@michael458 I was thinking of going with belted cases for both cartridges to avoid head spacing issues.
Lets consider "Head Space" for a few minutes and take into account something most do not consider.
It is the "Extractor" on control feed guns, that actually controls Head Space. Why is head space important, and what function does it serve? Head Space keeps the cartridge from going too far into the chamber for the firing pin to hit the primer, keeping the head of the cartridge in direct contact with the bolt face..... right? Basically.
Now comes this big claw extractor on control feed guns. Does anyone have a lot of experience with 1911 45 ACP handguns? Well, we know that these are control feed guns, the rim of the cartridge slides up under the extractor and controls feeding into the chamber. Now, what is the limiting factor of that cartridge going too far into the chamber? First, you will always hear that the 45 ACP head spaces on the mouth of the cartridge! That is the first answer, but that is not exactly correct. The cartridge cannot possibly go any further into the chamber, "than the extractor will allow it to"............ The rim cannot get past the extractor to go further into the chamber, the extractor holds the cartridge firmly against the bolt face so positive ignition is assured.
A Winchester M70 control feed rifle, with the big claw extractor serves exactly the same purpose, and that is regardless of whether it is belted, shoulder, mouth or other supposed method, it is actually the extractor that controls head space.
I love a 1911 45 ACP, always have. In 2005 when I designed the 50 B&M, a 2.240 inch RUM case, cut/trimmed, I never thought to use anything but a Winchester M70 Control Feed gun. This cartridge would head space off of the extractor, but like the 45 ACP, it could do so on the mouth of the case. Just behind the RUM 50, I designed a 50 B&M Super Short, a 1.65 inch WSM Case, and used a WSSM Win M70 Control feed gun for those, exactly like a Bolt gun version of 500 S&W, only we could hit 65000 PSI in the little bolt guns.
So now, we have a 2.240 inch RUM case and rifle, and we have a 1.65 inch case and rifle, both rimless, and to test the extractor head space, I decided to fire 50 rounds of the Super Short version in the RUM version chamber! Obviously Case Mouth with the 1.65 inch case, could not in any way head space on the mouth of a 2.240 inch chamber! Impossible.
Here you can see comparison between the .500s...........
So, off to the range with Winchester M70 Control feed 50 B&M..2.240 inch RUM case, and a box of 50 rounds of 50 B&M Super Short ammo............. I fired all 50 rounds, every round fired with zero issues.... accuracy at 50 yards was crap because of so much bullet jump, but without any issue at all the 1.65 inch case fired 100% reliably in the 2.240 inch chamber.
This is perhaps to me the most important reason to use nothing but a control feed gun for DG.
So what does actually control head space in a control feed gun...............
Now, I love other types of rifles as well, single shots, semi, levers and what have you. I had a few of those 1885 Winchester Single shots converted to various B&Ms as well, one of those is a 50 B&M. In this rifle the head space absolutely is on the case mouth. As long as I keep my cases trimmed properly, then all is 100%. I also have a Ruger #1 chambered in 50 Super Short, and again, it head spaces on the mouth. The only issue I have ever had with either of these guns is when I get too lazy to trim the cases properly after several firings and loadings, and it grows too long and will not chamber.
I have a big #1 chambered in 500 MDM. That is the big 2.8 inch RUM case. While it has a little ghost shoulder on it, it is a bolt gun cartridge using nothing but the extractor for head space. But I had this #1 and converted it regardless. The 500 MDM is one of the few B&Ms that needs to be crimped. So one day I am playing with the Ruger #1 with some standard 500 MDM ammo I loaded, with crimp! Low and behold, I experienced a few rounds that would not fire in the gun???? Since we needed the mouth to head space on in the Ruger #1, and I had CRIMPED these loads for the Bolt Guns....... well the cartridge slipped too far in the chamber to have reliable head space and be held against the bolt face......... After that, I had to remember loads for the #1 were not crimped.
I mentioned Semi guns, I have a .500 version in a semi, this cartridge is 2 inch RUM case, and yes, it head spaces on the mouth, since the semi is basically a push feed. Never ever an issue, except when I am too lazy to trim and a case gets too long to chamber, other wise zero issues......
Head Space is a very important part of deciding factors concerning the cartridge, I mean, if it won't fire, its a problem. But there are other things to consider beyond just the design of the case. In any control feed gun, it does not make a hill of beans how the cartridge head spaces, because it will always be the "Extractor" that controls that for you.
