Considering getting into reloading (303, 6.5x55, 30-06)

I've been tempted by reloading for my milsurps for a while now (I have three 303s - soon to be four - a Swedish Mauser, and about to get a 1903A3, fingers crossed).

I've been keeping my brass for the last few years in case I do so have a fair box of .303, mainly PPU once fired. My question is that I expected each chamber to be slightly different, so that once I start reloading I'd want to keep brass from each rifle seperate (which I've not done so far). Testing shows, though, that some of the brass now won't chamber in any of the .303s - but it must have been fired from one of them! (My range is closed lanes, so I will only have picked up my own fired cases).

How can that be? For the ones that won't, fit the bolt won't go all the way forward, so I'm assuming the shoulder has moved forward.

I'm a complete novice (in fact not even started) in reloading so I may be missing something entirely obvious!

Best to buy some good reloading manuals & digest what they are saying.
Buy good quality gear Redding & RCBS are pretty good entry level, Lee gear is not too shabby it's what I started with.

Reloading is just a sequence of events but you need to be meticulous in case prep and reloading as get it wrong and things go bad real quick.
You'll find the 303's shoot ok with BT's it will take a while to tailor their loads remember that the rifle was built around the ammunition not the other way.
Always start low and build up watching for pressure signs, have fun.

To agree with and add to Cinders: since you’re considering getting into reloading, watch plenty of YouTube videos and and buy at least one reloading manual. Note that necked cartridges involve an extra step or two over straight wall cartridges. Doesn’t surprise me at all that a spent cartridge won’t pass a fit check in the same rifle (hint: to your main point learn more about neck sizing vs. full-length resizing - each has pro’s and con’s and what your goal is will determine your path).
I like the ultimate reloader YouTube channel but there are others as well. Have fun

I find reloading a challenge and a way to tune out outside stresses.

Get yourself some plastic storage boxes and a labeller. Place some identifier of the rifle the ammo was shot in on the box. Keep your ammo separate that way, only neck size the brass and keep a log of how many times each case has been fired.

As the guys have said, reloading can open a lot of windows, if done properly and well researched. As to the problem you mention, others here will know a lot more than myself on the topic, but loading for Enfields can present several issues. It is possible that one of your .303's has a bit of wear (or mismatched bolt) allowing the case to push back farther from the chamber under the pressure of firing, and thus causing the shoulder to be reformed closer to the neck. Bolt face set back occurring during firing may not always be detected by reinsertion of a spent casing. Competent inspection is a good idea here with any suspect rifle. Case head separation is a possibility, at best a real annoyance. Accurate measurement of the cases might guide you to some conclusions, and if you get into reloading, you'll want a set of good calipers anyway.

When I first started reloading ~8 years ago, I bought a Hornady iron press kit. The first chapter of the manual that came with it had everything I needed to know to get started. I primarily wanted to reload rifle. At the time, there was very little savings, if any, in reloading 223, and it actually cost more to reload 9mm. But, I calculated I could save over 50% on 30-06, which was what I shoot most frequently. So, I bought a case trimmer kit (also Hornady) to go along with it, and a powder trickler. The biggest advantage to reloading is that if you are thoughtful and meticulous, the quality control in your shop will far exceed anything bought on the store shelf. So, when compared against premium match ammo (which yours too will of course shoot better), the savings go through the roof - even now with scarce components.

Components are starting to come back in stock. Don't wait until you are low or out of supply. Just grab what you can afford whenever you see it and you will soon weather the storms when they come up every 4 years or so.

I find the monotony of the process, and need for great attention to detail, both relaxing and theraputic.

All really useful stuff, thanks. I subscribe to ultimate reloader and a few others and will continue to watch and learn! First step is to start to work out what my bench needs to be and where, which involves some Domestic Authority negotiations…!

There is really no end to what gear you can get for reloading you've just got to figure out how far you want to go trim to length, clean primer pockets, annealing*, FLS V's Neck sonic cleaning V's Media tumbling on and on.
You'll find brass gets work hardened with firing and reloading processes like FLS/Neck sizing this is why I procured a Annealing Made Perfect machine as I reckon it saves the brass if you keep you brass separate then neck size only I also have body dies for my rifles in F class.
The subject is so vast but I run a excel spreadsheet to round count - rifles, and load data and results plus I have it all on a backup disc always take your time and double check, always reload above 50% saves double loads I usually have 95% in my rifles.
Never be afraid to ask a question either allot of us have tripped along the way so may be able to assist with our experience.
Cheers.

Another useful thing with those three cartridges is that hey are "forgiving" on powder types.

All three will work well with bullets in the 140-150 gn weight and powders like IMR3031 and IMR4895..

Yes, you can "push the envelope" with more exotic propellants, but "blips" on the learning curve can be "frustrating, exciting and "interesting"".

A good basic case trimmer is a must if you are cycling the same brass repeatedly; even moreso if you have several different rifles in the same calibre.. Not all chambers are identical, especially in vintage "Mil-Surps". and the result will be variations in the brass after firing.

Annealing is a whole different conversation, depending on the type and quantity of brass being reloaded, among other considerations..

The whole process may not save you a vast sum of money, but it is cheaper and better therapy than what you might get on a Shrink's couch.