Need advice; buggering up cases trying to convert .223 to .300

[PFDD]

First post here; been lurking, reading, ordering parts and have a 300/221 AR-based build almost complete but for cutting and rebending the gas tube. Thought I'd put a few cases together today. I'm relatively experienced loading .45 ACP, .308 and .223 ammo on both progressive and single stage presses but I've never formed cases before.

Here are the dies I'm using;
Redding form and trim die with extended shellholder
Redding small base (body only) sizing die
Redding 300/221 3-piece deluxe die set with FL sizing die, Neck sizing die and seating die and two sizing buttons; .270 and .30

I've ordered the Harbor Freight Chop Saw and it's inbound so I trimmed a few sample cases to both 1.400 and 1.355 using a tubing cutter. I tried trimming a few before and a few after running them through the form and trim die.

Here are the issues I'm having making cases;

I've snapped two case necks off when running the .30 caliber expander through the neck even though the outside and inside of the cases are lubed with Imperial Sizing lube. Fortunately I've been able to clear the necks using a tapered brass punch since I don't yet have the stuck case remover but I'll have to order it given my experience so far.

I've also crumpled three necks trying to run the .30 caliber expander through though I generally get zero resistance running the .270 expander through.

The brass I was experimenting with were South African cast offs (Berdan primed) that were of course disposable anyway, but then I tried the same process on a LC case and I snapped off a neck on it too. The brass was cleaned but not deprimed because I was using the Berdan primed cases initially.

Obviously I'm not doing something right but I'm using plenty of lube, and tried a couple of different sequences; trimming first, form and trim die followed by trimming, small base die first, then form and trim, FL resizing die with both expanding buttons in sequence; .270 and .30. Not sure where I'm failing but clearly I've not been successful.

I had planned on case forming using primarily LC brass and since I'm relatively close to the Quantico Marine Corp base some of the cases could have been fired in a SAW hence the precaution of the small base die.

I did manage to make a couple of decent cases but none would chamber fully even one that I seated a 220 Grain SMK to an overall length of 2.260. My barrel is from TP555 and though I forgot to order a chamber gauge with the barrel, I have one coming soon.

The press I was using was using was a Redding T-7. I used a RCBS Trimmate to clean up the chamfering on the case mouth. When I go into production mode after figuring out where I'm failing, I have a Giraud with the 300/221 case holder that I'll bring to bear for final trimming duty.

Clearly I shouldn't have the level of resistance I'm seeing with the .30 caliber expander and I checked it with the calipers to ensure it was indeed .30 (it was).

Seems there are two completely different schools of thought on the best source of .300 Whisper brass (.221 or .223). I've seen reasoned opinions in both camps so I'm unsure if all else was equal, but I'd like to use the .223 if possible since I have a fair number of cases already.

So in summary, what the heck am I doing wrong?

Your advice is more than welcome and appreciated.

Best,

Brad