.338 WM Precision Mic check - 225 gr HY SST

[huntinlever]

Bear with me.

This is more of a general question on the use of the RCBS precision mic, but here it applies to .338 WWM Hornady SST 225, which I may hunt for deer at lower velocity but more, just getting an idea for 225 grains at the range.

To establish freebore in my rifle, a couple of weeks ago I used the freebore tool on the mic and came up with a consistent reading of 0.030".

I note Hornady recommends a COAL for this SST bullet of 3.320, 0.020 off the SAAMI spec of 3.340. However, if I do a dummy round to this, and place it in the freebore tool, the bullet is so low it cannot be read on the mic.

If I do an easy jam of the bullet, I get a mic reading of 0.001. Already I'm confused - isn't freebore freebore, irrespective of the bullet, so all bullets would start at 0.030 (the mic freebore tool measurement)?

I backed it off to a reading of (-) 0.046), The bullet feeds fine. I believe I've sat it a further depth of 0.005, which is very little. However, that gives me a COAL of 3.431 - well above the SAAMI of 3.340.

I understand bullet shape will change the COAL, so the better measure is using the precision mic - but this seems weird to me. The bullet is sat higher than the cannelure depth, for one. Can someone help decipher what I might be doing? If it feeds well, should I just leave it here and try loads? Should I back it up more - say, 0.010-0.020, for a precision mic reading of (-) 0.041 to (-) 0.021 on the mic? Should I just back it up to the cannelure? Or back it up to Hornady's recommended 3.320" and forget the mic, as I can't read this short a cartridge anyway?

I feel like I must be screwing something up. Given Barnes recommends starting with 0.050 off the lands, which will put me at a (-) 0.030 on the mic, right? What if I find I need to go substantially lower - wouldn't this also mean it can't be read on the mic?

Thanks. Sorry for the turbid post.

[jmorris]

I am not familiar with that tool but there are several methods that can get you to “contact”, you then just need a repeatable way to measure off a datum (same diameter) along the ogive, so you can measure how much you vary and in what direction.

[huntinlever]

Thanks. I do have the OAL gauge and comparator coming, so hopefully I can get a better read.

Using the freebore tool, I just tried again, and I've got some concerns as what I found doesn't square with Hornady's own load data as to COAL.

As I mentioned, using the freebore tool I get a read of 0.030" - this is based on the mic's "dummy" round, which has a different bullet shape than the SST's. I'd thought this didn't matter - that the tool gives a freebore measurement that is independent of bullet shape - but now I'm unclear.

Setting the bullet depth so that it reads (-0.020") or 0.010 on the mic, the bullet barely chambers - the bolt will shut, but it's somewhat sticky.

I dropped it a further 0.010 down, so the mic now shows 0.000". That's a total of 0.030" "off the lands," if the freebore tool's figure, 0.030", is actually the "lands."

This set gives me a COAL of 3.417. This exceeds the SAAMI max of 3.340. Furthermore, the bullet is not sat to the level of the cannelure.

311362

This is also longer than Hornady's own data, which calls for a COAL of 3.320. This is the bullet set to Hornady's depth - at the cannelure, COAL 3.320":

311363

I note that SAAMI minimum is 3.215, so Hornady's rather short COAL indication is within the SAAMI spec (as I would expect it would be). I'd been leery of setting the bullet so short due to pressure issues.

So, hopefully that's a bit clearer - what should I do in this case? The round chambers fine at 3.417", but I'm concerned I'm taking the wrong choice if I go with this.

[Land Owner]

This may be the "round about" way to get where you are going. Assuming the bullet/powder combination has improved to its greatest potential, the following "jump to lands" fine tuning is recommended:

1.) Cut one deprimed case (reference) lengthwise with a hacksaw, through the case mouth and into the case shoulder.
2.) Put the reference case into the chamber with your bullet slightly embedded in the case neck.
3.) Close the bolt slowly.
4.) Remove the reference case/bullet slowly and carefully measure its length.
5.) Write that measurement down as "COAL Ogive against Lands".
6.) Do the same for the rifle's magazine. The magazine may be the controlling length.
7.) Write that down as "COAL Max Mag Length".
8.) Whichever is the shortest measurement, that is your max. length and COAL BASELINE
___8.a) for that bullet & case
___8.b) in that rifle
9.) Seat that bullet into a sized and deprimed case to the baseline length.
10.) Measure the baseline with the Precision Mic (PM)
11.) Note that in your records too (PM Baseline).

A.) You cannot load longer than the baseline as the round will:
___A.1) jamb into the lands
______A.1.a) the chamber won't close and
______A.1.b) closing the chamber will force the COAL to shorten to the baseline
___A.2) or fail to load into the magazine.
B.) Loading SHORTER than the baseline (increasing the jump to lands) is your only option.
C.) Use the Precision Mic to slowly decrease the COAL
___C.1) shorter than baseline
___C.2) until performance improves or
___C.3) performance continuous to diminish, then you find another bullet/case combo or
___C.4) start over with a different powder

The cannelure is made for crimping the case mouth into the bullet to preclude inertia changing COAL in the magazine at recoil (leaving you to wonder why accuracy is all over the target). Friction alone, between the case neck and the bullet, without a crimp may not be sufficient to overcome bullet inertia and COAL foreshortening (or lengthing) at recoil.

A roll crimp into the cannelure is recommended. The COAL of a cannelure crimped round may be considerably less than the baseline and may not be the most accurate for that bullet/case/powder/rifle combination.

Some bullets have no cannelure, and a taper crimp is necessary. A taper crimp can be used on cannelure (against the smooth wall) and non-cannelure bullets. The heavy recoil of a 338 may effect taper crimp shortening of COAL in the magazine.

[TurnipEaterDown]

Bullet profile changes cartridge overall length to contact rifling, as the bullet is basically bore diameter, and the contact comes as the profile of the nose engages the taper of the rifling (lands) coming in smaller than the bore.

The "cheapy" guy way of finding out how long of COL to touch is: Make a dummy, or several in multiple seated lengths, record length(s), take a Sharpie permanent marker and coat the entire exposed bullet surface, carefully chamber dummy after bullet is dry (don't scrape it going into chamber: start aligned w/ bore, not coming out of the magazine) and extract after chambering. The marker works better than candle soot. Sharper images of rifling abuttment, and more "clingy". Cleans up w/ alcohol. You should be able w/ some patience to get the contact length known within 0.010", and that's good enough given hunting rounds and commercial bullets.

(An aside: darkening a bullet w/ black marker is a trick I have used to improve readability on optical chronographs.)

Here's an educational "thing" you can do as well: If / after you get a comparator, load some rounds for something you have and measure a sample like 10 pieces. With the variability involved in the system, you will likely find 0.003" variation or maybe a little more is meaningless. Such is one reason seating to 'just touch' is not really a good place to be. Actually hard to know what that is, and to load to it repeatably.

As long as the loaded rounds are not touching or jamming for best accuracy, nor too long for magazine, I really don't usually care where they are when they shoot best. I just load to that, making charge adjustments based on pressure observations and expected velocity potential.
I have one gun that I load to best length for accuracy (not touching) and then quite literally file the tips (on Nosler BT / Bonded BT) to get to fit the magazine.

I very rarely ever crimp for a bolt rifle loaded round, so aligning w/ cannelure is mostly meaningless. The exceptions for me are odd things like wildcat where dies size fine for interference w/ cast but jacketed is insufficient tension, etc. I don't crimp on hard recoiling rounds for bolt gun cartridges either, and by hard recoiling I mean those that make 50+ lb ft of recoil energy.
Revolver, totally different thing. Crimp everything w/ a robust crimp. Different applications require different processes.

[M-Tecs]

. Already I'm confused - isn't freebore freebore, irrespective of the bullet, so all bullets would start at 0.030 (the mic freebore tool measurement)?


Not sure I am understanding you but distance to the lands is different between manufacturers and individual rifles. It is initially established by the reamer spec the manufacturer uses than as the throat erodes it gets longer.

With the RCBS Precision Mic it gives actual direct reads for headspace like an actual micrometer. For measuring headspace the RCBS Precision Mic is as good as it gets.

For distance to the lands it gives indirect readings (comparator) that are only applicable to that specific rifle and comparator unless a standard is used to calibrate the comparator. For this application the RCBS Precision Mic is workable but it's not my preferred method. I mostly use the Hornady Lock-N-Load® O.A.L. Gauges

SAAMI specs are nothing more than a voluntary guidance that ensure safe and interchangeable ammunition/firearm combinations. SAAMI spec doesn't mean optimal.



https://www.youtube.com/watch?v=ikkbi2HUSqA

[M-Tecs]

Three ways to find the lands


https://www.youtube.com/watch?v=90DOVa2cFNs&t=25s