There was also a 6mm TCU, a 6.5mm TCU (I shot one of those quite a bit), and of course, the 7mm TCU.

The 7emem version was everybody’s darling of course, although I always felt that 6.5 should have been just as versatile, but at the time it lacked the variety of bullet choices its bigger brother had available.

As I sort of recall, the .257 was capable of coming very close to .25-35 ballistics if you squinted hard and held your tongue just so. 

Never ever heard much about the 6mm TCU, but I used to run a couple of AR rifles in 6x45 that were probably well within spitting distance of the Thompson version.

Brass for any of them can be fiddly to create, especially if you aren’t conversant with Contenders.

I have a 7mm TCU on a XP 100, with a 15" Douglas barrel.

For cast bullets, I think the 7mm is the best choice. The bore to case capacity is very good for GC cast bullets. I did very well with it, using the RCBS 145 gr bullet (150 gr in #2 alloy) and 4895 or 2520. I never tried PB bullets in it but, I'm sure it would do atleast as well and probably better with them, as with the 2100 fps loads, the muzzle jumps, quite a bit.

Here are some development targets from it. It shot much better with 145 Speer bullets.

I have 6.5 14” that loves the 129 grain Hornady. It was extremely accurate and took out rams fine unless set real hard. Around 2000 fps in GI brass. That said a friend had a later barrel and could not get near as hot of loads without pressure issues.

Off topic, but:
back in my years of IHMSA , I briefly thought about getting into the 7 TCU, but I did get into the 7 International Rimmed in a Wichita production pistol.  Shot 135 cast bullets OK, but would not stabilize the 160 gr cast bullets.  Dumb Bert Stringfellow
built them with slow twist barrels. I idd build some XPs with fast 
twist barrels in 7 INt R, using 225 win brass. THey worked well.
After that I returned to my T/C contender 10" in 30-30.
160 gr Lyman 311466 clocking 1900 across the Oehler.
Deadly accurate and Never left a Ram standing.
Took High Production Cast Bullet at the 1982 Internationals

beltfed/arnie

Frank,
There were guys shooting certain ball powders
like 748 and especially Ball C2 that had flame thrower muzzleflash and blast.
My 30-30 load was with a case full of IMR 4895 under the 
160 gr cast bullet and had 
very little muzzle flash.
The recoil was not bad, what with having added a Pachmyer
forearm and Decelerator grip which added weight. A golf glove
was good to ease the skin sting of recoil.
beltfed/arnie

Frank,
I am confused. Are you talking about shooting a 30-30
Or a 7TCU or the 257. re. Muzzle flash?
Arnie

Frank

Thank You. I had not noted in your first post that you had the 
7 TCU caliber specified in your XP,
Yes, I have seen some very good match winning shooting with the 7TCU.  But "the guys" generally shot jacketed bullets in this ctg back in the heyday of IHMSA in the '80s-'90s
beltfed/arnie