of these moderate loads

Key phrase.

just under 39,000 CUP

As noted earlier this would be very close to original pre WW1 pressure levels, levels that were consistently exceeded by the vast majority of loadings since that time.

Most who handload for a Milsurp rifle start off using once fired brass that was loaded to modern pressure levels, with only a few modern sporting loads staying withing the pressure range you are using.
The first time the commercial round is fired in a loose Enfield chamber it will stretch just as most of the fired cases you showed sectioned earlier had stretched.

With my tightened up No.4 action with tight clearances I've never had a case stretch even with pressures around twenty percent higher than your loads. No case failures. Thats an exceptional result due to exceptionally tight headspace, not something everyone can expect from their individual rifle.
If my two groove bore shot as accurately with lighter loads I'd use them, but it doesn't.
I plan you use a load much like your own in my No.1 since it has a tight bore, and while its headspace is probably tighter than your rifle's its still looser than that of my No.4 with its No.3 bolthead.

Your headspace seems to be right on the suggested max. I've seen a great many Enfields out there who's headspace exceeds max by several thousandths and thin case rims add a few more to that total.

We get the point that if a new stronger than average case is used with pressures kept in the lower end of the scale for reasonably effective ballistics from the first firing onwards that you are getting exceptional case life.

As the shoulder strength thread with your illustrations pointed out cases vary greatly according to manufacturer and era of manufacture.

It would be irresponsible to suggest that every brand and lot of case has the same characteristics or that every casing and chamber combination would yield the same results.
The wide variety of bore sizes and number of grooves found in Enfield rifles of different types and years of manufacture, plus the individual bore conditions can effect overall pressures using the exact same loadings.

Another factor which I haven't seen mentioned is annealing work hardened case necks and how that might effect shoulder strength.
I'm not up on the more modern collet type neck sizing but I'm very familar with the need to anneal work hardened case necks, since they will sooner or later reach the point where they won't hold a bullet otherwise.

The Altered chamber shoulders of the Epps and Improved .303 were developed for a reason. That reason was that the results you are getting were not what everyone else was getting in those days.

I expect the alloys used in some casings today are stronger, but its highly unlikely that every manufacturer of .303 ammo has utilized the best available alloy.

temperflash, do you recall what you wrote on the subject a short time ago? "Well you can't successfully fire form .303 cases any way. No matter whether the case fills the chamber or not theres not enough of a shoulder to prevent the case being driven forwards by the primer detonation. The .303 case headspaces on the rim regardless."

This seems to me a broad assertion that's simply not justified by facts. You now say, "It would be irresponsible to suggest that every brand and lot of case has the same characteristics or that every casing and chamber combination would yield the same results." Isn't that exactly what you have suggested with, "you can't successfully fire form .303 cases any way"?

Broad assertion based on having never seen or heard of anyone successfully getting a once fired case to headspace on the shoulder in near fifty years of looking into this and other similar rimmed cartridges, and seeing no signs of it being feasible in any attempts I've made or been a part of.
When I see a Pig Fly I'll change my mind about that. Till then I'm not going to say that though I've never seen a pig fly and no one else has ever reported seeing pigs fly , and I've seen pigs fall from a great height without even attempting to fly, that ergo Pigs must Fly.
Theres always a very slim possiblity that one day a pig will fly past my window of course.

I could now say that although I've never seen it done successfully one fellow has posted convincing evidence that its at least possible in some cases, if the case is sturdy enough and loads are keep on the mild side and the shoulder area of the chamber has the proper conformation, but as a general rule you can't successfully headspace the .303 on the shoulder.
Not something to take to the bank just yet.

If this procedure gives similar results with my MkIII, which I plan to taylor mild loads for anyway, I'll have more confidence in it.
As I said earlier

Most who handload for a Milsurp rifle start off using once fired brass that was loaded to modern pressure levels, with only a few modern sporting loads staying within the pressure range you are using.

From what you've been saying earlier about lubed cases and such you don't seem to the reloading so much as custom tayloring loads. You are basically using the same methods as wildcatters do in fire forming, the process is similar in that the case was not designed to headspace on the shoulder and the chamber was not designed with that in mind.

I don't understand why you say that "Your headspace seems to be right on the suggested max."

Didn't you say earlier that using a headspace gauge your distance from breech to boltface was .074 which is .010 over minimum?

fire a FL-sized case

None of the cases I and those friends of mine who reload for the .303 were ever full length resized. most showed a noticable ring on first firing, Three to six firings was the average before case failure.
My No.4 ,before I discarded the worn non matching bolt for a new condition bolt and number three bolthead, measured .011 clearance using brass shims on a unfired military round whose rim I'd measured to be .063 + a few ten thousandths. Rather loose by my standards but apparently common enough in military use, not loose enough to bust a casing on the first firing even thin rimmed commercial cases, and no more than .001 over the .074 mentioned in the headspace thread.
That was the loosest action we tried out at the time. The others were two to three thousandths tighter in clearances.
Since it turns out the commercial rims are thinner than SAAMI specs indicated that increased the headspace accordingly.
The once fired Winchester cases we tried did very poorly in even the best of the rifles we tried them in, badly enough that I haven't tried them since tightening my action up. The Winchesters generally either partly separated or split long ways on the second and sometimes the first mild reload even if no annular ring was visible. They were very much undersized in diameter, the necks thin and flimsey. They certainly weren't headspacing on the shoulder after first firing.

I believe I did mention that a Long Branch No.4 belonging to a friend had an odd chamber and his fired cases had less taper and a noticably large diameter and clearly hemispherical shoulder. That particular rifle seemed to be headspacing on the shoulder but I consider that to be a fluke, since the chamber was far different than any other Military chamber any of us had seen.
Since the bore of that rifles was also exceptional, having been lead lapped at some point, and the chamber walls shone like chrome plating, we figured it to have been modified for range work by a previous owner.

It's a very accurate No.4. We glass bedded right here on the table this PC is sitting on in fact. I remember getting up late that night to give the action screw a quarter turn out to insure it would come out of the stock easily the next day.

PS
Now I've heard of a Rimless .303 cartridge before, a Canadian wildcat of some sort I think, and the 7.7mm Jap is often refered to as a rimless .303. I have never seen the specs on the former, possibly another case adapted to a rechambered Enfield.

And

No lubrication was used - case and chamber were kept dry.

Thats good. I see theres no more talk of lubing cases when fireforming.

PPS

Gauge	CDN Military	SAAMI/Australian
GO	.064	.064
NO GO	.070	.067
FIELD	.074	.074

Seems to me this would mean that .074 is maximum.
Though the Military cases at least can handle a few thousandths more without cracking on the first shot.

The friend with the odd chambered Long Branch had a unusual armorers gauge that the bolt was not intended to close on and the headspace could be determined by the angle of the bolt guide rib at the point where resistence was first felt. Only time I ever saw one of those. He had a lot of unusual Enfield Armorers stuff, I think he brought some of it back from North Africa.

Curled nylon monofilament makes useful spacers for fireforming new rimmed cases. The material is malleable enough to conform to the available space but sufficiently resilient to hold the case firmly against the bolt face.

Interesting. I'd thought of trying something similar using some very thin copper wire, fishing line hadn't occurred to me.
A thin band of cigarette or onion skin pare to center the case on first firing would insure centered firing pin strikes.

(How do you create a table on this board? That's a neat trick I can't recall having seen before - but I'm convinced it exists!)

I have no idea really. I'd simply meant to copy and paste the numbers then put them in proper order but instead the entire table was reproduced as you see it.

I've noticed that the cut and paste functions on this board operates very differently than it used to, the Yuku upgrade I suppose. It seems to pick up other elements besides letters and numbers.I also just got this Vista OS, still finding out things about it.