I have a 1873 SAA and the thing loves 255gr coated bullets and unique. I even load them down to 6.5gr or so. The problem is that 255's are expensive and I see that a lot of the cowboy shooters use 200's or even 180's or even 160's. Would I be able to use a lighter bullet going about the same speed and keep the POI close?

Yes its Long Colt for the haters

I loaded 185gr LSWC for my Bounty Hunter in 45 Colt, they shot POA. They were powder puffs, but very nice to shoot, and surprisingly accurate for the sights the gun has to work with.

I shoot a lot of 180gr Berry's plated bullets through my 45 Colt and yeah, the lighter the bullet, the lower the point of impact. In my case more than a foot low at 12 yards out of a Ruger Alaskan, and I'm pushing them pretty fast. What about these?

https://www.midwayusa.com/product/1018185579/berrys-superior-plated-bullets-45-caliber-452-diameter-250-grain-plated-flat-point

A friend of mine became the owner of a pair of Ruger Vaquero's in 45 Colt. He didn't like the recoil of factory ammo so I loaded some cast 185gr bullets.  At the range, I was the only one who could hit the five inch steel plates.  Finally I confessed that I was holding a good foot high.

To answer your question, recoil is what causes the pistol to roll in your hand and shoot to the point of aim.  The sights are regulated to do that. Even with a lighter bullet going the same speed as a heavier one, there will not be as much recoil. ( Newton's Law)  Also the lighter recoil allows you to control the roll more, which might cause you to shoot low.

Best idea is to try it and see if POI is acceptable with you shooting the gun. If you want to shoot lighter loads with less recoil you may want to file down the front sight until bullets hit POA.  But remember that removing metal is not reversible.

You may try looking at a different vender for 250-255 gr bullets.

The savings going from 180's to 250's on MBC's coated bullets is only about $.03 per bullet. That includes the shipping increase due to a 1500 max on 250 vs 2k max on sub 200's.

I fire about 1k 45colt a year. For $30 extra a year sticking with the bullet my gun likes.

I think most cowboy action shooters use the lighter bullets more for recoil reduction rather than cost savings. As with other action games at top levels anything to reduce split times and save 10ths of seconds gets considered and tried.

Used to shoot a lot of 200 lrnfp out of 45 LC.  Every pistol had to have it's front sight filed down, even the Rugers with adj sights.  Big change in poi at 25 yards.  Same deal shooting 180/200 grainers out of 44 mag.

Dont know about 1873 but my uberti SAA loved the lyman bullet button load. Its near 160. I used soft lead and sized down from the 460 (normally for 45-70) to a colt sizing. I ran over BP though. POI was on.

It was super pleasant to shoot compared to my 255gr over BP.

Stopped shooting BP in colt though. Too dirty. 44-40 is a much cleaner BP cartridge.

In any case, you can easily use trail boss with anything. No idea how to predice POI.

I have shot that and didnt notice that big of a change, maybe a couple inches at 25 but maybe my powder charge was different.