I can't keep all the colt nonstandard parts straight so... Will an upper advertised as an "SP1 large-hole upper" fit a Bushmaster lower without any modification or adapter? Thanks

Negative.  BM rifles are use the Mil-Spec or smaller pin, which is .250.  The large hole uppers run in the neighborhood of .312 - .315.  You will need an offset/adapter pin.

You may be able to fit an adapter into the upper hole-ie a cylinder with compatible internal diameter. Might be able to affix it with JD WELD ,etc. Haven't done it myself but its an idea. Bushmaster has a reverse offset pin for this. The internal cylinder might at least be useful.

Just needs a sleeve on the pin in the upper.

I had exactly the same problem, Colt big hole upper and a Bushy lower. I bought an off-set bushing and that just about corrected the problem. You have to screw around with the bushing to get the front end of the upper and lower receivers to make a nice tight fit. Try to make note of the bushing's position when it fits right. With the upper removed, I had to drill a TINY hole through the upper lug and into the bushing so I could "key" it into that position. I mixed up some JB weld, put some on the off-set bushing, pushed it in and set the drill bit in the little hole to hold it till the JB set up. the next day you clean it up and snap the upper and lower together. JUST A WORD OF CAUTION, drilling that tiny hole may detract from the upper's value.    

Troy has the ideal solution as the Colt large pivot uppers can be desirable, and it
saves using the adapter.  You might could find someone who'd trade you uppers.

However, if you want to use the upper don’t use JB weld!  Try Loc-Tite or
something similar that will dissolved in acetone.  Then the adapter can be
removed later on without damage to the upper.

HTH

RK    

I bought the upper at a gun show for $60. It was what some call a mid-ban, a screw on flash hider but no bayo lug. The original owner had cut the flash hider off with a hacksaw (I quess so he could use it on his post-ban lower) and later had it crowned. I needed an upper for a Ciener 22LR conversion unit - how could I pass this up! In my case the reply about the Loc-tite may have been a better way to go but I was not familiar with Loc-Tite's ability at the time. By the way, it also shoots the .223 stuff great!