I'll try to keep this brief, but need to provide some background so you're not starting off without the whole story as they say.
Longer? Ain't sure. A little background tho': started reloading when my B-I-L gave me a box of gas checked 150 grain .309 bullets for Christmas, 1973. I'd won a mdl 64A Winchester in .30-30 during our cruise to VN that year. I'd been saving my brass from practice sessions off our fantail during the later part of the cruise. I was a member of our ship's pistol, security and demo team. Might've been the only guy in the Navy with my own stash of ammo and firearms in the ship's armory! Wonder if that'd happen in today's Navy? Anyway, I went out and bought my first reloading stuff at The Grant Boys in Costa Mesa. I got dies in .44Mag and .30-30 along with a RCBS Jr. press, a Lyman #45 reloading guide, a lb. of 3031 powder and 1K primers. I also bought a box of Sierra 150s to match the factory ammo I'd been shooting.Back to your observation regarding sharing cast (gas checked) loads with jacketed ones of the same weight - that goes against everything that I thought I had learned - starting pressure, different friction and plasticity factors of the two materials, and even different diameters (frequently) between jacketed and cast bullets of the same "caliber."
But you've been at this longer than me. If you post the comment, I'd sure like to see what you have to say regarding these issues.
The gun guy at Grant Boys was a reloader and answered my hundreds of questions. One of which was about the range of powder charges shown in each powder for every caliber. The advice he gave me was to start by loading 5 rounds at about the mid point of the range, then load 5 more rounds at ½ grain increments up to the max. Shoot each of those in order until I get any signs of flattening primers or find a super accurate load. I found that 3031 wasn't a great powder for minute changes in powder charges. I tried several, including 4895, 4320 & 4350 before settling on RE-7. That has worked for me in my all my rifles. It's also the advice I tend to give for new reloaders... although on some rounds, like the .30-30, I tell 'em to start about ¾ of max, or around 20% below max.
Admittedly, I won't use this method in rounds like my .30-06 or 7mmRemMag. Mainly because I don't shoot lead bullets from either.
It may be counter-intuitive, as I've heard the same as you, but well after I'd established my loads. How can a can cast bullet sized at .309 have the same or lower pressure than a copper-jacketed round sized at .307 or .308? Here's my hypothesis: 1 - I use hard boolits (20Bhn), a good crimp, one which I think releases at a pressure above that which simply release an uncrimped bullet from the neck, regardless of whether it's copper clad or lead. 2- Even tho' the cast bullet is harder (arguably), than copper, it is lubed, and more malleable than the copper jacket; I can still make a mark in the lead bullet, but can't in the copper jacket...
Since I don't own a chronograph, the only way for me to check pressures is to watch for signs, and point of impact. After I'd established what factory load gave me the most accuracy outta my 64A, I worked on getting those cast bullets to shoot to the same POI. That also happened to be about 1-½ grains below the highest max load I've ever seen for a 150 grain bullet from any manual... about 27.5 grains of RE-7. It also happens to be the most accurate load I've found in my custom, heavy barreled mdl 94 in .30-30.
Admittedly, this is in a pistol cartridge, but... look at .45 Colt max loads, (from that same 1970 Lyman manual). It lists a max load for a 250 grain jacketed with Unique as 9.0 grains for 901fps... yet lists a max load for a cast in the same weight as 10.3 at 1028fps.
That lil' bit of data extrapolates into that, in fact, lubed lead bullets can be pushed harder than their jacketed counterpart without excessive pressure. What effect a gas-check has vs. a full jacket, I've never investigated. Maybe my logic is faulty, but... I don't hotrod anything... I've seldom pushed the envelope on recommended loads... although newer manuals and web sources don't go to my 27.5 grains of RE-7 for a 150 grain pill in the .30-30. My 1980 Speer manual #10 lists 29 as max.
I get nice flat primers, but no indications that they're "flowing", just nicely flat. If you want, I'll post up on the board and open up for comments... might be interesting to see what others think.
The answer to that last was "yes", so there it 'tis. What say the experts on this?