise the bullet nose may get off center within the first three inches of travel and not recover. If the throat is far oversized or too eroded for the bullet to bump up enough to fill it out , excessive nose canting and blowby damage occured.

[twildman]

Fascinating stuff, Multi. It has to be the grease that permits this performance in my 71, then; a mix of beeswax, paraffin, vaseline and white grease pushed into the grooves manually prior to sizing. I'm gonna stop ovethinking it and just enjoy my rifle.
Terry

[Kenny Wasserburger]

Its odd how Smokless does not bump up the bullet like BP does, i have plenty of recovered BP fired bullets both Paper patch and Greese groove and they all show bump up and rifling marks on the bullet.

My paper Patch load show rifling right up to the top edge of the patch but no rifling above the patch.

KW

[leadman]

twildman, I probably could push the Lyman boolit all the way thru the bore with a cleaning rod. Only takes light thumb pressure to push it into the throat or muzzle.

I haven't shot it enough to determine case life. I am only sizing about 3/8" of the neck and in doing so a little of the shoulder is also sized. I am using the Lee Universal Neck Expander but wish I had a Lyman M die as it is hard to start the boolit straight with the Lee Die just flaring the neck.

Only the base is in the case before loading and I can almost seat the cartridge by hand before closing the bolt.

I am lubing in an oversize .450" die so the is a layer of Carnuba Red on the driving bands. Maybe this is what is sealing the bore?? I need to go where I can shoot into something that will allow me to recover a boolit to find out.

I am also going to use a slightly harder alloy and see what happens.

[twildman]

Leadman: I think that you, I and Multigunner are on the same page about the grease; it can be the only explanation as to lack of gas cutting/leading. I am glad you have to do minimal work to the cases; at 2.50 each, we can only hope they last a long time this way...
Terry

[excess650]

I pulled a couple of original 1885ish military cartridges apart. The bullets were PP with flat or slightly beveled base, not hollow base. There was a grease cookie sandwiched between a couple of card wads, and shiny, coarse grained BP. They might have fired, but the cases and bullets were corroded.

I may revisit the '71 with some soft bullets from the RCBS 44-370 mould. The only rounds I tried through that rifle were with a .435" LBT LFN abround 400gr, and smooth so as to be PPed.

[leadman]

If I shorten the case I think I can load the Lee 300 grain .452" pistol boolit without gas check in the case and load it. The boolit will be in the case only where the gas check is supposed to be at the base.
Kinda like a heeled boolit for the older revolver cartridges.