Minimum case length for bolt action 6.5 Grendel?

[Howa]

I have a CZ MTR 6.5 Grendel rifle, I'm using both Lapua and PPU 'improved' cases.

My query is regarding the minimum case length for this calibre.
I have seen the figure of 1.520" quoted by Bill Alexander and other sources quote 1.515".
My rounds are trimmed to the smallest length taking as little as possible off the case length.
I'm puzzled by both the lengths on my Lapua and PPU cases.
After about 5 firings on both makes of cases, my Lapua cases are measuring about the 1.510" length and the PPU about 1.505" for the first batch I bought and about 1.508" for the second batch of PPU cases.
The best groupings I've found for my rifle are using 27.0 gr Tac, different primers don't seem to make any difference. loading to 2.250" COL , with Sierra M/K 107gr bullets, I'm achieving clover leaf results with these settings at 100 yards.

Why is the case length dropping so low and is it safe using cases of this length? No evidence on case or primer after firing to suggest anything wrong.
The rifle is great, enjoy the calibre.
I've found no difference in accuracy using either the Lapua or the PPU 'improved', no apparent faults on either cases after approx. 5 firings

 

[Bavarianbrit]

GRT drawing.

 

[phaedra]

Saami spec for case length is 1.520" (-0.020) so perfectly safe from 1.500" to 1.520"

 

[Howa]

Thanks for both replies, must say that I did not expect the decrease in case length to be so much after so few firings.
Glad to hear that all is ok at present though.

 

[Sharpie]

I am a little surprised that your cases seem to be shortening in length with repeated firings. The only mechanism I can think of is that when they are blowing out/fireforming to fit the chamber this is causing the brass to shorten, rather than as would be more usual, grow in length.

Which begs the question, are you FL resizing, or neck sizing ? Usually FL resizing causes the case to grow in length, as the brass displaced is extruded out towards the case mouth, there being nowhere else for it to go. It is essentially incompressible.

Personally, as I do not crimp, I am not over fussy about trimming every time, nor being too fussed about having all cases of precisely identical length, but let them grow until a few are approaching max. length, then trim the whole batch back to where the Lee cutter/length gauge/lockstud sets them, usually pretty close to the middle position, i.e. 10/1000" below max. Generally SAAMI tolerance min and max. case length the same, max. minus delta of 20/1000". I see no particular rhyme or reason for this figure, which seems to be pretty broadly applied across the majority of chamberings, large and small.

I see a downside, probably more imagined than real, to having cases towards the shortest end. Less neck tension, less neck length in contact with the bullet shank, maybe poorer obturation, and the gap between the case mouth and the end of the chamber neck portion more likely to build up powder fouling, in a ring that needs to be cleaned out periodically, lest it build up into something that might even jam against the case mouth.

Some barrel cleaning regimes don't really touch the chamber fouling much, which to be done properly does require say a different brush at least.

A cleanly trimmed case mouth, square and properly deburred will though give a cleaner launch to the bullet than something that has maybe grown a little un-square. over repeated firings between trimmings. So for me, something just slightly less than max, and no shorter than 10/1000" less seems about right. To keep things simple I tend to find myself just trimming every five reloads, whether they need it or not. It is one of the most tedious aspects of case prep.

 

[Howa]

I am a little surprised that your cases seem to be shortening in length with repeated firings. The only mechanism I can think of is that when they are blowing out/fireforming to fit the chamber this is causing the brass to shorten, rather than as would be more usual, grow in length.

Which begs the question, are you FL resizing, or neck sizing ? Usually FL resizing causes the case to grow in length, as the brass displaced is extruded out towards the case mouth, there being nowhere else for it to go. It is essentially incompressible.

Personally, as I do not crimp, I am not over fussy about trimming every time, nor being too fussed about having all cases of precisely identical length, but let them grow until a few are approaching max. length, then trim the whole batch back to where the Lee cutter/length gauge/lockstud sets them, usually pretty close to the middle position, i.e. 10/1000" below max. Generally SAAMI tolerance min and max. case length the same, max. minus delta of 20/1000". I see no particular rhyme or reason for this figure, which seems to be pretty broadly applied across the majority of chamberings, large and small.

I see a downside, probably more imagined than real, to having cases towards the shortest end. Less neck tension, less neck length in contact with the bullet shank, maybe poorer obturation, and the gap between the case mouth and the end of the chamber neck portion more likely to build up powder fouling, in a ring that needs to be cleaned out periodically, lest it build up into something that might even jam against the case mouth.

Some barrel cleaning regimes don't really touch the chamber fouling much, which to be done properly does require say a different brush at least.

A cleanly trimmed case mouth, square and properly deburred will though give a cleaner launch to the bullet than something that has maybe grown a little un-square. over repeated firings between trimmings. So for me, something just slightly less than max, and no shorter than 10/1000" less seems about right. To keep things simple I tend to find myself just trimming every five reloads, whether they need it or not. It is one of the most tedious aspects of case prep.

I am full length sizing my cases, I'll now be re-examining my sizing length and frequency.
Many thanks for your detailed reply, it's a great help in understanding my query.

 

[Sharpie]

I am full length sizing my cases, I'll now be re-examining my sizing length and frequency.

No trouble. I am still intrigued that your cases are apparently shrinking in length.

My suggestion would be, assuming that you batch your reloads, is that you take a sample , or even all, of each batch, and measure them after each firing but before resizing, then after resizing (but before trimming of course).

This is why I like the simple Lee cutter, length gauge and lockstud arrangement. It is practically impossible to over-trim, and the actual trim length is fixed by the case length gauge. As I said, they usually made to about max. minus 10/1000". Most other trimming methods can introduce variability, whether in the setup and adjustment, or simply imprecision in the tool.

Average these measurements out, then you should see a trend. I suspect that you might see little or no growth in length just during the firing. But some increase in length after the subsequent FL sizing.

This trend might even give you an indication of over how many firings trimming is likely to become essential. e.g. if you are consistently seeing say an average of 1/1000" growth after resizing, and are consistently trimming to max. minus 10/1000", you might be able to get away with five or more firings between trims. I have found that the spread between the average over the sample, and the longest one in the sample, is not more than 2:1 ratio, usually a lot less.

For my chamberings, never loaded to absolute max. pressure, or beyond.

As to the squareness of the case mouth, which can become irregular after repeated firings without trimming, I like to place my cases mouth down on a really flat surface, e.g. a glass mirror. They should all stand up straight, with no wobbling.

The mirror (I have a bathroom mirror tile for this) is also useful for checking loaded ammo for bullet and case runout. If you roll the round over it, and look at the reflection of the bullet tip, you can see any gross runout. Looked at obliquely, the reflection can almost double the effect compared with simply looking at the tip of the bullet directly. Optical lever effect. Quick and easy, and seems good enough for my purposes.