Article #1: "Fit is King"
- Thread starter Ian
- Start date
fiver
Well-Known Member
I'm gonna add some thoughts here.
you have all read the article Ian wrote up on what happens in the throat and down the barrel.
the powder you choose and how you place the boolit in relationship to the throat can also influence the powders burn.
now giving that powder an initial pressure spike with a faster powder is akin to using a different primer or to placing the boolit out in the lands and grooves to provide that initial rise.
some powders do not like that initial rise [7283 and 7383] and can just plain out change it's burn rate from doing this.
now think about backing that boolit off the lands and giving the powder a spike just long enough for the engraving to start [continuing that pressure rise] in a smoother longer transition that allows the boolit to get moving and into the barrel before more gas is applied to the base continuing that smooth push.
now you are engraving and accelerating with a longer 20-30-k pressure phase before putting the full
50-60k to the base of the boolit. [and getting the powder into a zone where it burns clean] you need that higher pressure to apply speed to the boolit.
you have to try a few things and really pay attention to the details here, but that's the goal, and is what powders such as rl-17 are trying to accomplish on their own.
you have all read the article Ian wrote up on what happens in the throat and down the barrel.
the powder you choose and how you place the boolit in relationship to the throat can also influence the powders burn.
now giving that powder an initial pressure spike with a faster powder is akin to using a different primer or to placing the boolit out in the lands and grooves to provide that initial rise.
some powders do not like that initial rise [7283 and 7383] and can just plain out change it's burn rate from doing this.
now think about backing that boolit off the lands and giving the powder a spike just long enough for the engraving to start [continuing that pressure rise] in a smoother longer transition that allows the boolit to get moving and into the barrel before more gas is applied to the base continuing that smooth push.
now you are engraving and accelerating with a longer 20-30-k pressure phase before putting the full
50-60k to the base of the boolit. [and getting the powder into a zone where it burns clean] you need that higher pressure to apply speed to the boolit.
you have to try a few things and really pay attention to the details here, but that's the goal, and is what powders such as rl-17 are trying to accomplish on their own.
Bret4207
At the casting bench in the sky. RIP Bret.
Charles- "Static fit" is the boolit as you make it and fit it into the case. "Dynamic" is what the boolits fit is after the primer goes "pop" and it passes the throat into the barrel. Static can affect dynamic of course, but not the other way around.
Rally Hess
Well-Known Member
Wonderful article and topic, which brings up a question I've pondered and not seen specifically in print.
Has the case completely expanded to chamber dimentions, before the bullet base leaves the case mouth? During ignition and expansion, it would make sense that the thinner parts of a bottleneck case would expand first, that being the shoulder and neck, but which expands first? If the bullet base hasn't left the case prior to case expansion it would partially protect that part of the neck from expansion and the shoulder and lower neck would expand first, effecting release of the lower neck tension on the bullet base. In my mind I'm picturing the upper third of the case and shoulder expanding in an upward wave, sealing first at the shoulder and then from the neck base upward until the bullet base passes out of the case. Has anyone a reference to case expansion rates?
Has the case completely expanded to chamber dimentions, before the bullet base leaves the case mouth? During ignition and expansion, it would make sense that the thinner parts of a bottleneck case would expand first, that being the shoulder and neck, but which expands first? If the bullet base hasn't left the case prior to case expansion it would partially protect that part of the neck from expansion and the shoulder and lower neck would expand first, effecting release of the lower neck tension on the bullet base. In my mind I'm picturing the upper third of the case and shoulder expanding in an upward wave, sealing first at the shoulder and then from the neck base upward until the bullet base passes out of the case. Has anyone a reference to case expansion rates?
Ian
Notorious member
That's a million-dollar question. I think it depends. Closest I've come to a real answer was with a test I did once using a take-off barrel and a case with the head cut off flush with the end of the chamber. I made a piston out of a piece of rubber to fit the inside of the case and packed the front part of the case with wheel bearing grease, seated a bullet over it, put it in the chamber, used a close-fitting steel rod to drive the rubber, and smacked it with a 20-oz hammer. If the bullet used fit and sealed the throat immediately, the neck seemed to open before the bullet moved much past the point of obturating the throat. If the bullet had to move a bit and/or deform before sealing the bore, a little grease would leak around it. That test really doesn't mean much since caliber, bottleneck, alloy, ignition pressure curve, engraving pressure and of course powder and primer types affect what happens. I think in general that the brass expands to the chamber confines behind the bullet right up to the point (if it happens) where engraving resistance becomes greater than the force necessary for the burning gas to expand the neck, then the neck probably blows all the way out ahead of the bullet base.
Rally Hess
Well-Known Member
Ian.
Thanks much for your quick reply.
I've often wondered about this while reading discussions about alloys, plain base or gaschecked, bullet shape, and type of grease grooves on the bullet. I've seen the slow motion film clips of bullets leaving the muzzle, and it seems there is always some residue exiting prior to the bullet, indicating at least some blowby, whether at ignition prior to obturation or during the trip down the barrel. I've also come to believe that explains some of the carbon deposits on the upper neck of bottleneck cases. Seems the milder the load(less internal pressure) the larger deposits on the case neck. It makes sense, in my mind, that because of the physical shape of a bottleneck case, and it's initial internal exposure to the expanding gases during ignition, that the shoulder would expand/stretch/seal prior to the neck expanding, at least until the bullet has moved forward, exposing the case neck to the expanding gases.
If indeed the gases have blown the neck out, prior to the bullet leaving the case neck, or upon initial engraving, wouldn't that leave the bullet base unsupported, or just surrounded by gases? If so, couldn't that also affect bullet/bore alignment?
Thanks much for your quick reply.
I've often wondered about this while reading discussions about alloys, plain base or gaschecked, bullet shape, and type of grease grooves on the bullet. I've seen the slow motion film clips of bullets leaving the muzzle, and it seems there is always some residue exiting prior to the bullet, indicating at least some blowby, whether at ignition prior to obturation or during the trip down the barrel. I've also come to believe that explains some of the carbon deposits on the upper neck of bottleneck cases. Seems the milder the load(less internal pressure) the larger deposits on the case neck. It makes sense, in my mind, that because of the physical shape of a bottleneck case, and it's initial internal exposure to the expanding gases during ignition, that the shoulder would expand/stretch/seal prior to the neck expanding, at least until the bullet has moved forward, exposing the case neck to the expanding gases.
If indeed the gases have blown the neck out, prior to the bullet leaving the case neck, or upon initial engraving, wouldn't that leave the bullet base unsupported, or just surrounded by gases? If so, couldn't that also affect bullet/bore alignment?
Ian
Notorious member
That last part is something I've pondered quite a bit. It doesn't seem like a good thing if the bullet base is left hanging in space with hot gas rushing around it, which may explain why chamber necks with a lot of loaded clearance are very difficult to make shoot well. With a very close case-neck/chamber neck fit (less than a thousandth) this seems less of a problem. Recently I designed a .30-caliber bullet for .308 chambers having at least .080" of parallel freebore such that the gas check will be entering the throat before pressure is sufficient to expand the neck ahead of the bullet base. The nose is of the "self aligning" type and engraves with minimal pressure. The bearing surface is also minimal, basically one driving band. I only shot a few of them so far, in an auto-loader, and the jury is still out on whether the idea is a win. I really need to do some hard-core, high-velocity testing in a bolt gun to see if the idea solves the classic "sloppy-neck" syndrome or not.
Rally Hess
Well-Known Member
That bullet design would keep me up at night!!! About half of my concern with this topic is what blowby gases effect have on lubes., and their distribution if those gases reach the lube groove in any quantity. With that large groove close to the base it seems like your testing fate. LOL
I remember some M60's we had at Ft.Hood that had extremely loose chambers and would really spit out some thilthy casings. Short bursts looked more like a round of 00 buck going down range. Our Armor said they were "within spec", which translates to: If you have to go, your going to get a new one, and as long as this one goes bang it's good enough to familiarize with.
I've been working in the shop today getting ready to attend a trade show this next week, and thinking about this subject. I've about got myself convinced that the neck of the case, in a chamber with reasonal dimentions, even with some blowby, would more than likely form/flow rather quickly as the bullet moves forward. I'd also tend to believe that with the alloys we use, in cast bullets, that some obturation occurs while the bullet base is still in the neck, which may also fill that minute space and help seal/expand the case neck.
I pondered the grease test you did and I'm not sure it would apply here. I believe your test would indicate the hydraulic expansion rate, which is nearly 1-1 as far as energy transfer in a confined space.Wouldn't gas, in this case expanding gas, have a lower expansion rate, with the heat having some effect on the brass also? Just a S.W.A.G on my part, but interesting fuel for thought.
In the few bullets you have fired of this design, were you able to recover any of them? If so, was there any evident gas etching inside the lube groove?
I remember some M60's we had at Ft.Hood that had extremely loose chambers and would really spit out some thilthy casings. Short bursts looked more like a round of 00 buck going down range. Our Armor said they were "within spec", which translates to: If you have to go, your going to get a new one, and as long as this one goes bang it's good enough to familiarize with.
I've been working in the shop today getting ready to attend a trade show this next week, and thinking about this subject. I've about got myself convinced that the neck of the case, in a chamber with reasonal dimentions, even with some blowby, would more than likely form/flow rather quickly as the bullet moves forward. I'd also tend to believe that with the alloys we use, in cast bullets, that some obturation occurs while the bullet base is still in the neck, which may also fill that minute space and help seal/expand the case neck.
I pondered the grease test you did and I'm not sure it would apply here. I believe your test would indicate the hydraulic expansion rate, which is nearly 1-1 as far as energy transfer in a confined space.Wouldn't gas, in this case expanding gas, have a lower expansion rate, with the heat having some effect on the brass also? Just a S.W.A.G on my part, but interesting fuel for thought.
In the few bullets you have fired of this design, were you able to recover any of them? If so, was there any evident gas etching inside the lube groove?
Ian
Notorious member
That bullet was designed to test several things that DID keep me up nights, still working on it. My thoughts on no base band: If gas is going to blow past the check while still in the neck (it does), what difference does is make if there's a large space or small space directly in front of the check? Part of what happens is the bullet nose obturates (corks) the throat after moving only a short distance. At that point, the lube groove is aligned with the gap between the case mouth and end of chamber. Resistance to bullet movement increases as the lands begin to cut into the shoulder of the bullet ahead of the full bearing surface, and at that point the bullet is fully-aligned and supported by the throat. The increased resistance, if played right with alloy and the pressure curve of the powder just after ignition, causes the gas check to crush the lube shank slightly. At the same time, metal is being swaged rearward from the driving band area as the bullet is squeezed through the taper of the throat. The metal closing in on the lube shank displaces lube into any void at the front of the chamber, effectively filling it and providing a seal and a bridge for the gas check or any other driving bands to follow into the throat. Think of it as a hydro-dynamic bridge. Ever notice rings of perfectly clean, unmelted bullet lube stuck to your fired case mouths? That's the hydraulic lube bridge in action. The swaging action of the bullet, if manipulated correctly through component choice, makes a perfect-fit, dead-centered bullet. The lube bridge helps it get into the bore straight by supporting the weakest point of the transition: The open space and diameter step-down between case mouth and throat entrance. The laws of hydraulics tell us that the lube bridge will form a uniform ring due to pressure equalizing within the "system" of the trapped lube bridge, so we get the added benefit of concentric guidance from the lube bridge, which is more than we can expect from our brass. Once fully in the throat, the lube groove should still be full, though smaller IF, and that's a BIG IF, the capacity of the groove is sufficient and the brass isn't trimmed too short. Think big-bore black powder cartridge rifles and the bumping of undersized, soft bullets to fit the throat perfectly, and how that method out-shoots almost any other at 1,000 yards.
Rally Hess
Well-Known Member
Wouldn't there also be some turbulance in the grease groove from the local gases, or do you suspect the hydraulic affect would exceed that in a larger grease groove that close to the base?
I've had some lube failure in years and projects past. I've recently been very impressed with Bens Red plus some beeswax to get the consistancy I wanted in pistol bullets. Hence my references to the possible blowby and it's effects/relationship to brass expansion and sealing. I indeed have noticed the grease on case mouths and necks, which seems to be most abundant on light target loads, that shoot bullets with large grease grooves. I've cast alot of the RCBS 44-245 for my .44 rifles and pistols. Seems the lighter the load the dirtier the casings. In my magnumitist days the casings I had to drive out of the cylinders were the cleanest.
I've had some lube failure in years and projects past. I've recently been very impressed with Bens Red plus some beeswax to get the consistancy I wanted in pistol bullets. Hence my references to the possible blowby and it's effects/relationship to brass expansion and sealing. I indeed have noticed the grease on case mouths and necks, which seems to be most abundant on light target loads, that shoot bullets with large grease grooves. I've cast alot of the RCBS 44-245 for my .44 rifles and pistols. Seems the lighter the load the dirtier the casings. In my magnumitist days the casings I had to drive out of the cylinders were the cleanest.
Ian
Notorious member
Turbulence in the groove is something I've pondered too, it makes sense that the lube would atomize within the groove and swirl in which ever direction the flow is going. Thing is, I don't see that happening when lube rings form on the case, or I don't think it's happening because the lube doesn't get dirty at all, it just gets pushed into the void at the end of the chamber and left there.
It's late or I'd take some pictures of living proof of how lube viscosity and friction characteristics affects the ignition and rate of pressure rise within the case. Just in the past two days I've tested two lubes that were night-and-day different with all else in the load the same. One left a black, flaky coating of powder residue inside the case below where the bullet was seated and case exteriors were slightly darkened. The other lube (with more "friction" for lack of a better description) left the inside and outside of the cases looking shiny-new. All cases had been cleaned to a bright finish and passivated with citric acid solution and stainless-steel pins. I didn't chronograph the two loads but I'm betting that the lube that produced the clean, well-obturated cases produced a lot more velocity.
It's late or I'd take some pictures of living proof of how lube viscosity and friction characteristics affects the ignition and rate of pressure rise within the case. Just in the past two days I've tested two lubes that were night-and-day different with all else in the load the same. One left a black, flaky coating of powder residue inside the case below where the bullet was seated and case exteriors were slightly darkened. The other lube (with more "friction" for lack of a better description) left the inside and outside of the cases looking shiny-new. All cases had been cleaned to a bright finish and passivated with citric acid solution and stainless-steel pins. I didn't chronograph the two loads but I'm betting that the lube that produced the clean, well-obturated cases produced a lot more velocity.
Rally Hess
Well-Known Member
Sorry for the long delay in a reply. I've been on the road.
Are you implying the "more friction" in your lube description, to be a thicker lube initially? If so, wouldn't that effect neck tension, initial bullet movement and pressure rise, at least somewhat?
Your description of the first lube would seem to indicate it didn't seal as well as the second lube from the description of the external deposits. The internal deposits, below the bullets seated depth, indicates some turbulance before the bullet base left the neck. Seems more probable with a thinner lube, but could also be present in the second lube. If the first lubes base was more flammable it would leave less residue.
Are you implying the "more friction" in your lube description, to be a thicker lube initially? If so, wouldn't that effect neck tension, initial bullet movement and pressure rise, at least somewhat?
Your description of the first lube would seem to indicate it didn't seal as well as the second lube from the description of the external deposits. The internal deposits, below the bullets seated depth, indicates some turbulance before the bullet base left the neck. Seems more probable with a thinner lube, but could also be present in the second lube. If the first lubes base was more flammable it would leave less residue.
quicksylver
Well-Known Member
HUMMMMM Great information, interesting discussion..
I hope you will indulge me for a moment.
It has been my job to think outside the box so I would like to apply some of that "expertise" here.
Fist off from what I can see here all the bullet designs and discussion thereof have been about cast bullets of more or less traditional design.
That being the use of traditional gas check, i.e. standard cup height and the use of lube grooves more or less located towards the base of the bullet.
What if, the gas check was made longer, a small lube groove immediately in front of that and then a large bearing surface with multiple lube grooves in front of it.
This design would (should) help some issues with standard cast bullets, 1. help relieve some of the gas cutting,2.put the lube ,I would think where it would do the most good, 3. place the bulk of the bullet weight towards the rear of the bullet ( think HP, I have heard they are more accurate than solids),4. it would also place the strongest portion of the bullet where the most stress is.
Think 315 top potion (with fewer grooves) , one of the pictured lower portions.
I only offer this because most of the discussion has been linear, perhaps a whole new way of looking at cast bullets could be beneficial.
I guess this would be like shooting most bullets backwards.
Who knows the Paradox wasn't supposed to work but it did.
Thanks for listening.
I hope you will indulge me for a moment.
It has been my job to think outside the box so I would like to apply some of that "expertise" here.
Fist off from what I can see here all the bullet designs and discussion thereof have been about cast bullets of more or less traditional design.
That being the use of traditional gas check, i.e. standard cup height and the use of lube grooves more or less located towards the base of the bullet.
What if, the gas check was made longer, a small lube groove immediately in front of that and then a large bearing surface with multiple lube grooves in front of it.
This design would (should) help some issues with standard cast bullets, 1. help relieve some of the gas cutting,2.put the lube ,I would think where it would do the most good, 3. place the bulk of the bullet weight towards the rear of the bullet ( think HP, I have heard they are more accurate than solids),4. it would also place the strongest portion of the bullet where the most stress is.
Think 315 top potion (with fewer grooves) , one of the pictured lower portions.
I only offer this because most of the discussion has been linear, perhaps a whole new way of looking at cast bullets could be beneficial.
I guess this would be like shooting most bullets backwards.
Who knows the Paradox wasn't supposed to work but it did.
Thanks for listening.
Rally Hess
Well-Known Member
Sounds like a bore rider with some forward lube grooves and longer gascheck. Getting pretty close to a Ranchdog with cup point maybe.