Sizing to fit the throat

[dogtired]

My .308 Win has a .3105 throat diameter (slugged). If I size my bullet to .312, would the gas check get scraped off in the throat? Should I size closer to .3105? I guess the real question is if the cartridge chambers and the bullet is larger than the throat, is it good to go?

[old turtle]

I am not sure as to what you are referring to as the throat. The critical dimension is distance between the groves. If it is a 308 Winchester it should be .308. I size my booits .309. If you are referring to the distance between the mouth of the case and the beginning of the rifling I would not worry about it and just seat the booit to just touch the rifling. In any case the gas check will not scrape off as the pressure of the gasses is on the base of the boolit which in this case is the gas check. In the old days, many people sized booits .311 and shot them in .308 barrels without any problem except perhaps accuracy. I generally size the boolts one thousand over groove diameter.

[runfiverun]

short answer: yes.
if you are bigger and mimicking the throats shape and have clearance for the brass to open and release the boolit you'll be good to go.
if you are grossly oversized then other problems will arise but .0015 should be no problem.
if you had a 310 or 311 sizer you'd most likely be okay also.
if i were ordering i'd go for 311 if i had a 310 or 312 i'd use it.
you can open a 310 if needed.

[rintinglen]

No, it won't scrape off the gas check, at least it did not appear to in my experiments. Formerly, Lyman made slip-on gas checks while the CCI and Hornady Checks were crimp-on. Back around 94, I fired 50 of each of Hornady and Lyman from a Ruger Redhawk That had .432-ish throats into a bucket of sand in an attempt to determine if the slip on jackets were falling or being stripped off on their way out of the gun. I found no evidence to suggest that this was happening. (Though I have to say that a few of the recovered boolits had shed their gas checks, it appeared that they did so in the sand after impact, and that both slip-on and Crimp-on gas checks appeared to be equally affected.) I did my testing at arms length ranges, and if I had it to do over, I would try to test at 25 yards, with a couple of paper targets between the point of impact and the firing line to see if the slip-ons were separating in flight.
Sine all gas checks are now crimp on, the point is moot,

[303Guy]

The burning powder gases would be acting on the gas check and the gas check would be acting on the boolit so I would suggest there would be no chance of the gas check being 'stripped off'. Besides, there are no sharp edges in the chamber or throat to actually catch the gas check. Or so I would think.

[littlejack]

Another DIFFERENT answer to your question about the throat being another dimension to size to, is a definate yes, with explanation. I do not think you will have any problem with your particular size bore and the .312 sized boolit, and your gc will not get rubbed off.
But, if you had a M/N like mine, and many others on this forum, chambered to have a .317 (mine) throat, and a bore of .313, you will, and I have had problems. The .317 throat allows the boolit to get "cocked" before being lined up by the bore riding section. This is the reason I have my name on the "Gang Buy" list for the 316299 mould by NOE.
All things have a bearing, not just the boolit.
Jack

[cbrick]

If it is a 308 Winchester it should be .308. I size my booits .309. I generally size the boolts one thousand over groove diameter.

I just gotta ask . . . You are "assuming" the groove diameter to be .308" because it's 30 caliber and you are sizing .309" . . . What if the groove diameter is not .308"?

My 30-30 should be .308" also but it is .3094". My 308 rifle should be .308" but it's .3076". My 308 XP should be .308" but . . Oh wait, that one is .308".

The point is that what "should be" and what is in the real world are two very different things. If I sized for my 30-30 at .309" because it's supposed to be .308" I'd never get the leading out of that bore.

dogtired, no, the gas check shouldn't come off but were it me I would size .310-311" as runfive suggested, a mild snug fit inside the throat is a good thing. What does your groove diameter measure?

Rick

[mroliver77]

I don't know why you would want to size the boolit to .312 if the THROAT is .3105. In this case I would use .310 die. If boolit ended up .310 that is close enough for me. If I had multiple .310 dies and one was closer to .3105 I would use it. If I only had a .309 and .311 I would go with the .311.

I would think that a boolit .0015 over the throat size would give some chambering difficulty.

Even a wimpy rifle charge has 20,000, 30,000 or more psi. That little ole piece of copper is coming out with the boolit! Like 303 guy says the pressure is acting on the check which is acting on the boolit.....
J

[303Guy]

It was of course a perfectly good question to ask. Very valid in fact. What goes on in a rifle chamber is not easy science! But it sure is interesting and a lot of fun! Might I suggest finding the fattest dimensions that will actually chamber? It's what I do but lack of range time and 'playing around' time has prevented me from testing some of my own suppositions, so I rely on the experience of others.

[Bret4207]

My .308 Win has a .3105 throat diameter (slugged). If I size my bullet to .312, would the gas check get scraped off in the throat? Should I size closer to .3105? I guess the real question is if the cartridge chambers and the bullet is larger than the throat, is it good to go?

It kind of depends on the boolit design, but overall, yes, you would size closer to .3105 than .312. If your throat slugs (impact slugged I assume) at .3105 and your grooves are at, say, .308 you will still be okay. IME you can get excellent accuracy with no pressure issues upwards of 2K fps in a 30 cal using boolits as much as .003 over groove if you play with it a bit. Of course your particular gun may not be cast friendly, so YMMV.

[Larry Gibson]

If the sized part of your bullet is to enter the throat on chambering then size to .310. Otherwise I suggest .311 size.

Larry Gibson

[dogtired]

The bullet is a tapered MX3-30 ARD. At the lube grove, it measures .3105, so if the rear band is sized .312, it should seal. Unsized, the rear band measures .314.

[Char-Gar]

I see nothing to be gained and several potential problems in trying to shoot a .312 bullet through a .3105 throat into a .308 barrel.

The entire bullets will be .3105 as it passes through the throat. The gas check probably won't come off, but that excess lead will have to go somewhere. It will either scrape off,or swage down into the base. Neither scenario would be something that would help accuracy.

I think you must be concerned about sizing a .314 bullet down to .310 and what that might do for accuracy. You are trying to keep it below the magic .002 max size, which is total BS anyway.

Size the bullet .310 in a nose first sizing die and shoot the things. I think you will find that will be a better solution to your concerns. Or, you could just buy a different mold.

[303Guy]

... several potential problems in trying to shoot a .312 bullet through a .3105 throat into a .308 barrel.

Indeed. The lead has to go somewhere as it gets swaged down and that could be rearward forming a cup base with uneven rifling land impression 'tails'. I would think that the gas check would alleviate that problem but maybe not. I cast a rebated base boolit to prevent the uneven base distortion effect and it seems to work. Mind you, I'm using paper patched boolits so the compressibility of the paper would probably be lessoning the effect also.

Here one can see the cupping effect. The boolit is too badly damaged to make out if the base was irregular at the muzzle. (The R/H sample shows powder peening - look at the L/H sample).