Please bear with me while I ramble and extrapolate....
I own a Ruger NM Flat Top Blackhawk (.44 Spec.) and a Ruger NM Vaquero (45 Colt) A few minutes ago it occurred to me that
if those cylinders are of the same external dimensions and
if they would interchange, it'd be fairly easy to figure out a reasonably safe pressure level for the 45 Colt NM Flat Top.
My calipers show the two cylinders to be
exactly the same diameter, and FWIW the .44 Special cylinder dropped right in to the Vaquero frame and functioned perfectly. The 45 Colt cylinder wouldn't quite fit in the Blackhawk frame but this was due to it being .003" too long.
So, with that said, it's fairly common knowledge the .44 Special in a Flat Top Blackhawk can safely be loaded to 25,000 psi. So....
(Fact #1)Given the slightly thinner walls of the same diameter cylinder, one could conclude the 45 Colt would be best/safest at around 2,2000-23,000 psi.
(Fact #2)This particular model Blackhawk is available in a convertible model whose second cylinder is chambered in 45 ACP. Of course we know that said cartridge loaded to +P pressures runs in the 23,000 psi range. So it seem reasonable to assume the 45 Colt cylinder could be loaded to similar pressures.
(Fact #3) Probably the most convincing argument was put forth by Brian Pearce in Handloader #234 where the subject was safe pressures in Rugers NM Vaquero (which we now know uses the same size cylinder as the Flat Top Blackhawk). He said:
"
This raises the question: What loads are safe in the New Vaquero? The weak link, so to speak, is the cutout for the locking bolt notches that leaves between .030 and .035 inch actual chamber wall thickness (which varied some from chamber to chamber). Considering the steels Ruger is currently using and with the above cylinder measurements, indications are that the gun is absolutely safe with loads that generate 22,000 to 24,000 psi. Certainly the gun will take even greater pressure than this, but with safety first priority, as well as longevity, I would advise against using loads that exceed that limit."
On the subject of 45 Colt loads, lately I've been working rabidly with my NM Vaquero 45 Colt. First having the cylinder throats reamed by the Cylindersmith, then installing the Bisley hammer, and fire-lapping it here at the house. Now I'm in search of a good day-to-day "working" load as well as a load for deer season next year. This evening just before dusk I loaded a dozen rounds consisting of 14.0 grs. of AA#7 under a 283 gr. cast SWCHP. It was too dusky for the chronograph, so I fired the last 5 rounds at one of my 50 yd. targets from a sitting position.
The cluster of four shots to the right were the last four consecutive shots. I was
delighted, as they say. I've found that you don't "accidentally" get good groups at 50 yds. so I have no doubt this load is a keeper. I'll chronograph them in the morning, but I estimate they'll be running right around 1000 fps.
Hope some of this rambling helps....
35w