Black Powder Cartridges in Conversion cylinder

[Tex Brushpopper]

I have a Rogers&Spencer replica with a .45 colt conversion cylinder. Cowboy smokeless loads work great; the advice I got was to keep the pressure below 12,500 psi. That is what I do.
So, I get the idea to maybe fire some black powder cartridges with a 35gr/255gr (Starline brass, CCI primers) load through this gun.
My question is, is this load below 12,500 psi? There doesn't seem to be much pressure data out there...

[Drydock]

Yes,  you're fine.

[Dick Dastardly]

My Ruger Old Armies with Kirst Konverter cylinders can easily handle full house 45 Colt loads, but I only shoot black powder cartridges from them.  Why?  Because if I wanted a full house 45 Colt, I'd buy Ruger Vaqueros.  Your guns should be safe and fun to shoot with the black powder loads you mentioned.  Forget the heathen fad smokeyless stuff.

DD-MDA

[reloader4410]

Schofield brass case full 2f or Goex cowboy, some conversions chamber some don't rim is a tad larger than 45 colt I just chuck up on a dowel & touch with a file until brass fits. HAPPY TRAILS>

[Coffinmaker]

I hang out at the opposite end of the spectrum.  Anointed, Card Carrying, Self Proclaimed, Dedicated, No Doubts Recoil WIMP!!  I would find 35Gr with a 255 to be just a mite stout.  I would personally think, shooting that heavy more than once "just for the fun" to be some silly.  However, that's personal.  Those that choose to shoot "bigguns" are more than welcome.  HOWEVER:

The 45Colt cartridge concept is super flexible.  Full 45Colt case, tone of gonne powder .. BOOM!!  45 Schofield case, some less propellent, lighter bullet, less BOOM.  Cowboy 45 Special case, way less propellent, itty bitty bullet,  gobs of FUN .... less Boomb.

I have several sets of Colt Pattern Cap Guns set up for conversion cylinders of .45 persuasion.  I shoot C45S cases, 150 or 160Gr projectiles and 2f in all of em.  Very pleasant to play with.  Although I still have some 45 Barnstormers carefully husbanded.  130Gr, hollow base wad cutters.  Really FUN!!  NO!!  I won't share the Barnstormers.  Nowhere to get more (sniff sniff).

[Rooster Ron Wayne]

If we are talking about anything besides a Ruger Old Army .
I always stayed with light Cowboy  Unique loads or BP light to mild loads .
If you need to shoot hot loads .
Shoot a Cartridge gun .

Rooster Ron Wayne

[Grapeshot]

Schofield brass case full 2f or Goex cowboy, some conversions chamber some don't rim is a tad larger than 45 colt I just chuck up on a dowel & touch with a file until brass fits. HAPPY TRAILS>

Had the same problem with an R&D cylinder for a 1858 Remington.  I took 100 Nickeled .45 Colt cases and trimmed them back to Schofield length and used a 165grain lead bullet so I could keep the POI close to POI.  Was only off by 3 inches high.