Danged Pinholes

[Chris C]

I do ignore the crimp groove. I don't crimp for this single shot rifle.........................................well, to be honest, I always have, but my friends on ASSRA have convinced me not to. So I purchased a Lee Factory Crimp Die to "just" close the bell.........not adding crimp pressure. If I don't close the bell, the round won't seat in the chamber. If I'm understanding correctly, fiver, that would seat the bullet back another .080" into the case. Is that what you mean?

Oh, and I just went out to the shop and checked and an un-sized bullet will seat in the chamber...........just barely and with a little nudging.

 

[Brad]

Another thing to try is just closing the bell on the mouth enough to allow the round to chamber. That slight bell can help center the bullet in the chamber and throat.

 

[Chris C]

That's what I strive to do, Brad. It's one of those "accomplish by feel" things. I full length size, so when I expand the neck for the sized bullet I'm sure to have a consistent grip from bullet to bullet but I try and close the bell just enough to slide the cartridge into the chamber and, as you say, help keep the bullet centered.

 

[Brad]

Feel is important. Some things are hard to measure so we must rely on feel and experience.
I think you are well on the path to some great groups.

 

[Chris C]

I appreciate that, Brad, thanks.

 

[Ian]

Fiver and I have asked every way we can think of for two pages how big of a bullet your rifle will chamber. Let me re-ask, one tiny question at a time and we'll get there eventually. First,

HOW BIG IS YOUR CHAMBER NECK?

 

[Chris C]

Wouldn't that be the O.D. of the fired case? If so, it's .402". (measured with C-Micrometer)

 

[Ian]

Chamber neck diameter is half the reason to make a pound cast (beautiful work, by the way). Measure the OD of the pound cast in the are where the bullet body would be seated. That will tell you what your chamber neck diameter is.

Step two is determine case wall thickness (sometimes a little tricky). Remove any burrs or bellmouth or crimp from a fired case and measure the wall thickness of the brass.

Step three is subtract two times your wall thickness from your chamber neck diameter, then subtract another thousandth for clearance, and the result will be the largest bullet your rifle will safely chamber.

Due to brass spring-back after firing, your fired brass should be about the diameter that you want your loaded brass to be, because the clearance will be established by the spring-back. So you can measure a fired case's ID along the length where the bullet is seated and that will be pretty close to the size you want to make your bullets.

REDUCE THE TOLERANCE STACKING. You want your loaded cartridge to fit close to the chamber walls so your bullet doesn't get crooked on firing. Make sense? Crooked bullets don't fly straight. Due to your chamber and bullet design, you will have to guide that bullet straight by minimizing the tolerances in the CHAMBER. Bore and groove measurements are entirely meaningless here, so forget about them.

Next post I will put up an illustration I just made showing the tolerance I'm trying to get you to measure and reduces here, and show you WHY you need to do that to get better accuracy. Bore/groove have no bearing on how you best fit the bullet to the rifle in this instance, you simply want to put the largest-diameter bullet that you possibly can in this rifle.

 

[Ian]

OK, here's the illustrations, pardon the crudeness and my ugly toe, first thing I grabbed was a Sharpie and some used typing paper.





So you can see where excessive "neck clearance" can let your bullet start crooked when the case expands to the confines of the chamber walls.

The third picture shows a slight exaggeration of what a bullet that started crooked might look like after it is squeezed through the throat funnel into the rifling. The tip of the nose won't be concentric to the bore, some of the lube groove "shanks" will be off center as well, and the base won't be square to the bore line. This bullet won't fly straight and will trace a looping, helical, corkscrew path to the target, causing wide group dispersion.

 

[Brad]

Those are some excellent illustrations Ian.

 

[Chris C]

As a side note, I just checked and I close my bell (sudo crimp) so the end of the brass measures .392".(correction: .394")

Measuring the pound cast the chamber is .394". To measure the fired case used in the pound cast, I get different dimensions because a .38-55 has a tapered case. But right at the point where the brass would end, the chamber measures .394".........but go back to the deepest point of the bullet in the brass, it would be .402". (that's measuring on the fired pound cast brass) All I have to measure case wall thickness is my dial calipers and it measures .009"-.010"..........all the way around. the lip. (yes, it's been carefully deburred.) If I subtract .021" for the brass and neck clearance, won't the bullet be way too small?

 

[Rick]

Because of the flat on the calipers they cannot be used to accurately measure the ID of a round hole. Add in that calipers are only designed/built with a +- tolerance of at best .001" they are the wrong tool for this job.

 

[Chris C]

I'm aware of that, Rick, I've no other measuring tool to use. I was using the very point where it's tapered down to what looks like about .020" thick.

I'm not sure I've measured this correctly................is 14 lbs of Pb and 14.18 oz a 20:1 mix?

 

[Ian]

On a .38 case you can get close enough for gov't work using your calipers to measure fired case ID. Scrub them around inside the "neck with pretty hard pressure to get a good firm measurement, then with that same reading showing, measure the calipers with a c-clamp micrometer to double-check the accuracy....if you had one.

I understand about the taper. Most chamber necks have taper, some (like 7.62 NATO) have a LOT of taper to the chamber. Try to find the middle and make sure you don't have any hard pinching going on when you chamber the cartridge.

Here's another trick: Just take a fired case and an unsized bullet and with your fingers try to seat the bullet in the case. This is that "feel" thing Brad was talking about. If the bullet falls in the fired case, slide it back out and wrap one layer of cellophane tape around the bullet with no overlaps and try again. That will give you an idea of how much too small your bullets are. If unsized bullets are a snug fit in the case neck, and require a little bellmouthing to get them started but can still be easily pushed in with your thumb, you might be in business just shooting them as-cast.

 

[fiver]

I know a lot of this is confusing and it might seem pointless.
I'm seeing numbers pop up everywhere.
but just looking at the op's numbers on the last page and the information I'm seeing show everything real close.
the only number that is missing is what does the boolit itself measure from the mold?
I seen 380 on the drawing but is that the real number on the boolit itself?
if so then things are not adding up.
but since the unsized bullet will chamber.
I personally would make up a small batch of them unsized and seated slightly deep and give that a run on paper to see how things go.
the harder alloy will take the over sized to proper sized no problem whereas the soft alloy will work in the other direction as long as the pressure is applied at the proper time.

 

[Chris C]

Okay, Ian. I tried an unsized bullet.....couldn't get it started even with a slight bell. So I sized one to .379" and it fell to the bottom. Sized another to .380" the base fits easily enough to be just snug so I'd say it fits........but the rest of the bullet didn't size all around, so the base must cast slightly larger than .380" and the rest of the bands mic at .379".

Now if you tell me to fire it "as cast"..........well, I'm gonna go nuts. A .380" bullet in my .3762" barrel goes against everything I've ever been taught!

I just went out to my reloading bench and checked through the bullets I've just cast and, indeed, to my disappointment, the bases measure .380" and the two bands and the max diameter of the nose is .379". Is that normal when I asked the mold maker to make the bullet .380"?

(what about the alloy mix question?)

 

[Ian]

Now if you tell me to fire it "as cast"..........well, I'm gonna go nuts. A .380" bullet in my .3762" barrel goes against everything I've ever been taught!

Yep, shoot them AS CAST.

You were evidently taught by people who still have a lot to learn about how to make a rifle shoot with smokeless powder. The Lyman cast bullet books are some of the worst in this regard, as well as a few precision enthusiasts on the internet who don't understand all the places where precision is needed. You could do all of this never having a clue about what your bore/groove dimension is. It is completely irrelevant to the measurements needed to make your bullets fit so they shoot straight.

If your base band is a little larger, go for it, just lube and load. I have to wonder if you don't have a case of shrunken bullet syndrome (a product of casting technique) because that bullet should have the same diameter bands from nose to base if I remember the drawing correctly.

Make a dummy round with an as-cast bullet seated to jam length (where the bullet touches the ball seat in the throat) minus about 20 thousandths in a fired case. If necessary to get adequate neck tension (.0015" interference fit between bullet and case neck should be about right) just bump (partial size) your brass in the sizing die until it will hold the bullet, then bell slightly withe your expander. The tapered cases are nice in that you can control the amount the neck gets sized simply by backing off your sizing die a few turns in the press. Size and measure, adjust the die, size and measure, until you get enough neck tension to be comfortable. No more than .002" interference fit between bullet and resized case neck. Any tighter than that and you're just ruining stuff. I'm betting your dies when set to fully size the brass will make the neck ID down around .370", which is WAY too tight.

 

[Ian]

Let's take another look at this. If your brass totals .021" and your chamber at the crack between end of case and the lead of the pound cast measures .394", your loaded rounds would pinch and not even chamber with a .377" bullet. That's why Fiver said the numbers aren't adding up. We don't want you to create unsafe pressure with a jam-fit on the outside of the case around the case mouth, but at the same time we're trying to get your tolerances to minimum so the bullet enters the throat STRAIGHT.

You have no bore-ride nose, so the case and chamber are the only things you have to steer your bullet straight. Minimize the area marked as "Neck clearance" in my drawings by using as large of a bullet as your rifle will safely accept, which is likely in the .379-.380" area based on what you've told us so far.

If you give the bullet nowhere to go but straight, instead of giving it room to get kicked a little bit sideways from behind when 'launched' from the case, you will be miles ahead in your quest for small groups.

 

[Chris C]

Okay, I just full-length sized a fired brass. I.D. was .370".........as you expected. After the expanding/flairing die, it was .375". That would still be too tight for a .380" bullet, wouldn't it? You are correct, Ian, the bullet was to have all bands cast at .380".

Talk to me about this "shrunken bullet syndrome". I'm running my PID at 700* and my mold at 450* +or- as it fluctuates. I'm on a 30 second cycle and kick the bullets out at 20 seconds. What in there might I be doing that makes the bullets shrink?

 

[Chris C]

Let's take another look at this. If your brass totals .021" and your chamber at the crack between end of case and the lead of the pound cast measures .394", your loaded rounds would pinch and not even chamber with a .377" bullet.

All I know to say is I've been sizing my bullets to .377" and shooting them some a long time now, so I must not be measuring things at the right places.