GC Cast limitations.......is it speed or pressure?

[Spokerider]

I keep reading that *generally* speaking, cast bullet accuracy begins to deteriorate after 1800-1900 fps or so. Of course there are many exceptions to this, with good accuracy being achieved with both faster and slower velocities.

What is the main limiting factor [ or factors ] with achieving j-bullet velocities with GC cast, in a 14" long barrel?


Assuming GC cast boolit fit, hardness and lube is optimum, is the accuracy limited by the amount of pressure needed to get the cast to equal j-bullets?


Why do I want to match J-bullet speeds with cast?........I just know somebody is going to ask........
Because I want to.

[caseyboy]

My understanding and also my observations is with the RPM of the boolit. Do a search on the RPM Theory/Threshold. Larry should be along in a bit to set you straight. :mrgreen:
caseyboy

[Green Lizzard]

after 30 yrs and 10000 rounds of 30-30 i am buying larrys theory.

[williamwaco]

You can read lots of stuff on these boards, and in the various gun rags.
There are two types of writers:

1) Those who have read a lot about reloading and shooting and think they know everything about it.

2) Those who have DONE a lot of reloading and shooting and know they don't know everything but what they do know is correct because they have done it.

Most of the stuff in gun magazines is of type 1. Most magazine articles I read these days are re-writes of articles I read twenty years ago. AND when I read them twenty years ago they were re-writes of articles I read fifty years ago.

We are fortunate to have quite a few type 2 writers on this board. You can learn rather quickly which they are. Listen to them.

You can match jacketed bullet velocities with cast bullets in most calibers. BUT: In some cases it is a LOT of work and can cause many headaches before you get it right.

Based on your comment, you have been reading too much type 1 writing.
My personal first experience with cast bullets in a rifle were with the .30-30 in about 1970. The first load I tried was around 1800 fps.
It shot three inch groups at 100 yards from a Marlin Micro-Groove 336. (Everyone knows those guns can't shoot cast bullets - Right? )
I stepped up to 2000 then 2200 with a stiff charge of 3031 from the jacketed loading data. It still grouped in three inches with iron sights.

I now have a TC Contender .30-30 with a 23 inch barrel and a 14x Nikon. That same load shoots 1.5" groups at 100 yards.

Yes, you can go higher, I just don't see any reason to ( for my use ).

PS: Air cooled wheel weights BNH 12.

[runfiverun]

Fit...

[Spokerider]

Thanks for the comments.

My barrel is a 1 in 14" twist, so it has medium-weight bullet velocity potential.......so I'm told.

I have some 155gr GC Loverin style boolits on the way......cast with Larry's alloy recipe, COWW with 2% tin / 50% pure lead, air cooled.

I have some Nosler Ballistic Tips for back up, if the barrel doesn't like the Loverin's, since I don't have an endless supply of .30 caliber molds to try / choose from.

[357maximum]

I would say that the quality of the castboolits (consistency), boolit fit, alloy toughness(I did not say hardness) and ultimate muzzle pressure (powder burnrate choice) has more to do with accurate j-word velocity cast boolit shooting than any other factors.

Twist rate (Larry's rpm theory),lube choice/quality, ultimate chamber pressure, rifling type,barrel length are also contributing factors along with several other, but I mentioned the ones first at the begiining of this post for darn good reasons. BaBore taught me how to make consistent boolits so that is no longer an issue for me....so......Fit/alloy/powder choice are the big three in MY book.

I use alliant MP-300 and Vhitvouri N-120 in my 14 inch 30/30. I still get good speed, a somewhat soft liftoff, but the powder is still fast enough to not have hurrendous muzzle pressure.

[Outpost75]

Fit, pressure and strength of materials.

[Larry Gibson]

I keep reading that *generally* speaking, cast bullet accuracy begins to deteriorate after 1800-1900 fps or so. Of course there are many exceptions to this, with good accuracy being achieved with both faster and slower velocities.

What is the main limiting factor [ or factors ] with achieving j-bullet velocities with GC cast, in a 14" long barrel?


Assuming GC cast boolit fit, hardness and lube is optimum, is the accuracy limited by the amount of pressure needed to get the cast to equal j-bullets?


Why do I want to match J-bullet speeds with cast?........I just know somebody is going to ask........
Because I want to.

You don't mention the cartridge(?).

Primarily it is RPM that is the limiting factor as mentioned. However, with a given cartridge and twist, it is also how you get to that RPM (speed) that matters also. All is not equal when we have barrels of greatly different lengths.

Let's take for example 2 different .308W rifles with 14" twist barrels; one has a 16" barrel and the other a 26" barrel. Lets say we want 2500 fps with the 160 gr 311466 Loverin cast bullet out of both. In the 26" barreled rifle we can use a slow burning powder giving 100% load density to achieve the 2500 fps. In the 16" twist barrel that load will not achieve the 2500 fps. Thus we will have to use a faster burning powder and accelerate the 311466 faster (give it a shorter time pressure curve) to achieve the 2500 fps. In doing so we increase the psi and cause more damage (imbalances) to the cast bullet. Since the bullet out of the 16" twist barrel is imbalanced to a greater degree the RPM threshold where the accuracy will decrease is at a lower RPM and hence a lower fps.

Basically the bottom line here is that the longer barrel will provide a higher RPM threshold with a given cartridge and twist because we can accelerate the bullet at a slower rate to achieve a given velocity.

So as to your question; "Assuming GC cast boolit fit, hardness and lube is optimum, is the accuracy limited by the amount of pressure needed to get the cast to equal j-bullets?" the answer is yes with the 14" barrel as odds are you will have to accelerate a cast bullet too fast. However, a lot still depends on the case capacity of the given cartridge which we still don't know(?).

Larry Gibson

[jonp]

Google Space Research in North Troy, VT. As larry said there is a reason the barrels they designed to launch satellites into orbit were not 5ft long

[mdi]

FWIW I'd like to help but my only experience with higher velocity lead in rifles is with a Handi-Rifle in .30-30 shooting 160 gr. gas checked bullets. I reloaded them to about 1800 fps and am getting about 2" at around 80 yds. (limited distance shooting area). I'm not a great shooter anymore, so that's about average in group size from my other centerfire rifle shooting...

[Spokerider]

You don't mention the cartridge(?).

Primarily it is RPM that is the limiting factor as mentioned. However, with a given cartridge and twist, it is also how you get to that RPM (speed) that matters also. All is not equal when we have barrels of greatly different lengths.

Let's take for example 2 different .308W rifles with 14" twist barrels; one has a 16" barrel and the other a 26" barrel. Lets say we want 2500 fps with the 160 gr 311466 Loverin cast bullet out of both. In the 26" barreled rifle we can use a slow burning powder giving 100% load density to achieve the 2500 fps. In the 16" twist barrel that load will not achieve the 2500 fps. Thus we will have to use a faster burning powder and accelerate the 311466 faster (give it a shorter time pressure curve) to achieve the 2500 fps. In doing so we increase the psi and cause more damage (imbalances) to the cast bullet. Since the bullet out of the 16" twist barrel is imbalanced to a greater degree the RPM threshold where the accuracy will decrease is at a lower RPM and hence a lower fps.

Basically the bottom line here is that the longer barrel will provide a higher RPM threshold with a given cartridge and twist because we can accelerate the bullet at a slower rate to achieve a given velocity.

So as to your question; "Assuming GC cast boolit fit, hardness and lube is optimum, is the accuracy limited by the amount of pressure needed to get the cast to equal j-bullets?" the answer is yes with the 14" barrel as odds are you will have to accelerate a cast bullet too fast. However, a lot still depends on the case capacity of the given cartridge which we still don't know(?).

Larry Gibson


It's the TC 30 30 1 in 14" twist barrel that was re-chambered to 30 40 Krag by SSK. It is the old 6 groove TC rifling. I was asking about loads for this barrel earlier if you recall.

Since the boolits that my friend has cast for me have yet to arrive, I have only time on my hands to contemplate my options of getting this barrel to shoot cast as well as jacketed. If I find cast boolit accuracy deteriorating before velocity is at, or near the ballistic tip velocity for identical powder type and similar powder weight, I want to know where and what to look at to correct.

I see where you are coming from, with the rate of acceleration differences in 16" barrels vs 26" barrels, and with achieving the target velocity. It would be unrealistic for me to think that I can achieve 24" barrel velocities with a 14" barrel......this I know. Rather, I would like the cast to match the J bullet velocities, in the same barrel, same powder [ or perhaps a different powder if cast boolits dictate a different burn rate ] and with similar pressures.......near 30 40 Krag maximum.

So, I'm reading that there needs to be a balance between rate of acceleration and muzzle pressure? What will ultimately become the cast boolits accuracy wrecker, too high a rate of acceleration [ faster burning powder? ] or excess muzzle pressure and boolit imbalance because of it [ slower burning powder ]? If it can be simplified to these two things.........

[runfiverun]

it can't because they are way down the list.
fit means just that fit,,,, static fit, it includes the case.
then you worry about the launch, and the muzzle pressure, and then other little things.
none of it matters if you screw up the first 1/2" of travel.

[Larry Gibson]

My TC 30-30 barrel like the 311466 sized at .311 (go figure!:D ). I suggest starting with the slower burners such as RL19 and slower if you have a selection/choice with the 30-40 case. If load density is below 80% use a Dacron filler. Ideally you want a powder that ignite consistently at 100% load density and gives the highest velocity with accuracy.....that's the trick anyways.......

Larry Gibson