Neck turning mandrel diameter?

[b money]

I have a project going and I need to know what diameter a 308 and 338 neck turning mandrel is, I first assumed it was .308 and .338 but then started to think that would ruin neck tension so does anyone have one of each they wouldn’t mind measuring with a micrometer?

[Foggy1111]

I turned a few case necks over time. IIRC....it was normally thought that the bullet diameter less .001 to 002" was the proper I.D. of the case necks to achieve a proper bullet tension. I think that target loads are good at .001" especially if loads are seated against the rifling......but I would opt for .002" for hunting loads.

[Rcmaveric]

Normally the mandrels go in calibers like 26 cal, 27 cal, 28 cal ext. So you would need a 30 cal and 33 cal.

Sent from my SM-G950U using Tapatalk

[b money]

Yes but I’m wondering if a 30cal is actually .307 or .3075 etc I want to turn a few

[country gent]

I have a die made to hold my turning mandrels. This allows me to expand necks to the mandrel before neck turning. The mandrel is a snug fit with the brass spring back. Makes for a very nice finish on the neck and very accurate neck wall thickness. When expanding I lightly lube mandrel necks with imperial die wax. and again when turning. The die wax also makes a good cutting lube to improve finish and cutter life. I turned necks for a tight necked 243 match rifle. I sized cases in a redding full length bushing die. expanded up roughly .001 on the mandrel and then neck turned. The chamfer and deburr and a trip thru the polisher.

[Outpost75]

Most resizing dies reduce the ID of the neck to 0.002" less than bullet diameter. Because case neck wall thickness varies, recommended practice is to gently inside expand and at the same time LIGHTLY lubricate the case necks with a die supporting a tapered arbor which is the exact same size as the mandrel on your neck turning tool. The expanding arbor is wiped clean of residue while applying a thin film of pure mineral oil USP, applied on a patch, repeating this step as needed, usually every 2-3 cases, such that the mandrel enters and turns inside the neck smoothly without galling the interior wall of the case neck.

On a .30 cal. the expanding armor and turning mandrel would both be .307". As stated by Foggy1111, hunting ammunition to be used in repeating rifles typically has case neck ID 0.002" lesss than the bullet, to prevent bullets from telescoping during the feed cycle. Target loads loaded singly only require 0.001" for correct neck tension.

[jsizemore]

I don't know if this will help but here goes. I use a K&M neck turning setup for my 6BR. I first size the brass. I then use an expander mandrel that measures .2433. The mandrel in the turning tool is .2423. I use a little Imperial Sizing Die wax for lube. My neck sizing bushing is 2-3 thousandths smaller then loaded round neck diameter.

Some of the lathe style neck turners use an inside neck reaming mandrel to support the neck while cutting the outside or just to inside ream.

[b money]

Well I have one I turned to .3065-307 and runout between the holding piece and the mandrel is between .0005-.001 so not horrible. Not great but ok enough to actually prove concept. I was going to put the mandrel in a tail stock Jacobs chuck in a collet lathe and have the brass in the headstock collet so I could pop them in and out but come to find out the headstock in that lathe is .004 out so I’ll have to indicate my mandrel in a 4 jaw and figure out how to hold the brass in the tail stock so it is free to turn and still quick to get in and out.

[jsizemore]

The problem with using a powered lathe is the heat buildup and it's affect on your final neck dimension. Maybe with a longer mandrel and a VERY slow speed you'd get what your looking for. Some folks have used an adapter in their powered screwdriver at minimum speed to turn their necks.

[popper]

The mandrel needs to be hard and smooth. Cutting is between the mandrel and cutter edge so exact dia. Shouldn't matter. Resize after trimming.

[HangFireW8]

B money

You can use a dremel or die grinder on your lathe to true up that 3 jaw chuck.

[lightman]

I don't turn necks on either of those calibers but the ones I do turn for are .003 smaller than bullet diameter. Like my 6.5mm (264) measures .261. This is the turning mandrel. Hope this helps.

[uscra112]

I use my lathe. I cut a new mandrel or "spud" each time, so that I know the mandrel is running dead true, regardless of chuck runout. Size isn't critical, but it must always be .002" BIGGER than the neck ID of the cases you will be turning. On large calibers like .40 maybe .003" or even .004". The case is pressed onto the mandrel with the tailstock - live center in the primer pocket. The friction will drive the case while turning the neck. The neck will of course be expanded somewhat. Take this into account when you start the actual turning operation, or you'll get the neck walls too thin.

Run the lathe slow, feed set to .003 per rev at most. Power feed direction AWAY from the headstock. Tool should have something like .090" nose radius, slight positive rake, and be very sharp.

Feed first from the chuck toward the tailstock. First case you do, you will do the cut repeatedly, advancing the cross slide by very small increments until you get a cross slide setting that cleans up the brass and/or gets you the wall thickness you want. Once there, LOCK the cross slide. Feed as far as you want up the neck, declutch the power feed, and move the carriage slowly back toward the headstock until it's clear of the brass. The tool will take a very fine cut on this return stroke, so don't rush it. I have a handmade fork tool that engages the rim (extractor groove if rimless) so I can tap the case back off the mandrel. Push another case on the mandrel, and feed out and back just once.

Easier done than said. Once cross slide is set you can do a case a minute, and the neck walls will all be as uniform as they can possibly be. You will probably want to resize the necks again to get the exact bullet pull you want.

I hope that made sense. Fatigue and pain meds have me a little fuzzy tonight.

Phil

The neck turning process is to get uniform wall thickness, and a thickness that matches bullet to chamber in the case of rifles with a tight neck chamber. Turning is NOT how you get a particular ID. THAT is done by your sizing die and expander mandrel.

[b money]

That’s a great write up! Makes total sense. I’m saving this for future use thank you!!!!

[popper]

I don't use a lathe (gee,wish I had one) but uscra said it very well.

[Moonie]

I know my K&M is too big to get a sized case on it. There is a mandrel they make to run a sized case into that will expand it to fit, I just neck turn them before sizing and I use a neck sizer that uses a mandrel so the neck tension should be the same regardless of thickness of the neck.

[Baltimoreed]

I have recently bought a Hornady neck trimmer tool to use on my military brass formed 300 blkout brass as the neck thicknesses are all over the map. In looking at how this machine works the mandrel is there to center the case in the machine. The cutter is going to cut any excess brass that is thicker that what the distance is between the mandrel and cutter blade. The case’s neck size isn’t really important as long as it slides over the mandrel as it will be correctly resized before loading anyway.

[oldhenry]

It's been a while since I had a tight neck gun, but dug out my Sinclair expanding die & mandrels to answer the question on .308 mandrel diameter.

My E30 mandrel measures a hair over .307 with my Mitutoyo (not digital).

I did my neck turning with the Sinclair tool using Sinclair's shell holder in a hand held electric drill. I also had a K&M & Hornady tool. The K&M is good: the Hornady not so good.Take it slow & either the Sinclair or K&M will do a good job.

The diameter of the chamber neck + the clearance you want determines neck thickness & this is why a neck thickness micrometer is so important (I just sold my Holland neck thickness micrometer on the FS section of this forum). I achieved the neck tension I wanted with the Redding bushings. At one time I had 4 tight neck guns: .308, 6.5X284, 243 & .223.

I didn't mean to ramble.

Henry