SLOW Twists and FAST Casts Using CUSTOM Barrels...Results, Please?

[MBTcustom]

Bjornb, how long is your Shilen 1in17? I chose the 15 because of Shilen's 30" finished limit and a desire to still shoot some heavies.

I can answer this and Bjornb cannot as the barrel was just recently chambered, threaded and crowned here at MBT and the rifle still resides here.
It is necessary to cut 1" off the muzzle end of the barrel (more or less) in order to achieve accuracy. The blank was shipped about 1" long (27.75") so the finished length after crowning is 26.75".
Also headspaced to this rifle is a 30" Brux in 30XCB, and a 29" Shilen in 35XCB, both of which are 1-14 twist. We have a twitch barrel Mauser.

This is being done on my own Felix rifle, and loaned to Bjornb to continue testing while the Bertha rifle is being overhauled.

[kens]

Quote Larry Gibson:
" However, with the 30 XCB cartridge (30x57) in at least a 26" barrel with a 14" twist will yield "Mauser" velocities of 2400 - 2700+ fps w/o such travails."

Quote Goodsteel:
"this rifle is a 30" Brux in 30XCB, and a 29" Shilen in 35XCB, both of which are 1-14 twist"

OMG !!!!
I'm having a vision!!!

One of us has a old RIFLE length Mauser (29"bbl) that we fit a replacement bbl (XCB), stepped contour, complete with original dress hand guards, bbl bands and all.
What a sleeper of a rifle!!!
Launching old WW's!!!

[Bjornb]

Yeah that would work I guess:

However the ruse would fall apart once I had to mount my Weaver T36 scope on it in order to see the target!

[runfiverun]

I'm wondering if the linotype alloy isn't too brittle at the higher speeds and is breaking at the crimp point of the gas check.
the boolits are still fling fairly straight [2.5' groups] but the gas check ain't really needed after it leaves the barrel.

I have heard of many high Sb boolits breaking off at the crimp groove during loading or if dropped.
the crush of the gas check could be causing this to happen.

[Larry Gibson]

No indication of any part of the bullet that may have broken off hitting the Skyscreens. All damage is being done by the GCs.

If the base of the bullet doesn't melt then can someone explain the soldered alloy on the inside of the recovered GCs?

I have mentioned numerous times over the years on several of these threads that with many bullet designs don't do well at HV when cast of linotype due to the brittleness of the antimony rich alloy. I have described how the back edges of the drive bands have chipped and broken off from the land engraving. This was found on otherwise undamaged recovered bullets shot into snow. My recent HV tests using the .308W and the 30x60 XCB seem to verify that linotype is not as accurate as is #2 alloy at the top end velocities above 2700 fps in the 14 and 16" twist rifles. I'll be switching back to #2 alloy and also COWWs +2% tin in the next series of tests. I compare the results using those softer alloys closely with the results of the linotype bullets using the same loads in the 2700 - 3000+ fps range.

As mentioned several times ; this is new territory shooting cast bullets with consistent usable accuracy above 2700 fps and into the 3000 fps range. Some different things are happening that we are not sure of what or why yet. Only further and more complete testing will tell us the answers. However it's a pretty good sure thing that the 30 XCB cartridge in a 14" twist barrel with at least a 26" barrel will get you very good results with cast bullets at least into the 2600+ fps range and higher with a longer barrel.

Larry Gibson

[Nrut]

Larry,
How does Leverevolution behave pressure wise in different ambient temperatures in the .308 case?
The reason I ask is Leverevolution is sometimes available at the LGS while I haven't seen any other powders there (other than Trail Boss) for almost 9 months..
Canadians have finally taken up "hoarding"..
Should add that I will be using jacketed bullets..

[Larry Gibson]

Nrut

Wish I could give you a definitive answer but I've not tested it enough across a wide temperature range. I can say that the same load here in Arizona picks up about 25 - 40 fps between 60+ and 85 - 90+ degree's. At 60 - 70+ degrees the velocity of my 49.5 gr load runs 2940 - 2960 fps and is quite accurate (averages 1.6" for 10 shot groups). At 85 - 90+ degrees that same load begins to lose GCs with the velocity in the 2965 - 2990 fps range.

Larry Gibson

[Nrut]

Thanks Larry..
It very seldom gets over 80*F here and I don't shoot then so I won't worry about temperature insensitivity..
I will be using 308 Marlin Express data to start with which Hodgdon gives data for..

[Larry Gibson]

I suspect LvR will work very well in the 308 Marlin, especially with the 160 gr Fex Tip Hornady makes for that cartridge.

Larry Gibson

[1johnlb]

Larry or Tim has anyone considered reducing the size of the 30xcb by 15 or 25 grs in order to make case efficiency better to lower pressures and accommodate the slower twist while increasing velocity? With the slower twist it seems to me that bearing surface for the purpose of strength could be reduced to yield higher velocities. But maybe my train of thought is wrong.

[MBTcustom]

Smaller case capacity with equal velocity = higher pressures, and I'm already at a case capacity that gives good speed at the pressure I want to be running. It's a pretty specific range that I'm trying to stay in.

For instance, the original 30XCB with it's 57mm case length was designed to give 2700FPS velocity with normal medium burn powders.
The barrel twist was 14 because that's the sweet spot if you want to shoot right at 2700FPS.
When you get up to 3000FPS you start running out of case capacity, flattening primers, and getting a stiff bolt lift. Reducing case capacity would exacerbate these issues, because you would have to go to even faster powders in order to get the same speed.

It's the same as asking why not use a 308 Winchester to get the same performance as a 300WinMag? Because by the time you get the pressures high enough to get that level of speed, you start to run into problems.

So, if you want to run faster but you want to keep the pressure low enough to shoot accurately with cast, you have to do what Larry did and increase case capacity.

Bjornb has done the same because he wants to shoot even faster. He had me ream the 17 twist barrel to the full length 30-06.

See what I mean?

Now if you want to get precision at a lower velocity, and a lower speed, and keep the case full of medium burn rate powder, then you need to reduce the case capacity as you have suggested. That's why 30-30 is such an awesome caliber. That's why 358 Winchester is such an awesome caliber.
When you start looking at what works and works really really well for cast bullets, you start seeing a shockingly common correlation of case capacity to bullet size and a matching twist rate that makes certain calibers just awesome.
Unfortunately, most of the time this only happens in certain shining moments in firearm history when all the planets aligned just right to give us something that is just easier to get results with.
The XCB project (and the 358Malcolm before it) is simply my attempt (with help from members like Larry Gibson) to just once, design a cartridge specifically for cast bullets, but not just for cast bullets at normal ho hum velocities. The XCB project was an experiment to see what would happen if we took all the scientific facts we know works for cast, put all of it together into a usable system, and pointed it specifically toward shooting cast at high velocity, what we can do with a standard cast bullet.
So far, it looks like if you get everything pointed the right direction, you can easily shoot 3100FPS with accuracy of less than 2MOA.

[1johnlb]

Ok, maybe I wasn't clear. I wasn't speaking of the case but the boolit. Reducing projectile weight and bearing surface reduced friction ( heat ) and gaining case effectiveness. Smaller boolit less force needed to move it. Slower twist less stress on the bearing surface thus less surface needed. What 14 twist needs may not be necessary for a 17 twist.

[Bjornb]

That would bring the bullet size down into LBT 150 territory. Sure it may work, but one problem would be to get a prototype made just to test it out. NOE is currently the only mould maker that can cut the kind of ogive used on the XCB, and we saw with the Accurate version, which is an excellent bullet up to about 2500 fps, that the two designs don't behave identically.

[MBTcustom]

Bjornb is exactly right. Making changes to the design is a very difficult and expensive proposition. We designed the 30XCB by carefully doing everything we know seems to work. The main objective was to fit the cartridge exactly and create a bullet that both fits and flies very well, and strengthen the design as much as possible.
I would think that reducing bearing surface would require reducing the weight forward, which would pretty much mean redesigning the entire bullet from scratch.
This is something that I would like to explore in the future, but I need more data before stepping out. I'm hoping we can get a few more calibers shooting fast so we can get a clear idea of what's the most important. Sometimes this is like trying to get a death grip on a piece of silly putty.

[Larry Gibson]

Larry or Tim has anyone considered reducing the size of the 30xcb by 15 or 25 grs in order to make case efficiency better to lower pressures and accommodate the slower twist while increasing velocity? With the slower twist it seems to me that bearing surface for the purpose of strength could be reduced to yield higher velocities. But maybe my train of thought is wrong.

As discussed it is not as simple as that. Numerous things to consider besides just reducing the psi, The center of gravity in relation to center of pressure, adequate OAL bearing surface % vs actual bearing surface %, sufficient lube capacity with available groove(s), etc. Additionally the gyroscopic stability of the shorter bullet at higher RPMs, etc, etc.

As Tim mentions the 30 XCB bullet was designed for maximum fit in a 30-06 length neck/throat. The OAL bearing surface, initial angle of the ogive and the shape and length of the nose were all designed based on what we knew worked. I designed the lube grooves of the 30 XCB bullet to give just a sufficient amount of lube with the modern softer lubes we have today. The number and width of the lube grooves was designed by me to provide maximum OAL bearing length and maximum actual bearing surface given the length of the case neck and the chamber throat. The depth of the XCB's lube grooves was purposely kept minimal to increase the strength of the inner bullet column to provide maximum strength at that point to resist collapsing of the bullet and torqueing of the bullet in the smaller and unsupported portion of the bullet in the bottom of the lube grooves during the required hard/fast acceleration and higher pressures needed for HV. Tim combined my design of the lube grooves and my insistence of the shorter nose with no bore ride into his concept of the 30 XCB bullet for initial use in the 30 XCB Cartridge (the 30x57 with tight specs and a tight throat). As they say; "the rest is history".............

Larry Gibson

[1johnlb]

I'm definitely not trying to kick the design of the xcb, I've been able to push it faster than any other design I've shot thus far with accuracy, in my own 14 twist. But I also realize that designs have windows of usefulness in their abilities. I'm a mechanic not a bullet designer and I highly value all of your opinions. Being a mechanic I deal with tools and for example a 3/8 ratchet has a useful range in doing both large and small jobs, but there are some jobs that a 1/2 or 1/4 " ratchet just do better. A 1/4" ratchet is still a ratchet just made to fit tighter areas.

I also, realize it's not that simple, but as Larry's been saying somebody has to pay the band, if you want to dance. I also realize that stability becomes an issue with the shorter body projectiles, but stability also becomes an issue with slower twist and longer bodies. My thinking was that the shorter body would produce less heat from pressure and friction, also increasing velocity from less mass, with the removal of any unnecessary weight for the slower twist. Still a xcb bullet just tweaked to fit a certain window of usefulness. Also, smaller mass absorbs heat faster, but less mass to cause heating. I was just sharing a thought to deal with the plastization problem. Even if it worked, may not be enough fps to justify another design.

But maybe I'm in left field here and just need to get back in the dugout .

[Larry Gibson]

1johnlb

All certainly worthy considerations and all are those I have been considering and working with in testing the slower 14" twist barrel for high velocity since April of 2007 when I first began reporting the results on this forum. The first bullets I tested were the LBT 310-160-SP, the RCBS 30-150-FN, 311291 and the 311466 (standard mould). Regardless of the "resistance" I received from some members here I persevered through testing several other bullet designs. I found 2 bullets that worked very well in the 2450 - 2600+ fps range in the 14" twist; the LBT 310-160-SP and the 311466U. Fortunately several other members saw the science, logic and results of what I was doing. We considered and figured out what it was in the design attributes of those 2 bullets that made them successful at such higher velocity. It was those design attributes that were incorporated into the 30 XCB design. The 30 XCB is a successful design because we designed it based on what design attributes were proven to work at high velocity not what we thought would work or what looked good.

I am wondering though where or what "certain" means in "to fit a certain window of usefulness"? Thus far the "window" has been quite large as the 30 XCB has proven, so far, as accurate as any other 30 caliber cast bullets and more accurate than most at lower end velocities such as 1700 - 1900 fps in faster twists and even at higher velocities in those twists. It has proven to be the top performer at truly high velocities in 14" twists rifles and in my own 16" twist rifle up through 2950 fps.

I have also tested 4 other lighter weight bullets fairly extensively in the 14" twist Palma rifle and am testing them now in the 16" twist. They are the ; 311359U, the 311465 (both NOE and Lyman) the 311466U and the LBT 310-150. Thus far 3 of them do not do was well as the 30 XCB above 2550 fps in the 14" twist. Only the 311466U does as well in the 14" twist as the 30 XCB does up through 2700 fps. I've also found the same 3 bullets (the shorter lighter weight ones) do not do as well above 2850 fps in the 16" twist rifle either. The 311466U appears to do as well but I have a series of tests loaded to find out but have not tested them yet.

Right now the answer as to why the shorter stubbier cast bullets do not do as well is simply a matter of RPM. It appears they just simply have a lower RPM Threshold. The additional problem with the lighter weight bullets is one of not enough mass for good ignition and burn with the slower burning powders. To get good internal ballistics most often requires a faster burning powder which give a faster acceleration which seems to offset any potential gains made by using the lighter weight bullet. Those faster burning powders work well if we keep the velocity and pressures down. What we've found for the high velocity end of the "window" though is the shorter lighter weight bullets do not have the mass for the slower burning powders to be used to keep the pressures down. There a "balance" that must be maintained between several factors. So far the XCB design maintains that "balance" across a wide "window".

Is the 30 XCB the best cast bullet design? So far it has proven to be for high velocity in 10, 12, 14 and 16" twist barrels of various cartridges. Is the 30 XCB the best cast bullet for lower velocities? While it certainly does very well the answer there lays in what it's used for. I'm certainly not replacing the 311041 with it in my M94 Winchester 30-30s......if you see what I mean. Will it prove to be a winner in CBA matches? Perhaps. Will it be a winner in Schutzen matches? I doubt it. Is it the end all of high velocity cast bullet designs with no room for improvement? Probably not. We are in totally new territory here. We have reached cast bullet velocities with repeatable and usable accuracy that was only dreamed of previously. We are still learning and I'm quite sure there is more to learn.

Larry Gibson

[HollowPoint]

"What we've found for the high velocity end of the "window" though is the shorter lighter weight bullets do not have the mass for the slower burning powders to be used to keep the pressures down."

I'm wondering if the lighter weight bullets would shoot better if they were cast in a slightly larger diameter from the same 30 caliber barrels ? That is if they drop from the mold slightly over sized to begin with.

The tighter fit might serve to compensate for lack of mass, no?

HollowPoint

[Larry Gibson]

HollowPoint

I've not found that it does. The XCB chambers require a .310 sized bullet anyway which is what was done. In the Palma .308W rifle the throat requires a .311 sized bullet. Any increase in bullet size regardless of the bullet above those in either rifle detracts from accuracy. An overly large cast bullet, if the chamber dimensions allow it, will be seriously deformed/imbalanced by being swaged down in the throat/bore. That is counter productive to accuracy.

Also it does not take a lot of psi to swage the bullet down. That happens very early in the time/pressure curve and I've not measured any increase in internal ballistic efficiency from using oversized cast bullets. Once the bullet has traveled one bearing length into the throat/bore it is sized down and thereafter has the same "resistance" as a properly sized bullet would have.

Larry Gibson

[1johnlb]

I am wondering though where or what "certain" means in "to fit a certain window of usefulness"?

Every tool has its window of usefulness or ability, some tools have larger windows than others, but when they reach there outer limits because of size or ruggedness they have to be refined or tweaked for better service, just like the caliper and mic, or the same reason that a 35 cal won't fit a 30 cal bore. The very same reason for the 30xcb case, it's ability to be adjusted. The projectile is just a little more complicated.

Thus far the "window" has been quite large as the 30 XCB has proven, so far, as accurate as any other 30 caliber cast bullets and more accurate than most at lower end velocities such as 1700 - 1900 fps in faster twists and even at higher velocities in those twists. It has proven to be the top performer at truly high velocities in 14" twists rifles and in my own 16" twist rifle up through 2950 fps.

I definitely don't dispute this and completely concur. Just can't help but wonder what would happen with the xcb lube grooves on the 466 or even the Lee 155 or NOEs 311414. I should have known you were already testing every avenue.

It was just a thought.

If I could ever get a cooling trend here, like 90 degs or so, I will continue in my testing. Thus far, I can definitely see a fps gap with my shilen throats not being cut with the xcb reamer, when compared to your results. Even with the others designs I've tested that fit my throat they didn't get up to the velocity that the xcb did or has in mine.

John