Don't drill out anything. Cut the spring to the same length as the stock spring. Install it, reassemble and then test it. You might discover it works perfect.

If the performance is poor, then remove the ARs and their springs. Put some padding on the plunger head. Some felt, a silicone disk, or rubber washer work really well.  Then reassemble and test it again.

But don't drill out anything right now.  You might find it works better with the ARs installed. If you drill it out, that won't be an option. 

Understood. I do plan on brassing the shells: should I friction-fit them or apply superglue? This is also for the lawbringer.

Always friction fit first. When you are satisfied with the end result, then superglue them in place. Some modders use hot glue instead. It's somewhat less permanent. 

With the k26 adjusted to the same length as the stock spring, range does increase a bit. However, the difference is only noticeable when you angle your shots (up to 10 feet difference). I didn't expect too much improvement since it is a shotgun after all, but I'm going to start brassing the shells to see what happens. However, I noticed that the ends of the brass are very sharp and can hurt the dart if you put them in quickly. Is there a way to soften these ends?

Yes, a simple way is to use a small piece of plumbers abrasive cloth or mesh and bevel the inside edge of the brass tube by simply sanding it down by hand. In fact you can dull the outside edge as well if it's going to be exposed. But that probably won't be an issue with a shell. 

Here's a crude not-to-scale drawing.

Alternatively, with soft metals like brass you can use a countersink bit, put it in the end, and just hand turn it to bevel the inside edge. It gives you a nice precise angle, if that's important to you.

So the brass won’t fit in the shells, they’re about the same diameter. Should I use a drill bit to make the brass fit?

Unless you are prepared to potentially sacrifice a shell or two, you’ll need a drill press and a 9/16th inch forstner bit to properly ream out each chamber. I’ve tried doing it by hand on a couple Nite Finders and ruined both.

The fact that you don't seem to have done your homework is concerning. Please be careful ! Let's plan to keep all ten fingers. Aight ?

Yeah, I messed up there. When my school opens again, I can use our drill press. Is the forstner recommended or required? Will the regular bits be too messy?

If you have a drill press and fasten everything down really tight, you might be able to use a standard bit. Even then it's hard to get a standard bit started precisely in an existing hole. The object being drilled cant help being  bounce around.

Only a forstner bit allows you to drill a precise hole over the top of an existing hole.