Gunsmithing Why does my bolt face look like this?

[glock24]

I have a new Howa 1500 mini-action in 6.5 Grendel, and I started working up a load with Lapua brass, AR-Comp, and a 123g Hornady ELD.

My first charge weights ranged from 27.5g to 29.0g. During testing, bolt lift felt gritty and sticky on all the loads, and I did have ejector marks on the highest charge weights. I decided I was overdoing it, so I cleaned the rifle completely, and conducted a second test today using charge weights between 26.3g and 27.5g. Again, every load made the bolt lift feel gritty and sticky.

Below is what I'm seeing with the bolt face and the case heads. That bolt head is after only 18 rounds. It is happening on every load, and I no longer feel it is related to excessive pressure. It seems like the ejector hole is somehow scrapping brass. I was thinking about disassembling the bolt, and polishing up the bolt face. I was also thinking about trying to break the edge of the ejector hole with some polishing compound and a Dremel. Before I do so however, I'm hoping for some second opinions on what I'm seeing.

Thank you

 

[Rthur]

Lapua brass could have lost some case capacity and your loads are warmer than you think.
It would be pronounced in a smaller case capacity such as the 6.5 Grendel.
Did you run these loads over a chrono for a velocity caparison?

R

 

[AIAW]

Yeah, good point Rthur. Also, how slick does the chamber look? If it's too slick you could possibly be getting bolt-thrust from the case not being able to expand to the chamber wall. Lapua brass is a tad harder than most. Might not hurt to put a light cross-hatch in it with a little green Scotch Brite. Just enough to get the mirror finish out of it (if it exists).

I had a similar issue to yours from someone going crazy on a chamber polishing and taking the mirror finish out of it made a big difference.

 

[my human host]

Something like that is going to perpetuate itself. Most likely nothing to get worked up about.
Few possible causes that come to mind:

Shoulders need to be bumped.

Extractor has a sharp edge that's shaving your cases a bit. Said shavings stick to the boltface where they cause a bit of galling.

Boltface is a smidge out of square.

 

[fastarget]

Something like that is going to perpetuate itself. Most likely nothing to get worked up about.
Few possible causes that come to mind:

Shoulders need to be bumped.

Extractor has a sharp edge that's shaving your cases a bit. Said shavings stick to the boltface where they cause a bit of galling.

Boltface is a smidge out of square.

Agreed, the most common would be the shoulder needing bumped. Does it feel reel tight to chamber the round, and less so after you fire it?

 

[glock24]

Thanks for the insight, guys. I think I'll take the bolt apart and give it a good cleaning. I'll also look or burrs on the extractor hole, and the condition of the chamber.

To be clear, this is brand-new Lapua brass that I ran through a FL sizing die initially to bump the shoulders another 001" and fix any out-of-round necks. I also checked velocity on a few loads this time, and I was around 2350, which is quite modest.

 

[honesthop]

AIAW- I completely agree with 'too slick a chamber finish' topic. if the brass doesn't 'grip' the chamber walls upon expansion, the case can be forced excessively against the bolt face. now you've increased strains on the locking lugs. using too light an action in souped-up wildcats, 338 lupes in 700 actions, and other excesses then A.I. THAT! I have scoured chambers a little for grip, but I polish dies that need it for sure J B paste & BORE BRIGHT. less case lengthening when sizing. I use hornady one shot, but imperial sizing lube is tops for sizing 'tuffies'.

 

[Timo Turl]

I had similar. Bumped the shoulder another 2 thou and it went away

 

[181backup]

Also not cleaning the case lube off may cause some of this if it's chamber grip problem

 

[glock24]

Update:

So the chamber isn't particularly smooth or shiny . . . but I now remember that my cases do have residual Hornady One-Shot on them. I didn't clean it off after running these new cases though a FL die.

So can someone explain what is happening? As the case is fired, it is thrusting backward into the bolt face instead of expanding and sticking in the chamber? And this phenomenon pushes the face of the case head into the bolt face surface with enough force to cause some scrapping when the bolt is turned and opened?

 

[Geno C.]

You got it. That is unless there is resistance when you close the bolt before firing. If that's he case then you have brass that is a bit too long for your chamber. All of my ackley guns do this while I'm forming. You can minimize it it with lube on the bolt face and the bolt lugs. If the bolt closes easy, then I would think the excess lube on the cases could be your problem

 

[scudzuki]

Definitely a bad idea firing ammo that has lubricant on the case. Without the assistance of the grip afforded by the brass expanding against the chamber wall, the bolt, lugs, and action are the only thing(s) holding the brass in the chamber and preventing the dreaded kaboom.

 

[Rthur]

Update:

So the chamber isn't particularly smooth or shiny . . . but I now remember that my cases do have residual Hornady One-Shot on them. I didn't clean it off after running these new cases though a FL die.

So can someone explain what is happening? As the case is fired, it is thrusting backward into the bolt face instead of expanding and sticking in the chamber? And this phenomenon pushes the face of the case head into the bolt face surface with enough force to cause some scrapping when the bolt is turned and opened?

Took it for granted there wasn't any case lube on the cases.
A factory chamber is rarely to slick also.

The one shot isn't allowing the case to get a grip on the chamber upon firing. As it moves rearward the bolt takes the hit.
This also allows the case to fire form a little longer. Thus upon primary extraction the bolt rubs the case a little upon rotation.

R

 

[AIAW]

Yeah if we knew about the lube it would have been a dead giveaway early on! Still, the diagnosis of bolt-thrust was in the right area at least. 181backup nailed it with the lube post.

Hopefully glock will test and let us know. Sure-fire resolution is always good for these threads.