Cast bullets and bore size question

[Bitman]

I know you're supposed to shoot cast lead bullets that are .001 to .002 over groove size.
But I have 3 different 38-55 rifles, all with different bore sizes.
What would happen if I shot a cast bullet in a rifle that was .004 or .005 over bore size?
Just wondering. Thanks

[DougGuy]

What do the throats measure? Optimally you would want to size to whatever will fit in the throat.

[Cris T]

I have a similar question. I know that the softer the lead the easier it is to obturate into the bore, but with a harder bullet and maybe 4 or 5 thousanths over bore size, could it cause too much pressure and blow up the gun?

[Larry Gibson]

I have a similar question. I know that the softer the lead the easier it is to obturate into the bore, but with a harder bullet and maybe 4 or 5 thousanths over bore size, could it cause too much pressure and blow up the gun?

I have conducted pressure tests in the .308W and the 30-06 using the same load in each rifle with the same bullet sized in .001 increments from .308 up through .314. The increase in average psi all but the .314 sized bullets was within the expected test to test variation. The .314 average psi was only a little outside that variation. The very small increase in psi was not enough to get concerned about and, considering, since cast bullet loads seldom are at the maximum MAP level for the firearm.

Keep in mind a bullet, especially a cast bullet, is swaged down to groove diameter or less if lubed in the first bearing length of travel in the barrel. That one bearing length of travel is very early in the rise to pressure curve and si often not even measurable with cast bullet loads. After that one bearing length of travel the swaged down bullet is just like any other bullet of groove diameter traveling down the barrel. Ergo, if there was any initial psi increase, the swaged down bullet does not cause any further psi increase past that intial one bearing length of travel.

[Wilderness]

I have shot .360" hard cast bullets through a .353" 9x56 MS (throat started at .359") and currently use .3125" in .30-30, both with superior accuracy. The latter are left at .3125" after sizing down from .322" in two steps. These are full speed loads for hunting.

My theory for .30-30 is that with such an abrupt throat providing minimal guidance for a cast bullet heading into the rifling, guidance from the case neck is the next best thing, achieved by taking up all but the last .001" or two of available space. Case neck guidance is especially valuable if the bullet cannot be seated on to the rifling, for instance due to COL limitations for magazine feed.

I think it was Richard Lee who is quoted as saying a cast bullet does not need to be sized if the rifle will close on the cartridge without resistance.

I believe the .38-55 also has the old style abrupt throat.

P.S. Good stuff Larry, gives me confidence in what I am doing.

[BLAHUT]

If soft no problem, hard is where things can get interesting... With some of my guns, I kept stepping up, as long as they would chamber, till I got the accuracy I thought was good for me...

[Rockindaddy]

I have experimented with boolits sized to various groove diameters in my 50-70 Sharps rifle. The groove dia was .514 Found that cast 450 gr Lyman shot ok about 2" at 50 yards. Wanted to kill a fat West Virginia buck so I wanted a soft bullet. I alloyed my boolits to 1:20 and made up a .518 size die and opened up the mold to .520. The old rifle shoots like it has eyes. Got clover leafs at 50 yds with a tang sight! Was shooting Reloader No.7 It is my firm belief that most rifles like cast boolits at least .002 over groove dia. I have an 1893 Marlin in 38-55. Its a great shooter! Been shooting 250 gr cast and sized to .380 with a 1:20 alloy boolit. Lynotype alloyed boolits don't work on game. They don't expand. One of these hunting seasons I plan to smoke a buck with the Marlin 93.

[Snakeoil]

My 03 has a 0.312 groove diameter. Did not know it at first and shot .311 and 312 bullets with dismal accuracy/repeatability. Friend handed me 2 rounds with .314 bullets that he uses is his shot-out Enfield. I drilled a bowling pin at 500 yds with those two rounds. Bought a 4 cavity 314210 GC mold from NOE and never looked back. Original 1921 barrel on that 03 and it is a tack driver all the way out to 500 yds.

I don't think that going over .002 of groove diameter will hurt you. But it won't help you either. Will probably shoot the same as a .002 over bullet. If you want to check pressures, download GRT and run the numbers.

[Bitman]

Thanks for all the replys. Some very interesting ideas. Gives me some things to think about.
I plan on sticking with .001 or .002 over bore size. But had no idea what might happen if I can a round mixed up.

[Rodfac]

I plan on sticking with .001 or .002 over bore size.

What I think you mean is groove dia, not bore size.

Bore size is the dia. from the top of the land on one side of the bbl. to the top of the opposing land.

Groove dia. is measured to the bottom of the rifling grooves and is generally somewhere around 0.008" larger than bore size. In a .308 caliber rifle, for example, the groove dia. is ~0.308 with a bore dia. of 0.300.

Rifles differ and each is an individual...in my Sako, manuf. in 1970, the groove dia. is 0.308". Over many years, I've had my best accuracy with bullets sized to 0.310" which is all that the throat will accept. In one of my Krags with a much larger throat, 0.314" dia works wonders.

HTH's & HTH's Rod

[Bitman]

Okay, here's the deal. I have a Ruger #1 in 38-55. I measured the groove diameter at the muzzle with dial cailpers. It's .379"
I took a .381 cast bullet and was able to tap in thru the barrel with a wood dowel fairly easy. Shoots almost one hole at 50 yds.
I have a 1906 built Win. 1894 in 38-55 I just bought. The groove dia. at muzzle measured .379, I tried to tap a .380 cast bullet thru the barrel.
It was very hard, like 10 times harder. Thought maybe bore was leaded. I cleaned and cleaned and it appears to be clean. It's bright and shinny with good rifling. So, I tried to tap a .379 cast bullet thru the bore and it too was very tight. Almost seems to tight to safely shoot in the rifle.
What am I doing wrong here. Any advice is appreciated.

[RickinTN]

Okay, here's the deal. I have a Ruger #1 in 38-55. I measured the groove diameter at the muzzle with dial cailpers. It's .379"
I took a .381 cast bullet and was able to tap in thru the barrel with a wood dowel fairly easy. Shoots almost one hole at 50 yds.
I have a 1906 built Win. 1894 in 38-55 I just bought. The groove dia. at muzzle measured .379, I tried to tap a .380 cast bullet thru the barrel.
It was very hard, like 10 times harder. Thought maybe bore was leaded. I cleaned and cleaned and it appears to be clean. It's bright and shinny with good rifling. So, I tried to tap a .379 cast bullet thru the bore and it too was very tight. Almost seems to tight to safely shoot in the rifle.
What am I doing wrong here. Any advice is appreciated.

What did the bullet you drove through the bore of the Ruger #1 measure after you tapped it through? I also have a Ruger #1 in 38-55 and about as large of a bullet that will fit in the case and still chamber is .377". I do need to do more measuring and testing on mine.
Take care!
Rick

[DougGuy]

What am I doing wrong here. Any advice is appreciated.

Calipers would be the first thing to question, you can get close but also introduce errors in your math. Use a micrometer that reads in .0001" and has a ratchet or friction stop on the thimble.

The problem with using anything but dead soft lead to slug the barrel is springback. Once you size a cast boolit in a sizing die, they always come out of the die bigger than a soft lead boolit sized in the same die, and it matters not whether it is a sizing die, or a barrel. So here's another factor to consider.

[Bitman]

What did the bullet you drove through the bore of the Ruger #1 measure after you tapped it through? I also have a Ruger #1 in 38-55 and about as large of a bullet that will fit in the case and still chamber is .377". I do need to do more measuring and testing on mine.
Take care!
Rick

Well, I don't know, as dial calipers are no good. I'm shooting a 160 gr. LFNGC bullet, that's .380 dia. With the longer Starline brass and 29 grains of IMR3031.
I have to very slightly resize the loaded round, in order for it to chamber. But it's shooting 5 shots in 1.5" at 100 yds.

[DougGuy]

Well, I don't know, as dial calipers are no good. I'm shooting a 160 gr. LFNGC bullet, that's .380 dia. With the longer Starline brass and 29 grains of IMR3031.
I have to very slightly resize the loaded round, in order for it to chamber. But it's shooting 5 shots in 1.5" at 100 yds.

I wouldn't think you are doing anything wrong to be honest. And measuring things only gives the human brain a numerical value that can be used in comparison with another numeric value, it doesn't translate snug fit, hard fit, loose fit, etc. You did the exact process to find out what your barrel's THROAT will accept and when I mentioned earlier about sizing to fit the throat, this is exactly what I meant. You are doing it. You are there.

There might be one drawback to fitting to the throat this way, if your alloy has much antimony in it, after sizing and loading cases boolits often grow in diameter after sitting for a while, and then they will be hard to chamber or not want to chamber at all, so there is that to watch for, might be a short shelf life for assembled ammo..