Obturation compensation?

[Doug]

I haven't slugged the barrels but was told it wasn't necessary cuz of consistency of known data, so here's my question. All my moulds drop .458" bullets, which I understand to be .001" over groove dia of the 45-70. Just received the 45-90, wh/ was supposed to be .001"-.002" larger bore dia than the 70, but at the muzzle lands at least, this isn't the case--it's actually tighter by maybe .005" but noticeably so. I understand these are just the lands, and that the (relative and/or actual) groove depths of these two rifles may differ. That said...IF I had a .458" groove dia. (wh/ the 90 is supposed to be and may be), and I can't get my bullets to drop at .459", will obturation make up the .001" deficit if I use a softer alloy? Another option would be to beagle moulds then resize for .459" but I'd rather not resize, unless you guys would recommend that as against going from 20:1 to, say, 30:1. (Black powder charges only) Thanks

edit: Just ordered some 3M hi-temp flue tape. Have read that it only adds .002" to the drops. And think that I'll be ordering a .459" sizing die to go w/ the .458" sizing die that I originally got. But first I'm gonna slug the barrels. Appreciate that what matters is chamber dimension, but that's all the more reason to gain some dia. here.

double edit: Rather than beagling for out-of-round bullets/noses (even after resizing), think I'll be lapping if I do anything. Suppose my question is now reduced to how snug a fit I should be after b/t the bullet nose and chamber throat for 1/18 twist @ 20:1 alloy under BP pressures. All I know is that this tiny amount of nose play @ the muzzle lands (of the 45-70) indicates an unacceptably small nose dia., regardless of obturation effects.

 

[9.3X62AL]

Doug--

Keep in mind before I start here that i AM NOT a scientist or metallurgist, and that what follows is based only upon experience and research I've done over the past 40+ years of shooting--reloading--and casting.

I have yet to encounter a 45 caliber rifle that had throat or grooves at the .457" spec. This population sample size is about 20 examples; 2 were 458 Win Mags, the rest were 45-70s. The majority showed throats in the .459" ballpark--one was .461", the other .463" (both original Trapdoors).

So, why do mould makers persist in making cavities that cast @ .457"? Well, "that is the way it has always been done". Precedent runs for President, and often wins elections. Well, I can be a nosey jerk, and I don't retreat from asking The Next Question. In my past life I got paid to do that, and I got pretty good at it. The "next question" became "Why intentionally undersize your bullets?" Where is the vigor in that?

I spent a couple years on this chase, certainly not full-time but in the manner of case work at my old shop I would pick it up and run with it for a time until some other cataclysmic distraction took precedence. I didn't let it go, though. One of the larger weighing standards an investigator employs in assessing information is to balance data commonalities and inconsistencies--place them in a context if possible, and attempt to arrive at either valid conclusions or to better direct further inquiry.

I have loaded 25/20, 32/20, and 44/40 WCF calibers for a long time. Their commonalities were/are 1) all began as blackpowder chamberings and 2) all generally spec an undersized lead bullet for (relatively) over-sized throats and grooves. Just like the 45/70. What does this tell us? I had to dismiss my smokeless powder/modern bullet fit biases and set my clock back 125 years to arrive at what I believe to be a reasonable explanation.

I had a little help from my shop's Hazardous Device Team on this inquiry; in my role as a fire cause/origin investigator, we did a lot of work that crossed paths. Their input and the established literature show that smokeless propellant and black powders work in VERY different ways--the principle difference being that smokeless propellants burn in a progressive elevation of pressures under confinement, while black powder literally explodes with a low-order detonation. These BP detonations are far better at causing obturation than are the pressure propagations of smokeless fuels. Given the pure-lead or soft lead/tin alloys of the blackpowder era, an undersized bullet did not pose the disadvantages for the BP user that can frustrate a modern shooter of castings with smokeless fuels. In short, these undersized bullets could be viewed as a "blackpowder bias" of their own. Given the gritty fouling left by some black powders in throats and bores, a slightly undersized bullet might even be advantageous in a prolonged string of firing.

This is NOT to say that undersized bullets are the best answer for BPCR applications--not at all. I still believe that diametric fit as close as possible to throat specs is the "ideal", regardless of fuel type, barring other variables that might crop up. My sole motive was to offer an explanation of a current and persistent anachronism that causes questions and confusion in some quarters.

 

[Doug]

Well 9.3x,

You did rightly discern the grumblings of my internal dialogue, so kudos for that. Truth is, and after reading further, for my conditions I will not be happy with anything less than .001" over groove dia., and I also wipe b/t shots so don't need any extra space to accommodate. Afraid I'm taking this long range target shooting a little seriously, but oh well.

 

[9.3X62AL]

I hear ya, Doug. Fit is king, metallurgy is queen, and lube is the first-born son whose heart is right but still enjoys drinking and wenching to excess. He'll grow up, sooner or later.

 

[Bret4207]

Okay, so I'll preface this with the admission I HATE the idea of "obturation" to achieve fit. Before you go to Beagling, lapping, slugging, etc., pop off a full power round and simply measure the mouth of the fired case. That's as fat as you can go without case reaming. See what that measurement compares to what your mould drops. If the bullet is at least a half a thou bigger, then start shooting and see what happens. Why people start making assumptions before they even shoot 'n see I don't get. If the mould drops way undersize, you can get a mould cut correctly for a lot less than your time screwing with a drastically undersize is worth. USUALLY you want a bullet running a good thou, if not more, over bore diameter at the chamber end- but not always. Sometimes a .459 bore will work just dandy with a .4592 bullet. All the "rules" and "recommendations" don't mean squat to the barrel. It just wants what it wants with the load/bullet you have. IME every single time I think I know what I'm going to have to do to get a particular gun to shoot cast I get proven, if not wrong, at least not so bright. As Backasswards liked to say, "It only matters if it does." Start at square 1 and proceed to square 2, then repeat.

 

[Doug]

Yes Brent,

Have laddered, have about 400 rounds through it w/ fire formed brass, and it also occurred to me that the necks are of course fully expanded. What prompted all this was the comparison w/ the new rifle and the surprise that followed as stated above. Now, in this time the groups are still very loose, and as I prepared loads w/ the new mould that I wanted to use for long range, I noticed that the bullets canted slightly after thumb seating and when turned on their side. So I put a taper crimp down enuf to prevent the bullet from sliding up and down and canting. Then I went back and looked at the other (Postell) cartridges I had waiting...same thing. I also seat depth to bein off the lands, so no "artificial" help there for correct alignment. Occurred to me that these (all my drops f/ 5 moulds) are dropping small for the chamber, and that neither should there be movement when the nose is inserted barrel-end. Now, I'll be trying these as freshly crimped to see whether groups tighten from better concentricity, but what you've said and what I've observed all adds up to too small a bullet, no?

 

[Bret4207]

Well, it's more complex than just the bullet is loose in the fired case and the nose seems smaller than the bore. The case mouth dia. is just as large as you can easily go- no more, no less than that. I use it as a guide to the dia. I hope to start with. But that doesn't mean it HAS to be that fat. Your concentricity (sp) issue is going to give you poor grouping no matter what. The neck is going to want to have at least, IMO, one calibers worth of area capable of holding the bullet in proper alignment. Without that it's going to be a real tough row to hoe. Can I inquire as to why you aren't seating against the leade? If the nose is significantly smaller than the bore, and it's not the result of a taper, then bullet alignment is critical and seating into the leade can help. If you can't or if the bullet is just plain too skinny, I'd be back to having a mould cut. We live in the Golden Age as far as mould makers go. You can get exactly what you want to fit your barrel for relatively little money. I wouldn't be looking to obturation, the process I describe as taking the bullet you lovingly and carefully crafted and smashing it into putty in the faint hope it will somehow "fit better", to solve your problems in long range rifle. It's fine for popping rocks in the sand pit, but not for anything serious. It sounds like you have some bucks tied up in this, might as well do it right.

Are you shooting black or smokeless?

BTW- it's Bret, not Brent.

 

[Doug]

Strictly black, as stated above. And after ladder testing I tested for seating depth and it preferred off the lands. As for the molds, I purchased according what others were using and winning with in this rifle. (Well, I did buy one used mold for the handles, but it helped in other ways too, so...) And regardless of whether the molds came with a 457/458/459 prefix, they all drop the same. So here's another thing I just did--I dropped this new money bullet into the chamber of the new (and narrower) 45-90 and it makes not a sound. BUT, drop it into the 70 and it's like a spray paint can when you shake the rifle. No...I'm afraid that it does all seem to add up to an insufficient bullet dia. for the 70, but the surprise was that it's supposed to be the other way 'round, at least from what all the pundits have related online.

edit: Just tried the other 2 main bullets for the 70 and they also rattle. Think I'll make a pound cast.

 

[9.3X62AL]

Bret's description of reliance upon obturation to take up diametric slack is my view as well. I can't over-emphasize the need to get FIRM ideas of what specs a rifle's throat--grooves--and lands actually are. Once you start learning these details, some surprises can emerge. Assuming = groping in the dark in a sandstorm.

One such surprise......my CZ-550 in 9.3 x 62. I am fond of bore-rider designs like Lyman #311291, and when I set out to design such a bullet for the 550's bore I got a couple surprises, actually. Throat is about .3665", grooves are right at .366", but lands are VERY tall--diameter is .352". That = .007" land height, roughly double what most of my firearms show. Also noted was that the throat's freebore extended .300" before the rifling leade began rising up. All of this meets up with bullet length restrictions owing to twist rate--3 turns/meter (about 1-13"). The short answer.......FORGET bore rider, but have a tangent ogive commencing at .600" up from bullet base to enable almost one full caliber of seating depth (neck length is about .290") and having a 70% meplat with finished weight of 270 grains. The bullet shoots WONDERFULLY, and the rifle has about 2500 of them downrange to date. Pure, unadulterated luck--thanks in no small part to educated guesswork and hopeful supposition. I ran out of omniscience many years ago, and my early estimates of its depth were wildly exaggerated.

 

[Brad]

Depending on obturation for a good fit is, at best, iffy.
When you use a bullet of proper dimensions you eliminate a large potential problem. Why leave it to chance?
CAN black obturate a bullet for good fit? Absolutely. It is a matter of using an appropriate alloy.

I prefer to use a bullet I know fits. I don't like leaving things to chance. In my mind I eliminate a source of uncertainty. Heisenberg didn't mind uncertainty but I sure do.

 

[Doug]

Yes Brad,

I couldn't agree more. I'm taking every opportunity to learn and every chance to do things right. I'm honestly thrown by the discrepancy wh/ was supposed to be the other way round, but maybe more properly stated, I'm surprised by the (excessive) bore dia. of the 70. All my equipment was gotten initially to accord w/ everything everyone was reporting for the rifle and I encountered no deviations from that "consensus reality." That said, it's not as though any of it matters. As 93x pointed out, and as I've experienced, the molds seem to come in the same sizes anyway. Now I could go as Bret suggested and order a custom mold, but I could also lap three very "accomplished" molds that I already have and simply size these down for the 90. It isn't recommended acc. to the really successful competitors, and neither is the seating die wh/ was why I avoided any crimp after fire forming, but I chose the molds for some versatility and have to use these for now. Thanks everyone for letting me think out loud.

 

[Brad]

Try beagling the mould. It works far out of proportion to expectations. It is also entirely reversible.

 

[JWFilips]

I have beagled a number of moulds: & for some crazy reason it has always worked...and as Brad said it is reversible
I'm thinking the beagle process make a slightly out of round bullet But when sent down range the out of round gets quickly changed to "Round"

 

[Doug]

Well that was my original intention. Even ordered a roll of 3M hi-temp flue tape this morning, but then was concerned about roundness. And although we only have access to 200 yds locally, we have a 600 yd match in 2 weeks and a 1k match in 4 weeks. Just figured that roundness was gonna show up at distance.

 

[Bret4207]

Okay, now we have a better picture. The "moulds not being as advertised" is nothing new. That's why you run into so many guys that have 4 or 5 examples of the same bullet- it took that many tries to get one that fit. Black will obturate a bullet, no doubt, but it's not the "right way" IMO. Yes, Beagling is worth a try. You have to remember that when you size the now hopefully oversized Beagled bullet, that the metal doesn't just get shaved off, or at least it shouldn't. The oblong shaped Beagled casting should round out to an extent. It will lengthen too. And if you're going that route, be aware you can "bump" the nose to make it fatter too. That's basically using the sizer to push down on the nose when it's against the stop and swage it fatter. This is a much more controlled form of "obturation".

As far as seating off the lands, once you change the diameter of the bullet, that may well change too. One of my best thought out suggestions over the years, if I had any good ones, was to just change one thing at a time, take copious notes on your observations and then take the next step. Don't change diameter, bump the nose and seat to a different depth all at once or you'll never know what the issue was.

Yeah, try Beagling before you sink money into a mould. But before you go to lapping and trying to get a round bullet 2 or 3 thou larger, be advised there are people that can cut you present mould EXACTLY 3 thou larger, EXACTLY where you want it larger. Lapping can work. Lapping can also fail miserably. Like I said, this is the Golden Age as far as moulds go.

As far as roundness, yes, round is nice. But you have to keep in mind that if you're going to use the obturation method of fitting a bullet, then perfectly round is only going to last until the primer lights. Then the bullet is going to go plastic and hit the leade at an angle, more or less, and form to whatever part of the barrel it's in when it stops being plastic. That's why I mention concentric loading, the nose being into the leade if the rifle wants it, etc. Every tiny variable adds up. And if you want to shoot for score at 1k yds, then you need to think about every little variable.

 

[Doug]

Thanks Bret, I'll know more after the weekend. The new bullet is at least concentric now (after the undesirable use of crimp), and it never occurred to me to bump the nose--and that was a big (unspoken) part of my obturation Q too (how far up the bullet obturation occurs). Maybe it doesn't matter since I'd rather get all the ducks in a row before firing anyway. Just finished prepping the 90 brass and will begin looking at what I have to be able to bump back the nose. No Q that it'd be better if I can get away w/ beagling, and as steps go, it's most logical to begin there. Cheers

 

[James W. Miner]

You need to understand mold expansion and contraction. As the mold gets hotter the boolit will get SMALLER 90° to the blocks but will expand at the parting line so when you measure you will see the largest portion is at the parting line.
If you Beagle a little, you can get an even ROUNDER boolit.
I agree with Bret, to expand a boolit to seal has never worked for me, even with a Minie' ball in a ML. I lapped a lot of them for a perfect fit with thumb pressure to go from missing a 4'X4' board at 50 yards to hitting the 200 meter gong, just FIT
Lapping is not easy and you can screw up fast. I got away from using a boolit and grit and made brass rings for the drive bands. Need a bunch cut because the rings wore down faster then the aluminum or mold metal.
Point is, NEVER will I make a boolit softer to OBTURATE. Best to call it SLUMP since obturate means TO SEAL.
Seal first with the right boolit and you don't need to SLUMP a boolit.
I have worked with many guns and mostly the revolver which is the hardest ever. Nobody has made cast shoot like I have from a revolver. It is to understand what happens when the primer pops, what primers do, what lube does and how brass works. I have hundreds of groups smaller then 1/2" at 100 yards and 1" targets can be hit.
Here is my .44 SBH at 200 yards. 1-5/16" doing a drop test and yes that heavy boolit dropped 35" with a 50 yard setting. Notice the case tension.
Then a 100 yard shotgun shell with my BFR in .500 JRH. You need fit and an alloy that withstands what you shoot.
Bret has known me many years and many have found my findings true. I have never told anyone to soften a boolit but never said soft can't shoot, it can but you have your work cut out for you.
Do you know a faster powder does more boolit damage from instant pressure so what you call a LIGHT load is worse then a full power, slow powder load. I shoot cast from rifles with rifle powders, never see me use Unique or any fast powder.
Bring your .44 loads here for me to shoot and I can tell you what primer you used.

 

[Rick]

Nobody has made cast shoot like I have from a revolver.

Nobody Jim? Really? Nobody?

.

 

[quicksylver]

I'm kind of late to the table, but I stopped being concerned about "Beagling " a mold when I realized what a two groove barrel does to a bullet.

And speaking of grooves, how does the type of rifling affect all of this?

 

[James W. Miner]

Nobody Jim? Really? Nobody?

.

Not that I have found. But if you are the old IHMSA shooting friend from other sites, I can't argue with you.
You know I will with the use of new brass for every shoot though!
New brass the worst for boolit pull. One of many tests with 50 rounds at 50 meters using new brass. Benched scoped .44. I would not shoot these at 200 meters. You know I sorted loads by boolit seating pressures but not for theses tests.
IHMSA sorted shooters and loaders fast. Nobody ever out shot my Ruger.