Last purchase for the build is a comp and some modded Lancer mags. Which comp should I run? I have the Tromix lighter weight barrel that is .750 gas block. I assume it is the standard .458 threads 5/8 x 24? Any input would be great! Thanks....

Revised below***

She shoots great! Got a fair amount of kick but overall I had the most fun building this caliber...

Thanks guys

For a .458 brake you can do no better than the TROMIX 4 port fish gill brake.  It is the best brake I have used on the .458 and it is ready to go, no mods.  You are correct in that the TROMIX barrel uses 5/8 x24 muzzle threads.

The MI 3 port fish gill is excellent also but it will need to be opened up to clear the .458 bullet.

The TROMIX Radial Compensator is by far the best looking but it does not reduce recoil as much as the TROMIX 4 port or the MI 3 port.  The difference is not great but it is less than the fish-gills.

4 gill it is then....

Are Tromix Lancer mags necessary? Should I order the single stack followers too?

IMO no, nice, but not necessary.  The TROMIX followers are really only for use in states where the magazine needs to only work with the .458 SOCOM cartridge.  If you use the TROMIX .458 followers with the 5.56 it will hold and feed them until you get down to the last two or three rounds then they dump them all, therefore, they only really work properly with the .458S.  In states that do not have such goofy restrictions the followers don't bring anything to the party.

I use the standard 5.56 followers in mine so I can use any mag in any 5.56 or SOCOM I have, but I really like the TROMIX floor plates with caliber and capacity markings, not because they are required in my state but only because I like them.  Again, the marked floor plates only real use is for states requiring caliber and capacity markings; Indiana does not require such nonsense.
The only mods I make to my mags is the half moon cut in the front of the mag and that is ONLY for making removing live rounds easier.  If shooting wide flat meplat bullets the the half-moon cuts can aid feeding but they do not enhance feeding at all with the bullets I shoot, 300 gr TTSX, 250 gr GMX, and 500 gr RN.

So, short answer (too late), if your state requires capacity and caliber markings and restrictions, then the TROMIX followers and floorplates are a must have.  If your state does not require such BS then it is purely a personal desire to use them or not, they really will not aid function any.

FYI, cutting the half moon in the front of a Lancer mag takes about 20 seconds with a Dremel and a half-inch sanding drum.

Plain lancer mags have been the best I have used. Tried a multitude of others. The Lancers week by far the best.

Lancer with the semi-circle cut work darn well

Comp and mags are ordered... for factory ammo where do you guys turn?

Thanks for the help on this guys!

I have not bought factory loaded ammo for decades so I can't help you there.  SBR makes good ammo and is correct to spec chambers but it is expensive so buying direct from them is very expensive.  You might try Midway, Graf and Son, Midsouth... all the usual discount ammo sellers.

There is subsonic ammo out there - $2 a shot, IIRC.

SBR has been making 458ammo for a long time. They have the best proven track record.  Detroit ammo has also been around a while and they have the 500gn subs for around $2 each.  Black  Butterfly is pretty new to the Socom.  They have a wide selection.  Make sure you get standard 458 loads and not the ones loaded for the mutant.

Edited OP