I'm a new owner of a Ruger Bisley 45 Colt, thrilled to own one of the Acusport 5.5 barrel stainless models.

This is my first experience reloading lead bullets, I want to load some Cast Peformance or similiar 300g bullets. Do I need gas checks or not? What is the rule for using gas checks vs plain base LBT bullets?

I have the fortune to get a couple of the Accusport 5.5 " over the years. I generally use lead bullets w/gas checks as they keep leading down to a minimum. Depending on the antimony or tin content/hardness of the bullets, some plain based bullets are accurate enough. For all my revolvers, I prefer the LBT type of bullet and the the WFN semi-wadcutter also. First thing you should do is shove a bullet through your cylinder throats to see if they will fit. If not, they'll have to be polished to accomodate the bullets.

Any time I use cast bullets, I use gas checks. I don't like cleaning lead out of my barrels.

If you plan on a hunting load, use gas checks, if you are loading a plinking load plain cast will work.

For me the only rationale for using 300gr and heavier bullets is for hunting and in that case you really need gas checks at the velocities you will want to run them at. In my experience leading comes into play at higher velocities or with heavy charges of fast powders. My standard plinking load is the 255gr Lasercast over 10.0gr Unique, this load is not gas checked and has never leaded my guns. Ditto for 18.5gr #2400 over the same bullet (great load). I second the comment about checking the cylinder throats, they may need to be reamed, but I would only do so if accuracy is bad. I have a .45 Colt/.45 ACP Blackhawk convertible that had such undersized throats that I wouldn't even shoot a group at 25 yds, more like a pattern. I reamed the throats, did a trigger job, and freespun the cylinder (worth doing if you will be shooting a lot of heavy loads) and now it shoots into an inch at 25 yds with the right load. Good luck, the Accusport Blackhawks are great pistols.

I'd go shoot the revolver before worrying about tricking it out or making modifications.

Unless you are going to be hunting really big game, 13.0 grains of HS-6 or any other load that gives around 1,000 fps with a 260 grain weight bullet is going to be adequate for deer, or most anything else in the lower 48. I shoot plain based cast bullets and do not have problems with excessive leading; so I don't have a need for gas checks. Some of the bullets I have shot are gas-checked design and that is fine as well.

Every gun is different, and that goes double for Rugers. Some will shoot PB fine while others require a GC.

As always, check the Ruger's throat diameter. Older Ruger 45's had oversize throats, newer ones are sometimes undersize. You want a throat around 0.453". Some Ruger owners report a constriction where the barrel screws into the frame. All of these things can play hell with cast bullets.

My Bisley Blackhawk shoots well (at least better than me) with and without gaschecks and does not seem to lead without them. For the most part I choose gaschecks only when they come on the LBT bullet shape and weight I've chosen.

No, you do not need gas checks! If you cast and the boolit is designed for one, then you need it. PB and gas checked boolits will shoot the same depending on boolit design more then whether a gas check is used or not.
Boolit fit is much more important and they should be a snug, push through fit in the throats. The throats MUST be at least .0005" over bore size. One or two thousandths is OK too but fit the boolit to the throats, not the bore.
Then a good lube like Felix, LBT magnum, LBT Blue and Lar's Carnauba Red will make it sit up and talk. Do NOT use Lee liquid Alox

A lot of the Rugers had throats too small, smaller then the bore, and need lapped or reamed to get anything to shoot. You should slug the bore and throats to get measurements.

Ditto what bfrshooter said about Ruger throats. My Bisley would would lead the first few inches of the barrel before getting the chambers opened up to .451 from the factory delivered of .447. Once done, no more leading.