300blk and 308 win size to 308 or 309?

[moorvogi]

i'm about to get a mold that's 312 and plan on squishing them down a bit but.. do i want to go to 308 or 309? is there a "standard" or "typical" dia w/o slugging the barrels?

[Hannibal]

In order to correctly size your cast bullets, you need to know the diameter of the throat, which requires a pound-cast or a cerrosafe cast. There are stickies and threads on both.

To do otherwise is to simply hope for the best.

[John McCorkle]

i'm about to get a mold that's 312 and plan on squishing them down a bit but.. do i want to go to 308 or 309? is there a "standard" or "typical" dia w/o slugging the barrels?

I will say off the bat the "cursory" answer is to slug it... however the good reason behind that is because manufacturer differences between what is listed and what is makes a ton of difference... especially in older firearms.

Better yet is to chamber cast and see what the throat diameter is...fitting to the throat will give your bullet a concentric start and increase accuracy a ton.

If you have a modern firearm from a well respected manufacturer you can go with a .309 and see if you get leading or accuracy issues. Always +.001 over for lead. It will obturate (squish in on the size) under the pressure of the burning powder but if it is too small...even by a little bit...the pressurized gas will cut by the bullet and cause lead to deposit in the rifling. Accuracy will be awful too

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[DougGuy]

You could alternately ask for a few sample boolits sized .308" and .309" possibly .310" here on the forum and if an obliging member will send you those, you can seat them in some empty brass and see what will and will not chamber. You could use these dummies as a go no-go gauge. If you fit to the throat, you would obviously want the largest boolit diameter that will chamber. Here is one inexpensive and quick way to find out..

Matter of fact I need to do the same thing, when my 300 BO upper arrives.. I have pin gages but you can't check the throat and chamber neck at the same time with a pin gage, you can with a dummy..

[roharmon]

I have an 8 inch pistol and a 16 inch upper and size mine to 310 from a NOE mold. I have a Lee mold, but can’t get them to drop larger than 308.

[Moonie]

I size mine to .310 for a 16" carbine, a 10.5" SBR and a 7.5" Encore pistol in 300 BO

[kir_kenix]

Default 30 cal (american) sizing for me is .310. I will take a chamber cast and tweak from there if I have problems or am trying to eek out that last bit of accuracy.

[popper]

I will suggest getting sizers from 309, 310, 311. Actual finish size will depend on the sizer AND alloy. My AR10 takes 310, 30/30 311 and BO varies.

[StratsMan]

My $.02.... There's good advice here, but don't make a sizing decision until you have received the mold, settled on an alloy and have cast enough from it to know the actual diameter of the casts it throws. I have purchased large-for-caliber diameter molds only to find that they cast small so I don't size them down at all...

My two blackouts both work fine at 309, but YMMV....

[6622729]

i'm about to get a mold that's 312 and plan on squishing them down a bit but.. do i want to go to 308 or 309? is there a "standard" or "typical" dia w/o slugging the barrels?

I size to .309. I am getting excellent accuracy and no leading with copper gas check and 45/45/10 lube.

[moorvogi]

Amazing feed back! I feel like i should "reply all" but i hope this post will do that.

Accuracy is nice but my first concern is safety, then leading. Most (if not all) of the cast bullet shooting i'll be doing is going to be under 50 yards, maybe 100. Past that, i have a different field/range and a different firearm.

More than one person mentioned this and i'm not sure how/why. How does the alloy composition affect the diameter of the cast bullet? I can see that it may affect the WEIGHT but it's unclear to me how it would affect the diameter of the bullet.

[popper]

First, soft alloy doesn't spring back like the harder when sized. Second, alloy composition will change the as cast diameter. Moulds normally are spec'd for wheel weight alloy. Mould/cast temp plays a part also. Lee sizers are IMHO, made for WW.

[Moonie]

Generally harder alloys cast bigger in a given mold. This does not count heat treating, just the casting alloy.