I thought the reasoning for the five shot cylinders was because six shot cylinders had to have the chambers bored on an angle.
That is correct.
The distance from the center of the cylinder to the bore on the 1858 Remington is too small to allow six 45 Colt cartridges to seat without their rims interfering with each other.
So Howell bored the chambers at a very slight angle so that the rims could clear each other. Yes, he patented the idea.
Later he sold the patent so when he started a new company to make cartridge conversion cylinders he could no longer make angled chambers without violating the patent.
So, for a while, Howell's 45 Colt conversion cylinders for the 1858 Remington only had five chambers.
More recently, the patent has expired, and Howell is once again making 45 Colt cartridge conversion cylinders with six angled chambers.
https://www.howellarms.com/1858-remington
As you can see in Driftwood's picture of the 6-shot cylinder, the rims are partially exposed. I don't know if this is a weakness, or not.
I actually had a gunsmith open up the counterbores on this cylinder ever so slightly to accommodate the slightly larger rim diameters of 45 Schofield cartridges. Schofield rims are about .520 in diameter, 45 Colt rims are about .512 in diameter.
Before he did so we mocked up the cuts on AutoCad and it became obvious that only opening up the diameter of the counterbores would leave a wafer thin area of steel between the counterbores and the outside of the cylinder. So we milled straight across to create 'viewing windows' that can be used to see if there is a cartridge in the chamber or not.
This later became a feature on the 45 Colt conversion cylinders being made by the company that supplies them to Taylors. If you check out Howell's website, he is doing the same thing now.
Why in the world would this be any weaker than a typical Single Action Army cylinder that leaves the entire rims exposed?
Remington 1858 with 45 Colt cartridge conversion cylinder showing rims visible in 'viewing windows":
Colt SAA with rims visible. This is in fact the way one can see if there is an empty chamber under the hammer. This one is fully loaded with six rounds.
Anyway, I feel more confident in the strength of my 5-shot than I would with a 6-shot. But another factor is the strength of the host pistol. I have these conversion cylinders both for my Euroarms (Armi San Paolo, pictured above) and my Uberti NMA '58. The Uberti is more robust in every way. Given a choice, I would prefer to use a conversion cylinder in the Uberti rather than the Euroarms.
That argument does not hold any water. It may make you more comfortable, but I have said many times that the cylinder, not the frame, is the pressure vessel in a revolver. As long as both revolvers have steel frames, it does not matter that the Uberti is slightly larger than the EuroArms (Armi San Paolo). I have both, my old EuroArms 1858 that I bought way back in 1975, and a stainless Uberti 1858 that I bought used a number of years ago. The Uberti came with a cartridge conversion cylinder just like the one I use in my old EuroArms 1858. It does not matter that the Uberti is slightly larger than the old EuroArms. Yes the conversion cylinder for the stainless Uberti is blued. Who cares? I don't.
Even better would be the gated conversion Howell sells ( or Kirst). That gives you full support (conversion ring/plate) for the cartridge during firing.
But, I understand the ease of a drop cyl and not wanting to modify the frame.
Which is exactly why I went with the R&D style conversion cylinders all those years ago. No alteration of the frame necessary. I can pop the original C&B cylinders back in and they are C&B revolvers again.