Why I don't size bullets larger than the throat entrance diameter

[Ian]

Browsing through one of my albums, I came across this picture. The two bullets on the right were pounded partway into the throat of one of my bolt rifles with a brass rod and back out again from the muzzle end. Note the bullet second from right where the ring of displaced metal actually sheared off....and the front bearing surface wasn't even fully past the throat entrance. These are about .3125" as they come out of the mould cast from water-quenched 50/50 alloy, and the throat entrance is around .310".

Some people get away with larger-than-throat diameter bullets, but I don't know how they do it without getting lead in the bore from the sheared metal. I'd kind of like to know, actually. Throat reamer?

 

[Joe7436]

I don't size larger then the throat specs. I know you XCB has a tighter throat so can understand why you size like you do for it, but I'm questions why you size small for your MIA? A lot depends on what condition or shape the edge on the ledge at the very end of the neck case mouth area is. A rifle that's fired a lot erodes that sharp edge and that will allow a slightly larger bullet diameter to squeeze in that throat.

 

[Ian]

I size .3098" for my M1A's .310" throat and get just the faintest scuff. Some of the alloys I've used come out around .3095" or a titch larger from the die I use, so there has been some slight variance amongst all the things I've used in it.

My .30 XCB has a .3106" throat, just like it's supposed to have, and I size .3105" for it (.310" marked die with aged, WD bullets cast at .311+ come out that way and it's perfect).

My other two .308s have .3095" or smaller throats, so I have to go even smaller with those.

 

[Joe7436]

Are you positive your M1A has a .310 throat? Mines a National Match with the stainless barrel and I'm using the bullets unsized at .3135. I see I will have to do a pound cast on it. I had no reason to do so before.

 

[Ian]

My M1A has a .3100" by .075"-ish-long, parallel freebore. I did two pound casts to be sure. As I said a .3098" front band just scuffs, that's as in scuffs when chambered/dechambered. .301x/308 bore/groove, .3425x .344" chamber neck (tapered as typical of .308s). It's NOT a NATO chamber.

 

[smokeywolf]

Ian, did you get a good representation of your leade when you did your pound (or Cerrosafe) cast of the chamber? No step, shoulder or burr in the transition from freebore to leade?

 

[Brad]

I too have changed how I look at sizing. The idea that we can always just size a few thou over expected bore size and call it good seems silly to me now.
I do,know my 375 has a throat tighter than the .379 bullets I have been feeding it. Accuracy is good with lower end loads but gets squirrelly as velocity exceeds 1900 fps. Might be bullet size troubles?
I made a sizer that runs .3755 to try. The bore is a nice snug .375 so I will see what happens. I can easily make another sizer or open this one up more until I find just the right size.

 

[45 2.1]

My M1A has a .3100" by .075"-ish-long, parallel freebore. I did two pound casts to be sure. As I said a .3098" front band just scuffs, that's as in scuffs when chambered/dechambered. .301x/308 bore/groove, .3425x .344" chamber neck (tapered as typical of .308s). It's NOT a NATO chamber.

On the last sentence, just what makes you say that?

 

[Joe7436]

I too have changed how I look at sizing. The idea that we can always just size a few thou over expected bore size and call it good seems silly to me now.
I do,know my 375 has a throat tighter than the .379 bullets I have been feeding it. Accuracy is good with lower end loads but gets squirrelly as velocity exceeds 1900 fps. Might be bullet size troubles?
I made a sizer that runs .3755 to try. The bore is a nice snug .375 so I will see what happens. I can easily make another sizer or open this one up more until I find just the right size.

Ian did mean "size a few thou over expected groove size instead of you said bore size"? I pay no attention to bore size unless I'm using a bore rider bullet or a bullet which has such a configuration that bore size is important.

I haven't found the same things you have sizing fat. How about the many that feed revolvers the fattest bullets they can with excellent results?

 

[Ian]

What about revolvers? The picture I posted was not from a revolver or from a worn, rounded-off, funnel-shaped throat, though I get that those don't have this problem necessarily. Those shaved bullets were from a typical, modern, bottleneck rifle chamber in good condition...something a lot of us have. And it will shave lead rings if you don't size correctly. If it didn't cause a lot of leading in the bore or other issues when shooting, I wouldn't worry about it, but sizing larger than throat entrance diameter has caused me some troubles in these types of rifles and that's why I avoid it.

Smokeywolf, the throat is smooth, it's seen enough jacketed to be polished pretty well, though it isn't worn. The freebore/leade angle is definite but not razor-sharp, same for the leade angle/land transition, definite but not rounded off. The chamber/freebore transition angle is still very sharp, and the freebore is most definitely parallel and exactly .310" in diameter. The chamber body is also very tight, FL-sized brass and factory-loaded commercial and military surplus ammo all fits very tight with virtually no wiggle in the chamber. Measuring a pound cast, I come up with SAAMI minimum specs for .308 Winchester, that is, the drawing measurements exactly without the tolerances applied.

Bob, I said what I said because (in part) of the measurements in sentence before it.

 

[45 2.1]

Bob, I said what I said because (in part) of the measurements in sentence before it.

I don't think you'll find any listed chamber or throat dimensions for the 7.62 Nato with anything Government on them. From the folks I've dealt with, the word is they're still classified. SA does say that their M1A is safe with all military and commercial ammo under 180 gr.

 

[Joe7436]

Ian I know those bullets were from a rifle. I was referring the statement you made about not going to shoot oversized bullets anymore....so how do you explain shooting big fat revolver bullet or even pistol for that matter? On the revolver they jump out of the cylinder and hit that awful tight barrel hole getting swaged down even more, but yet they shoot pretty damn well. While we're at it how about the base of a rifle bullet getting bumped up before clearing the neck? What do you have to say about that? Boils down to I don't agree with you on "fat" bullets. I have not seen accuracy degrade, but have seen it go up. Kind of what you are saying is almost like Lyman sizing to groove size, yeah yeah I know, not quite as bad though.

 

[Brad]

Comes down to personal experience Joe. Just because it works for you doesn't mean it works for everyone.

 

[Rick]

so how do you explain shooting big fat revolver bullet or even pistol for that matter? On the revolver they jump out of the cylinder and hit that awful tight barrel hole getting swaged down even more, but yet they shoot pretty damn well. Boils down to I don't agree with you on "fat" bullets.

That is extremely easy to explain . . . I size to fit the revolver. No big fat about it.

 

[Joe7436]

That is extremely easy to explain . . . I size to fit the revolver. No big fat about it.

Apparently it's extremely easy to misunderstand what I meant. Have to use an example. You have a 45 Colt with say .456 cylinder throats. Your groove diameter is .452. That's a fat bullet hitting the forcing cone, bore/groove.

 

[Rick]

Apparently it's extremely easy to misunderstand what I meant. Have to use an example. You have a 45 Colt with say .456 cylinder throats. Your groove diameter is .452. That's a fat bullet hitting the forcing cone, bore/groove.

Actually I don't, I have a 45 Colt with .4525" throats and .452" groove diameter and I size to fit the revolver.

 

[45 2.1]

Actually I don't, I have a 45 Colt with .4525" throats and .452" groove diameter and I size to fit the revolver.

Congratulations, you have a near perfect spec. dimension........ However, there are 45 Colt revolvers out there that have 0.449" to 0.459" cylinder throats and rifling between 0.450" to 0.457". Sizing to fit some of the odder combinations of those won't fit what you said. That shows there are quite a variety of dimensions to fit or rebuild the firearm in question.

 

[Brad]

I had such a revolver. A Ruger BH in 45 Colt. It has a .452 barrel and had .449 throats. A .452 bullet leaded like hell.m the throats, forcing cone, and barrel all leaded. I learned that a revolver cylinder throat is a piss poor excuse for a bullet sizer. Bullets are sized down but accuracy suffers from the leading and shooting a .449 bullet in a .452 barrel.

I lapped the throats to .452 and it shoots just fine now.

I was smart enough to know that no matter how big the bullet started the throats would size them down. I was also smart enough to know that there was no way in hell is was conducive to any level of accuracy.

What that all has to do with sizing rifle bullets I am still trying to figure out.

 

[Joe7436]

Okay let's tie it in with rifle bullets. Plain and simple....if your bullet is under throat size your rifle isn't going to shoot those small groups that everyone likes.

 

[Brad]

Read the thread title Joe. Nobody is suggesting to size under throat size, it says why Ian doesn't size over throat size.
There is over, there is under, and there is right to throat diameter. Pretty simple really.