Hey gang. Got my first kit from DM yesterday. It's made from SS and Ti. Gonna be used on 556 guns, including some SBRs. Questions:

1. after I drill/mod the baffles,  is heat treating either of these materials a good idea relative to longevity / durability? If so, optimal process for each?

2. the endcap is Ti. I'm thinking maybe just darkening it with Oxiclean. Question about that though: will it affect durability at all?

3. If the spacers, especially the blast baffle area, are Ti, is there anything I can do to the inside that will help it withstand the abuse / heat?

1. Not unless they're 17-4SS in Cond A. Not sure what flavor of SS you have.
2. Shouldn't
3.a Use a SS liner, instead.
3.b Have interior ceramic coated.

The inside of .mil barrels, gas keys, and carrier bores are hard-chromed.

Not sure why suppressor parts are not commonly hard-chromed too, since it's tough enough for a 556 chamber even during full auto fire.

you have a point. Some silencer internals actually are chromed...I forget the manufacturer but I noticed a few cans that use chrome when adding them to the PWD. Is it possible to only chrome the inner-surfaces of small parts? I need to keep certain dimensions the same for fitment...

Run 17-4 SS baffles for the first 2 or 3 and you'll be fine.  The Ti spacers will be fine also unless you're planning on mag dump after mag dump.  Muzzle brakes also help distribute the blast more evenly in the blast chamber, and the brake takes the brunt of the blast.

I personally use Griffin Armament Taper mount muzzle brakes due to the repeatability and how well they work.

I think DM uses 17-4ph SS so you don't need to try and heat treat them.

If I recall DM's 17-4 cones X are in condition A so they aren't as hard to drill. That's still pretty good as a baffle but heat treating for an hour at 900 F after final machining will harden them and they should have a slightly golden color tinge after hardening.  Some people can get a lead pot up to 900 F or if you know someone with a pottery kiln that can be used but in either case you want the temp control to be pretty good.

You are correct dm cones are annealed. Even if the cones from zmachineworx are h1150 they can be annealed and heat treated to h900.

Hardening steel is not something the average DIYer is outfitted to do. There are steps critical to achieving a quality final product. While any Youtuber seems adequate enough to do so, no one truly knows the final hardness or if actual tempering has been achieved w/o quality testing.
Turning H1150 into H900 is not an easy process. The metal must be heated to over 1000C, pickled in 2 separate caustic acid baths, rehardened to desired condition, and followed up w/ a final pickle or electro-polish to achieve maximum corrosion resistance.

Using 17-4SS in A or H1150 is not a terrible thing; they're still a very strong and resilient conditions.

I never said it was easy just that it could be done

I gotcha