Expander size for proper neck tension?

I've heard various reports of people using expanders that are anywhere from 0.001" to 0.007" under bullet diameter. Personally I've always preferred 0.001" because it minimizes runout and generally allows the bullet to seat straighter, without risk of deformation. Lately I've been very OCD about this, and made special expander plugs for all of the calibers I'm loading, with different sizes for cast boolits and jacketed bullets. For 357 SIG, I use 0.354" for jacketed bullets, and 0.356" for cast (I size my cast boolits 0.002" over bore, which is 0.357"). For 300 Blackout, I have a 0.307" plug for jacketed, and a 0.309" plug for cast (0.310" boolits).

Some people claim that you'll get more neck tension by using a smaller expander, but from what I've seen, brass cases tend to only have around 0.001" of elastic deformation, which of course can vary a bit depending on the hardness of the brass. So if you size the case mouth/neck any smaller, it seems that you're basically just using the bullet as the expander, and not adding any additional neck tension. What do you guys think? I've yet to see any dislodged bullets by loading into brass sized 0.001" under bullet diameter, for both jacketed and cast bullets.

And of course there's also the issue of springback after the expander plugs are pulled out, meaning that the mouth/neck will probably end up 0.002" under bullet diameter anyways. Again, this varies based on brass hardness, but I'm not sure to what degree.

Is there any reason why I should expand less?

Are the rules different for cast boolits versus jacketed?

Last edited by Jumbopanda; Yesterday at 06:31 PM.

.003" under for most cast, .005" for bigger cals. that kick a lot.

I shoot for .004 under boolit dia. or .002 under groove for my finished expanders depending on the partiular arm and cartridge w/w/o paper patch etc. I sight the difference as I load for a rifle that is supposed to be 308/312 and is infact 305/316 w/a 320 throat my expander for that 1 is 316 that averages 315 id finished for the 320 lead and 317 PP'd . The brass for naked boolits finished is softer by lot . The brass for the PP is harder by lot .

My 284 dia 7mmRWH has an expander that finishes at 282 for 286 boolits but should be tighter for naked boolits but is perfect w/PP.

I think neck length plays in to it also and is dependent on your neck sizing details. I load for 2 06's w/huge necks and nominal throats so I size only 3/4 of the neck so less tension than the 3rd that has to be full lengthed every time w/tight neck ,throat and bore. There's 1 more that gets just barely sized 1 time in 3 full house shots as that particular seating die just off the crimp sizes the neck perfectly during seating (fat neck and grooves).

The answer is going to be the .001-7 is a guide w/.004 being the average answer ,but its really what Mr Arms wants. Larger bores probably tend to need closer to .007 and smaller bores closer to .001 . Imagine trying to get a 225 boolit in a 218 hole while a 460 is a slip fit in 459.

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Some target shooters use a slip fit. You can actually insert and remove boolit by hand from case. They have a custom tighter chamber cut and turn the case necks to acomplish this slip fit (no resizing after firing). Boolit ogive also touches against rifling when chambered (less wobble during flight). You also want your boolits to be concentric.

Boolit Man Join DateJan 2013Posts134

It also depends on case neck wall thickness and the state of work hardening or anneal.of the brass.

With jacketed match bullets for benchrest shooting an arbor 0.001" less than bullet diameter is fairly standard. If soft cast bullets are used in unturned necks which have been work-hardened by firing and reloading several times, chamfering and only flaring the mouth to prevent shaving lead, the rest of the neck having been expanded with an arbor dimensioned for jacketed bullets, you will get deformation of bullets softer than about 12 BHN, which can be measured, if you assemble dummy rounds and pull them.

Where a heavier bullet pull is desired to improve ignition, a snug bullet fit is desired. In .30 cals. I like a stepped plug with .312" portion 1/8" long below a 15? flare, with a 3/8" long .306 cylinder with hemispherical end termination.

With harder alloys a stepped expander which permits hand starting the bullet, aligning it straight to the top of the GC or base band works well, seating the bullet into a tighter neck beneath the mouth expansion. The Lyman "M" die facilitates this. Or alternately the RCBS cast bullet neck expander, with plug of appropriate diameter. I use the. 308" plug with 30? flare for. 311" diameter cast bullets, with good results. In the 7.62x54R a .309 plug for. 312 bullets in my Finn M39s.

Last edited by Outpost75; Yesterday at 09:02 PM.

It also depends on case neck wall thickness and the state of work hardening or anneal.of the brass.

With jacketed match bullets for benchrest shooting an arbor 0.001" less than bullet diameter is fairly standard. If soft cast bullets are used in unturned necks which have been work-hardened by firing and reloading several times, chamfering and only flaring the mouth to prevent shaving lead, the rest of the neck having been expanded with an arbor dimensioned for jacketed bullets, you will get deformation of bullets softer than about 12 BHN, which can be measured, if you assemble dummy rounds and pull them.

Where a heavier bullet pull is desired to improve ignition, a snug bullet fit is desired. With harder alloys a stepped expander which permits hand starting the bullet, aligning it straight to the top of the GC or base band works well, seating the bullet into a tighter neck beneath the mouth expansion. The Lyman "M" die favilitates this. Or alternately the RCBS cast bullet neck expander, with plus of appropriate diameter.

Good info. I have done some experimenting with loading dummy cartridges and pulling bullets. I've measured the size of both the bullets and the cases they were pulled from, and found that cases don't spring back more than 0.001", which is why I now expand necks that much under bullet diameter. And I can confirm what you said about deformation of soft bullets; my pulled cast bullets measure 1 thousandth smaller after pulling if I use an expander meant for jacketed bullets.

In 357 SIG, I have gotten bullets seated terribly crooked if I don't use a 0.001" expander. I suspect that the short neck contributes to this problem.

I never anneal any of my brass, so I suspect a lot of it is pretty stiff at this point. I've been thinking about building an annealing machine...but I have too many other projects I want to do.

for my M1A I use a Redding S-bushing die with a .336 Ti-N bushing with the expander polished down to .306 gives me a nice clean .002 neck tension when using LC-07 brass.

There are too many variables to give a short answer that applies to all situations. Are we talking handgun or rifle, sixgun or pistol, hard, soft or medium alloy and what kind and burning rate of powder. These all make a difference in neck size/tension.

Disclaimer: The above is not holy writ. It is just my opinion based on my experience and knowledge. Your mileage may vary.

Boolit Man Join DateSep 2011LocationWellington, NVPosts79

When I started to reload for 30:30, I had a standard RCBS 2 die set to load with. Not having the expander die body or plug that is included with the Cowboy set, I spent a little time on my lathe and just made my own. The expander plug I made for that die was @ .307, I size and cast @.309 for my old Winchester 94. .002 press fit seams to work out well so far. When reloading for my old Winchester 94 in 38-55 with the larger .380 bore, I do the same. Cast and size for that one @.381 and expand with a .379 expander plug, it to seams to work well. The largest stock expander plug for the 38-55 that RCBS has in stock is the .3775 they include with the Cowboy Die set, and that was to small for the older large bore 38-55's, so I made that plug to. So far .002 seams to work well for me, is it ideal ?? I sure don't know that answer.
Chris

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Abbreviations used in Reloading
Pointed Soft Point "Core Lokt"

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