A mould for the .30 XCB/.30-'06

[geargnasher]

For those involved in the XCB project, I'm going to work out the final design here for my uses and any input would be appreciated so we can all have access to the same design. Since the XCB has a standard '06 throat with zero freebore, .3106" throat entrance, and 3-degree included angle to .300" lands, this would be an excellent light-medium boolit for the '06 as well.

I'm going to modify my original design slightly to better fit standard dimensions. The Accurate 31-185G was worked out for a rifle with .309/.301" figures and from my mould engraves a touch too hard in my XCB, and the leade angle is a bit too steep.

Here's what I'm starting with:



I'd like to slim the nose to .299" at the ogive break and taper through to .301" for the portion that rides the bore. The next section would be a straight taper at 3-degrees included angle to front band diameter, which I'm planning to reduce to .310". I'd also like to make a single, radiused lube groove rather than the steep, square-bottom groove because I think a radius flings lube better and crushes less, but I might just leave it alone. My "patented" ZeroDisplacement gas check shank will remain as designed for Hornady gas checks. Other things we might keep the same are weight (180-190 grains, for those inclined to run a twist slower than 1-in-12"), and the approximate driving area. I'll put up some drawings of the modified design once we hash this out a bit.

Input? Yeah, I know, it needs a .080" or so meplat, but Tom has his ways about that. I'll probably fix one cavity with a ball mill at a later date and let everyone know how it shoots.

Gear

[btroj]

I think that sounds about right. My barrel is gonna be snug enough to need a .299 start.
As cast is looking to be .310? Sizing to .310 I assume?

What is a "zero displacement" check shank? Looks like it has to do with the fact the shank will be snug on the bottom of the check and has a slight taper on the shank to give room for the check to crimp. Also looks like the shank is short enough to leave little room for lube in front of the check.

I like it. 2 cav brass is what I will be going for.

Brad

[geargnasher]

OK, who did the 31-200G? It has EXACTLY a 3-degree included nose taper. Somebody has a SAAMI-spec. '06 and knows what they're doing. My only concern is there is ZERO wiggle room without distorting the boolit, and they're a lot to go wrong in the neck behind the tapered section. I'd like to see less in the neck and maybe more out front.

Gear

[geargnasher]

Pretty much so.

The '06 throat is essentially a Morse or Jacobs taper. Translated to boolit proportions, that's .382" length for .010" diameter change. The total throat should be about .400" long from .3106" to .3000" at the lands, but I like a boolit that's just a fuzz smaller than throat entrance diameter and neck clearance that is the same or just a couple of tenths more than the boolit/throat entrance clearance.

I'm going to sleep on it, but right now I'm leaning toward the straight taper rather than a parabolic one like the 185G.

Gear

[btroj]

If the throat has a straight taper why wouldn't you mimic that with the bullet nose? If the angles are the same or very similar then a straight taper should allow maximum contact between bullet nose and throat. With a snug neck and strong contact between nose and throat how can the bullet do anything but enter the bore straight?

A straight taper from .298 or .299 to to .310? I like the short body length of the 195G but I think the nose taper needs to leave at least 40 percent body.

[geargnasher]

If the throat has a straight taper why wouldn't you mimic that with the bullet nose? If the angles are the same or very similar then a straight taper should allow maximum contact between bullet nose and throat. With a snug neck and strong contact between nose and throat how can the bullet do anything but enter the bore straight?

A straight taper from .298 or .299 to to .310? I like the short body length of the 195G but I think the nose taper needs to leave at least 40 percent body.

That's the idea. If one wants to remove all doubt and is extremely, super-duper careful about exact neck thickness and uniformity, AND uses well-aged boolits that are finished growing, there is such thing as zero or even slight interference neck fit, but don't tell anyone on cast boolits 'cuz they'll freak out

Gear

[HARRYMPOPE]

Just order an Eagan style taper die from Erik Ohlen and make any bullet fit.Or have a bump die made from the throating reamer that cut your chamber. A 3 deg throat is too steep IMHO,Try a 1-1.5 deg angle per side.

[Larry Gibson]

If you are designing the mould to take full advantage of the 30x57 cartridge (even with the faster 12" twist) to push the RPM threshold up then I suggest you've the cart before the horse. You are establishing a given weight and then making that weight of cast bullet "fit". I suggest the design should be full diameter (.311) to fit the case neck and throat length with the GC at the base of the neck and the full diameter to the leade. The nose should simply taper from there. Let the weight be what it will be. That way the bearing surface should be 60 - 65% of the bullet's length which is best for HV in that twist. Actually that bullet has already been designed and is offered by LBT as the 160 gr. I is is an excellent one as it was Bass Ackward's favorite for his HV loads in his '06 and proved excellent in my own 12" twist target M70.

However, if you are just wanting a heavier bullet to shoot pigs with then drive on and ignore this post.

Larry Gibson

[btroj]

I like it Gear. It will fit OUR needs quite well.

[SSGOldfart]

Maybe I'm jumping in the middle here, but wouldn't.311 give use more room to size if needed.

[btroj]

.311 would allow for more sizing but if you KNOW the barrel dimensions why not make a bullet that needs minimal sizing?
Bullets that allow for sizing are a good thing when bore sizes are unknown or could vary over multiple rifles.
My barrel is a true .308 x .300 bore and that tells me why size I want bullets to drop at.

This is a specific bullet for a specific rifle. That means it can be made to be just what is needed.

[geargnasher]

SSG, yes it would, IF needed, as many of the '06s out there do. One of the many advantages of having Tom at Accurate Molds work up a design is he lathe bores them and will cut the bands a thousandth or two larger on an existing design at your request. In fact, the exact size and alloy are things you specify on the order form.

The XCB, with fresh throats, does best at a fuzz over .310". I already have one and have shot it a bunch with two different boolit designs, including the 31-185G

Gear

[btroj]

And how did the 185G do? Bet it was good but not quite good enough, hence the tweaking.

What powder you running Gear?

[SSGOldfart]

Thanks Gear, that makes sense as I'd want a true.311

[geargnasher]

And how did the 185G do? Bet it was good but not quite good enough, hence the tweaking.

What powder you running Gear?

H4350 at first, switched to WW 760 and it seemed to do much better. Also tried H4831 and 3031, but the 3031 hit a little too hard too soon. I also did a little WC 872/booster trick just for giggles and to use up the half-cup or so of that old stuff I had laying around. Thinking hard about WW780, that was THE powder for 190-grain PP boolits (2650 fps in an '06) and there was about 8 grains of room left IIRC, but I'd have to check my notes. Some of my HV work with the '06 is what led me to settle on the case capacity of the XCB, just about a case full of WW780 Supreme should get a 190-ish boolit up to 26-2700 without booster or buffer and still plenty within the pressure limits of the '06/Mauser family of cartridges.

My limit was alloy. 2400 fps is about it with WD 50/50 and a rather ill-fitting nose shape. Same alloy poops out in my ten-twist .308 at about the same velocity, but is turning about 30K rpm faster. Need something just a hair tougher for good accuracy at 2600 and a better-fitting boolit, either copper-enhanced or Lamar's high-velocity alloy. That is, unless you want to go to a 14-twist and shoot 160-grain boolits into an inch and a half. I'm already keeping 186-grain boolits well inside an inch at 2400. Did someone mention that Tim builds excellent rifles?

Gear

[btroj]

I figured a bit of copper would be a good thing. I need to order a bit of copper Babbitt from Rotometals and mix up some alloy. A few hundred pounds of a single alloy would be easy to mix up and set aside for this rifle.

Yeah, I had heard something about Tim making good rifles. Seems some even burn out barrels in a few months....

[Larry Gibson]

Yup, 1 1/2" groups at 2600 fps with a 160 gr cast bullet. That's 10 shot groups BTW. 3 and 5 shot 1" or less are common. You hit a wall ( your way of saying RPM Threshold) at 2400 fps because it is what it is. To hit 2600+ fps with a 195 gr cast in the 30x57 will require 60,000 + psi. Think your bullet will hold up to that?

Larry Gibson

[geargnasher]

Yup, 1 1/2" groups at 2600 fps with a 160 gr cast bullet. That's 10 shot groups BTW. 3 and 5 shot 1" or less are common. You hit a wall ( your way of saying RPM Threshold) at 2400 fps because it is what it is. To hit 2600+ fps with a 195 gr cast in the 30x57 will require 60,000 + psi. Think your bullet will hold up to that?

Larry Gibson

As much as you'd like everyone here to believe what you made up about that, I never wrote or implied that the "RPM" was the issue. The alloy I was using is failing at the pressure and poor fit, a simple limit I've tested and found many times in many guns. It failed even sooner with 3031, I was just curious to find the failure points in THIS rifle. Got about 50 FPS more than usual for a .30-caliber, probably because of the long barrel. Tougher alloy and better fit will fix it right up.

My boolits can take a lot more pressure than yours because I use different techniques from yours when building my ammunition. I haven't seen any groups from YOU out of YOUR 10-twist .308 at 169K RPM that go into 1-1/4" for repeatable, ten-shot groups at 100 yards, and yours is a much better rifle. Don't get me started on what I've done with cast in the 7mm-08 or the military Swedes in the 200K+ arena (not at 60K psi, either). You've proven repeatedly that you can't shoot past 144K rpm and hold under an inch, even with a harder alloy, in any of your guns that I know of, and until you do and can tell ME what I already know about fit and alloy strength, leave the random "It's the RPM!!!" axe in the shed, because that dog won't hunt and you can bring nothing constructive to the conversation.

Gear

[Larry Gibson]

geargnasher

The alloy failing from the pressure and the acceleration is what is causing the imbalance in the bullets inside the barrel (that's internal ballistics). It is the centrifugal force of the RPM that is the cause of the inaccuracy in flight (that's external ballistics). If there was no centrifugal force from the RPM your imbalanced bullets would be just as accurate as balanced bullets. Thus it is indeed the RPM that is causing the inaccuracy. You back off on the RPM by lowering the velocity (because your rifle barrel twist is a constant) and accuracy will improve. Or you can change to a slower twist and lessen the RPM which lessens the centrifugal force and it won't affect the imbalanced bullets as much and accuracy improves. It's as simple as that.

Regards what you've not "seen" from me. I have posted numerous groups from rifles of various twists here on this forum, including those from 10" twist rifles. It is you and a couple others who fail to demonstrate what the claim. I, just in the last 45 days stopped at Kerrville wanting you to show me how you do what you claim. You declined to demonstrate your shooting claims. So what does that tell us? The fact is I will shoot with anyone to demonstrate what I say is correct and what I can do. You won't. So why don't you slip on over to AZ and we'll go shooting some cast bullets and I'll show you. Maybe frank will come on down also.

If you remember correctly I came up with the concept of the 30x57 (which you have renamed the 30 XCB) and the 35x57 some years back and was discussing the concepts here back in '07 and specifically '08 (http://castboolits.gunloads.com/show...0-call-bullets & http://castboolits.gunloads.com/show...0x57-amp-35x57).

I came up with that concept of the 30x57 for several reasons the most of which was to use a 14 or 16" twist barrel to keep the RPM under 140,000 for shooting cast bullets accurately at HV w/o all the fuss and frustration such as you've encountered. Since you have chosen not to follow the original intent and are using a 12" twist barrel you have run up against the RPM Threshold (you are calling it a "wall" here) at just about 2400 fps. That is what RPM........144,000 RPM is what. That is just where the RPM Threshold is with your rifle and loads. So obviously that "dog" has hunted.

If you want to push cast bullet velocity higher than 2400 fps in your 12" twist rifle with accuracy then use a better designed bullet with less nose hanging out in front of the bearing surface. That is what I'm bringing to the conversation. As I mentioned in my 1st post if you simply want to push a 195 gr cast bullet then you are heading down the right path. The 30x57 with slower powders will max out at or under the RPM Threshold at 2400 fps anyway. It should work just fine.

Larry Gibson

[geargnasher]

On paper, anyway. Order up if you like them, and remember, you spec the alloy and should have some latitude on size.



Many thanks to Tom who whipped this up two days from my request.

Gear