Where does undersizing matter?

[Thompsoncustom]

Just a quick question, does it matter if the base of the bullet is undersized as long as it is over bore diameter somewhere? That should stop leading correct, but what about accuracy?

[mdi]

JMO, I would think the base of the bullet should seal the barrel to keep gasses from cutting/melting the sides of the bullet. Also, a bullet base should be "square" so as it leaves the barrel it is not pushed off "center" by gas more on one side than the other (?? make sense?). Mebbe some one can explain it better...

[ku4hx]

I've shot tumble lubed gas check boolits without the gas check many times. That's about as undersized a boolit base as you can get. Results were fine, but as soon as I replenished my GC stock I stopped doing it. I'd do it again if I had to but I'd rather not.

[RobS]

Yes and No. As individuals have mentioned a person can shoot a GC'd boolit without a check and things are ok or even think about a beveled base boolit, same principle. However from the base up if there is no drive band(s) of proper diameter prior to the lube groove "too small" then gas blow by will start in immediately on the boolit lube and likely burn it off giving undesirable results. Boolit fit is king where as other techniques are merely secondary, but important, to shooting cast boolits.

[MtGun44]

If it is only the base, not too serious, but not ideal. You DO have to have the
majority of the boolit properly fitted to have success.

Bill

[Thompsoncustom]

Well I ran into some leading problems after turning down one of my expanders here the other day and it got me thinking about the 168gr SWC I load in 9mm. So I pulled the bullet and the base was .335 but this is my most accurate bullet and never has any leading. I'm guessing that the bands farther up are not undersized and engaging the rifling really well as I have used normal pan lubing and the hi tek coating and this bullet works great. Haven't tired to load any with my undersized expander just when it sized the .355ish because I only have calipers :).

[williamwaco]

In my experience you cannot achieve any reasonable level of accuracy unless the base is absolutely perfect.

I will not offer an opinion on leading. I just don't know.
I think you will find those bullets to be relatively inaccurate.

Note: Bevel base bullets are a different matter.
I assume that is not what you mean by undersized.

[Thompsoncustom]

I think you will find those bullets to be relatively inaccurate.

Well the farthest I have tested these is 50ft with a CZ75b and they were all touching so that might not be great but good enough for me.

[williamwaco]

Well the farthest I have tested these is 50ft with a CZ75b and they were all touching so that might not be great but good enough for me.

Are you talking about gas check shanks being "under sized"?

[Old Caster]

I have been testing a Pardini 32 ACP with lead bullets recently and the only mold I had was a Lyman 85 grain that is supposed to throw a .313 bullet. I sized them to .313 and some of the bullet isn't touched so it is smaller than that amount. I got consistent 10 shot groups around 2 1/2 - 3 inches at 50 yards. The bore of the Pardini is .3137 so they were too small, not to mention not quite round either, and while they may shoot even better if they were the proper size, they were not anywhere near awful and most people would be quite satisfied with that accuracy. I once fired some lead 9mm that were sized to .357 in an accurized Beretta FS. The groups were fair at maybe 5 inches at 50 yards but with an occasional one sideways maybe a foot or 18 inches out of the group. I checked and sure enough, they were swaged down by the brass not being expanded to around .352 if I remember correctly. I haven't slugged that barrel but expect it is .356. Until you try, you don't know for sure what will happen but by following sizing rules, you have a lot better chance of success.

[Thompsoncustom]

Are you talking about gas check shanks being "under sized"?

The mold I cast these from is the lyman 358429 so no gas check.

[BAGTIC]

In order for gas to "blow by" it has to have somewhere to go. That means a passageway leading from the base end to the front end.. A tight seal anywhere along that route will stop "gas blow by". My feeling is that the number one cause of "blow by" is too small a bullet. If a bullet's diameter is marginal for effecting a tight seal and it gets started slightly misaligned it will be more prone to leak gas. A 'tight plug' will stop that. Better a thousandths too large than a thousandths too small or as Rooster Cogburn said, "Fill your lands you S.O.B.'.