Having switched from H870 to Retumbo in my 300 RUM I am now seeing very black sooty necks. Loads were from 96 to 100 gr.
What gives here?

You just need to purchase some 0000 steel wool. Sooty necks are part of playing the game.

I could care less what the brass color is when I fire as long as it's not over max and the accuracy is there.

Good Luck

Reloader

Sooty necks are usually indicative of low pressure-the brass is not sealing against the chamber wall.

Might not mean much, but I neck turned all my 300 Win. Mag. brass, now after firing...100% no sooty necks, indicative I believe in a very complete seal........time will tell if it stays that way....r in s.

YOU will get soot from work hardened brass if you have reloaded the brass a couple times.

got the same on my 30-06 brass-load is a 150 at 2700 fps. way low pressure ,but very accurate.I could easily add 1-2 more grains next time.

Soot around the necks can mean several things as already mentioned . Also brass ( Necks )are trimmed to short or bullets not seated out far enough .Fire forming cases to your chamber will normally stop it .

Curious as to accuracy your getting changing from H870 too Retumbo and Velocity differences , can you elaborate on that ?.

Shoot Straight Know Your Target . ...

Thanks for all the responses. In my 40 some years of reloading I've never experienced that.
Loads were from mild to hot. Some brass was used 5 times and others was new. Bullets were seated to specs.I'm starting to wonder if something happened to the chamber ie corrosion.
As to accuracy, with H870 I had the load dialed in to give me slightly less than 1/2" groups. I haven't achieved that yet with Retumbo - around 3/4". But velocity increases are promising- around 150 to 200 fps.

Sometimes slow powder will cause "sooty" necks.
Not a real problem.
Good Luck!

Herb, I once had several bathes of 338-378wby ammo show me soot all the way down the case. These were rounds that were loaded with 122grs of H870 and we normally right on the money for pressure and velocity. I had used the load for two years, and never once experienced any sooting. I was concerned until I fired some loaded in new brass, and they came out clean with no soot what so ever. I re-annealed the old brass and the sooting stopped. I have also seen sooted cases when shooting at very very lows temps. Around here we get to -40 in late December and January and I'll be damned if I'm going to be locked up for two months watching idiotic television.

I was always getting right around 3250fps with H870 and then I switched to Retumbo and went all the way up to 3450 with 225 gr bullets. Great powder and it doesn't seem to change properties when the temp drops severely.

if the brass is okay, its from lower pressure than unsooty necks.. but not a problem

Question ; Low pressure or slow powder some how back pressures the chamber ? , before the neck forms to the chamber wall and releases the projectile and exhaust gas ?.

Are you using Mag primers so as to insure as complete a burn as possible ?.

I had that happen to me a long time ago , I screwed up and used Std. primers in a Mag case and powder didn't burn completely . With Mag primers it never happened again .

Shoot Straight Know Your Target . ...

Might not represent a low peak pressure, but a slow early pressure rise so that the base of the bullet is out of the case mouth before the pressure's high enough to obturate the neck against the chamber wall and seal it.

I was with a friend one time trying out his new K & M neck turner and he had some cases that had soot going down the neck. It was lower toward the shoulder in some places and closer to the neck mouth in others, sorta like a sine or cosine wave.

Anyway I checked the neck thickness on a gauge and the variations in neck thickness exactly matched the variations in the soot on the neck, with the soot that went farthest toward the shoulder being the thinnest neck thickness.

I would think that the amount of soot would depend upon the difference between the chamber neck dimension and the outside of the loaded shell. Also the softness of the neck brass and the load dynamics such as type and amount of powder that would determine how quickly the neck brass will seal against the chamber walls.

I mistakenly assumed that enough pressure was needed to open the case mouth press it against the sides of the chamber before the projectile left the cartridge .

In saying this , I have experienced sooty necks as well as shoulder area , when my brass was in need of annealing . Also when to much pressure was exerted from the die on the neck when seating bullets . Other than those two scenarios I can't recall that having happened to myself '

Wait on an old military rifle I had an eroded throat chamber it also did that !.

Shoot Straight Know Your Target . ...